=============================================================================
Puzzles - Simon Tatham's Portable Puzzle Collection on RISC OS   Version 1.00

(C) Simon Tatham, 2004-2025; Stephen Fryatt, 2024-2025        24 October 2025
=============================================================================

Contents
--------

  1. Licence
  2. Introduction & Installation
  3. Playing Puzzles
  4. Net
  5. Cube
  6. Fifteen
  7. Sixteen
  8. Twiddle
  9. Rectangles
  10. Netslide
  11. Pattern
  12. Solo
  13. Mines
  14. Same Game
  15. Flip
  16. Guess
  17. Pegs
  18. Dominosa
  19. Untangle
  20. Black Box
  21. Slant
  22. Light Up
  23. Map
  24. Loopy
  25. Inertia
  26. Tents
  27. Bridges
  28. Unequal
  29. Galaxies
  30. Filling
  31. Keen
  32. Towers
  33. Singles
  34. Magnets
  35. Signpost
  36. Range
  37. Pearl
  38. Undead
  39. Unruly
  40. Flood
  41. Tracks
  42. Palisade
  43. Mosaic
  44. Known Issues
  45. Version History
  Updates and Contacting Me

1. Licence
----------

  The RISC OS-specific code in this port of Puzzles is licensed under the
  EUPL, Version 1.2 only (the "Licence"); you may not use this work except
  in compliance with the Licence.

  You may obtain a copy of the Licence at
  http://joinup.ec.europa.eu/software/page/eupl.

  Unless required by applicable law or agreed to in writing, software
  distributed under the Licence is distributed on an "*as is*" basis,
  *without warranties or conditions of any kind*, either express or implied.

  See the Licence for the specific language governing permissions and
  limitations under the Licence.

  The source for the RISC OS port of Puzzles can be found on GitHub, at
  https://github.com/steve-fryatt/puzzles.

  The core Puzzles code is copyright Simon Tatham and others, and is
  licenced under the MIT Licence: see
  https://www.chiark.greenend.org.uk/~sgtatham/puzzles/ for details and
  access to the source code repository.

  Portions of the code are copyright Richard Boulton, James Harvey, Mike
  Pinna, Jonas Koelker, Dariusz Olszewski, Michael Schierl, Lambros Lambrou,
  Bernd Schmidt, Steffen Bauer, Lennard Sprong, Rogier Goossens, Michael
  Quevillon, Asher Gordon, Didi Kohen, and Ben Harris.

  This manual is based on the original by Simon Tatham, with RISC OS-
  specific modifications where required.

2. Introduction & Installation
------------------------------

  Puzzles is a RISC OS port of Simon Tatham's Portable Puzzle Collection, a
  collection of 40 one-player puzzle games which are mainly focussed on
  logic and maths. More details about the collection can be found on Simon's
  website, at https://www.chiark.greenend.org.uk/~sgtatham/puzzles/.

  I discovered the collection through the Android port, and have also played
  the GTK version available from Simon's site. Since it is described as
  "portable", I decided that RISC OS needed its own version -- and so here
  it is.

  In his introduction to the manual, Simon wrote:

      "I wrote this collection because I thought there should be more small
      desktop toys available: little games you can pop up in a window and
      play for two or three minutes while you take a break from whatever
      else you were doing. And I was also annoyed that every time I found a
      good game on (say) Unix, it wasn't available the next time I was
      sitting at a Windows machine, or vice versa; so I arranged that
      everything in my personal puzzle collection will happily run on both,
      and have more recently done a port to Mac OS X as well. When I find
      (or perhaps invent) further puzzle games that I like, they'll be added
      to this collection and will immediately be available on both
      platforms. And if anyone feels like writing any other front ends --
      PocketPC, Mac OS pre-10, or whatever it might be -- then all the games
      in this framework will immediately become available on another
      platform as well.

      "The actual games in this collection were mostly not my invention;
      they are re-implementations of existing game concepts within my
      portable puzzle framework. I do not claim credit, in general, for
      inventing the rules of any of these puzzles. (I don't even claim
      authorship of all the code; some of the puzzles have been submitted by
      other authors.)

      "This collection is distributed under the MIT licence. This means that
      you can do pretty much anything you like with the game binaries or the
      code, except pretending you wrote them yourself, or suing me if
      anything goes wrong."

  The rest of this RISC OS manual is based very heavily on the original by
  Simon and his contributors, with small tweaks where necessary to make the
  content match the RISC OS experience. And while the licence of the RISC OS
  port is different (due to the libraries used to port it), the spirit of
  the puzzles is the same.

  2.1. Installation
  -----------------

  Puzzles is designed to work with RISC OS 4 or later; it can be used on
  RISC OS 3, but a copy of the Nested Wimp must be installed. It is 26/32-
  bit neutral, and should therefore run on all hardware systems as long as
  the 32-bit Shared C Library is installed. If required, this can be found
  in the System Resources on RISC OS Open Ltd's website at
  https://www.riscosopen.org/content/downloads/common.

  However, note that it may be slow on older hardware, such as a StrongARM
  RiscPC.

  To install, copy the !Puzzles application from the archive into a suitable
  place on your hard disc. It can be run from a floppy disc, but this is not
  recommended. Once installed, double-click on the !Puzzles application in
  the Filer window to load it on to the iconbar.

3. Playing Puzzles
------------------

  To play the puzzles, double-click on the !Puzzles application to load it
  on to the iconbar. Once loaded, a Select click on its iconbar icon will
  open the Puzzles list window, showing all of the available games. To play
  a game, click Select on its icon (or Adjust to open the game and close the
  Puzzles list window).

  The window supports interactive help, to provide a few more details about
  the individual games. The layout can be changed from large to small icons
  using the window menu. If the menu is opened over a puzzle, selecting
  Puzzle -- Help... will open this manual at that puzzle's entry (this only
  works for the HTML format documentation).

  As part of a suite, there are many features which are common to all of the
  games in Puzzles, and these are described here. Details of the individual
  games are in the following chapters.

  3.1. Game Window
  ----------------

  The Game window contains one of the individual puzzles. The keyboard and
  mouse can be used to interact with the game, as defined in each of the
  games' descriptions.

  The Game window menu offers a number of options for interacting with the
  game, which are common to most of the games. If the window has input
  focus, some standard keyboard shortcuts can also be used. The available
  actions are described in the list here, and in the following sections.

  Game

    Leads to a sub-menu containing a number of actions relating to the
    current game.

  Type

    Leads to a sub-menu containing a number of preset game configurations
    and an option to create custom configurations.

  Specific game...

    Open a dialogue box allowing the specific game ID (see Section 3.2.
    Specifying games with the game ID) for the current game to be copied out
    or a new game to be created from another ID.

  Random seed...

    Open a dialogue box allowing the random seed (see Section 3.2.
    Specifying games with the game ID) for the current game to be copied out
    or a new game to be created from another seed.

  Undo (U, Ctrl-Z, Ctrl-_, or *)

    Undoes a single move. (You can undo moves back to the start of the
    session.)

  Redo (R, Ctrl-R or #)

    Redoes a previously undone move.

  Preferences...

    Where supported, brings up a dialog allowing you to configure personal
    preferences about a particular game. Some of these preferences will be
    specific to a particular game; others will be common to all games.

    Within the Preferences dialogue, click 'Set' to store the options and
    apply them to the associated Game window -- this will cause a new game
    to be generated. The 'Save' button will save the preferences for future
    instances of the puzzle as well. Preferences are saved separately for
    each of the puzzles in the collection.

    One option common to all games allows you to turn off the one-key
    shortcuts like N for new game or Q for quit, so that there's less chance
    of hitting them by accident. You can still access the same shortcuts
    with the Ctrl key.

  Help...

    Open this manual at the section for the puzzle (if the HTML manual is in
    use).

  The Game entry leads to a submenu containing the following additional
  options.

  Save

    Saves the current state of your game to a file on disk. A saved game can
    be re-loaded by double-clicking on the file in a Filer window, or
    dropping it on to the Puzzles iconbar icon.

    The Save and Load operations preserve your entire game history (so you
    can save, reload, and still Undo and Redo things you had done before
    saving).

    The saved files are compatible with those generated by versions of Simon
    Tatham's Portable Puzzle Collection running on other platforms. When
    loading files from other platforms into the RISC OS port, either set
    their filetype to 'Puzzle' or drop Text or Data files on to the Puzzles
    iconbar icon.

  Save sprite

    Saves the current game board as a sprite file.

  Copy as text

    Copies the current state of your game to the clipboard in text format,
    so that you can paste it into (say) an e-mail client or a web message
    board if you're discussing the game with someone else. (Not all games
    support this feature.)

  New (N or Ctrl-N)

    Starts a new game, with a random initial state.

  Restart

    Resets the current game to its initial state. (This can be undone.)

  Solve

    Transforms the puzzle instantly into its solved state. For some games
    (Cube (see Chapter 5. Cube)) this feature is not supported at all
    because it is of no particular use. For other games (such as Pattern
    (see Chapter 11. Pattern)), the solved state can be used to give you
    information, if you can't see how a solution can exist at all or you
    want to know where you made a mistake. For still other games (such as
    Sixteen (see Chapter 7. Sixteen)), automatic solution tells you nothing
    about how to /get/ to the solution, but it does provide a useful way to
    get there quickly so that you can experiment with set-piece moves and
    transformations.

    Some games (such as Solo (see Chapter 12. Solo)) are capable of solving
    a game ID you have typed in from elsewhere. Other games (such as
    Rectangles (see Chapter 9. Rectangles)) cannot solve a game ID they
    didn't invent themself, but when they did invent the game ID they know
    what the solution is already. Still other games (Pattern (see Chapter
    11. Pattern)) can solve /some/ external game IDs, but only if they
    aren't too difficult.

    The Solve command adds the solved state to the end of the undo chain for
    the puzzle. In other words, if you want to go back to solving it
    yourself after seeing the answer, you can just press Undo.

  3.2. Specifying games with the game ID
  --------------------------------------

  There are two ways to save a game specification out of a puzzle and
  recreate it later, or recreate it in somebody else's copy of the same
  puzzle.

  The Specific game... and Random Seed... entries in the Game window menu
  each show a piece of text (a 'game ID') which is sufficient to reconstruct
  precisely the same game at a later date.

  You can enter either of these pieces of text back into the program (via
  the same Specific game... and Random Seed... menu entries) at a later
  point, and it will recreate the same game.

  The difference between the two forms is that a descriptive game ID is a
  literal /description/ of the initial state of the game, whereas a random
  seed is just a piece of arbitrary text which was provided as input to the
  random number generator used to create the puzzle. This means that:

  * Descriptive game IDs tend to be longer in many puzzles (although some,
    such as Cube (see Chapter 5. Cube), only need very short descriptions).
    So a random seed is often a /quicker/ way to note down the puzzle you're
    currently playing, or to tell it to somebody else so they can play the
    same one as you.

  * Any text at all is a valid random seed. The automatically generated ones
    are fifteen-digit numbers, but anything will do; you can type in your
    full name, or a word you just made up, and a valid puzzle will be
    generated from it. This provides a way for two or more people to race to
    complete the same puzzle: you think of a random seed, then everybody
    types it in at the same time, and nobody has an advantage due to having
    seen the generated puzzle before anybody else.

  * It is often possible to convert puzzles from other sources (such as
    'nonograms' or 'sudoku' from newspapers) into descriptive game IDs
    suitable for use with these programs.

  * Random seeds are not guaranteed to produce the same result if you use
    them with a different /version/ of the puzzle program. This is because
    the generation algorithm might have been improved or modified in later
    versions of the code, and will therefore produce a different result when
    given the same sequence of random numbers. Use a descriptive game ID if
    you aren't sure that it will be used on the same version of the program
    as yours.

  A descriptive game ID starts with a piece of text which encodes the
  /parameters/ of the current game (such as grid size). Then there is a
  colon, and after that is the description of the game's initial state. A
  random seed starts with a similar string of parameters, but then it
  contains a hash sign followed by arbitrary data.

  If you enter a descriptive game ID, the program will not be able to show
  you the random seed which generated it, since it wasn't generated /from/ a
  random seed. If you /enter/ a random seed, however, the program will be
  able to show you the descriptive game ID derived from that random seed.

  Note that the game parameter strings are not always identical between the
  two forms. For some games, there will be parameter data provided with the
  random seed which is not included in the descriptive game ID. This is
  because that parameter information is only relevant when /generating/
  puzzle grids, and is not important when playing them. Thus, for example,
  the difficulty level in Solo (see Chapter 12. Solo) is not mentioned in
  the descriptive game ID.

  These additional parameters are also not set permanently if you type in a
  game ID. For example, suppose you have Solo (see Chapter 12. Solo) set to
  'Advanced' difficulty level, and then a friend wants your help with a
  'Trivial' puzzle; so the friend reads out a random seed specifying
  'Trivial' difficulty, and you type it in. The program will generate you
  the same 'Trivial' grid which your friend was having trouble with, but
  once you have finished playing it, when you ask for a new game it will
  automatically go back to the 'Advanced' difficulty which it was previously
  set on.

  3.3. The Type menu
  ------------------

  The Type submenu, if present, may contain a list of preset game settings.
  Selecting one of these will start a new random game with the parameters
  specified.

  The Type submenu may also contain a Custom... option which allows you to
  fine-tune game parameters. The parameters available are specific to each
  game and are described in the following sections.

4. Net
------

  I originally saw this in the form of a Flash game called FreeNet[1],
  written by Pavils Jurjans; there are several other implementations under
  the name NetWalk. The computer prepares a network by connecting up the
  centres of squares in a grid, and then shuffles the network by rotating
  every tile randomly. Your job is to rotate it all back into place. The
  successful solution will be an entirely connected network, with no closed
  loops. As a visual aid, all tiles which are connected to the one in the
  middle are highlighted.

  Note 1

    http://www.jurjans.lv/stuff/net/FreeNet.htm.

  4.1. Net controls
  -----------------

  This game can be played with either the keyboard or the mouse. The
  controls are:

  Select tile

    Mouse pointer, arrow keys

  Rotate tile anticlockwise

    Select click, A key

  Rotate tile clockwise

    Adjust click, D key

  Rotate tile by 180 degrees

    F key

  Lock (or unlock) tile

    Ctrl-Select click, Shift-Select click, S key

    You can lock a tile once you're sure of its orientation. You can also
    unlock it again, but while it's locked you can't accidentally turn it.

  The following controls are not necessary to complete the game, but may be
  useful:

  Shift grid

    Shift + arrow keys

    On grids that wrap, you can move the origin of the grid, so that tiles
    that were on opposite sides of the grid can be seen together.

  Move centre

    Ctrl + arrow keys

    You can change which tile is used as the source of highlighting. (It
    doesn't ultimately matter which tile this is, as every tile will be
    connected to every other tile in a correct solution, but it may be
    helpful in the intermediate stages of solving the puzzle.)

  Jumble tiles

    J key

    This key turns all tiles that are not locked to random orientations.

  (All the actions described in Section 3.1. Game Window are also available.)

  4.2. Net parameters
  -------------------

  These parameters are available from the Type -- Custom... option in the
  Game window menu.

  'Width' and 'Height'

    Size of grid in tiles.

  'Walls wrap around'

    If checked, flow can pass from the left edge to the right edge, and from
    top to bottom, and vice versa.

  'Barrier probability'

    A number between 0.0 and 1.0 controlling whether an immovable barrier is
    placed between two tiles to prevent flow between them (a higher number
    gives more barriers). Since barriers are immovable, they act as
    constraints on the solution (ie. hints).

    The grid generation in Net has been carefully arranged so that the
    barriers are independent of the rest of the grid. This means that if you
    note down the random seed used to generate the current puzzle (see
    Section 3.2. Specifying games with the game ID), change the 'Barrier
    probability' parameter, and then re-enter the same random seed, you
    should see exactly the same starting grid, with the only change being
    the number of barriers. So if you're stuck on a particular grid and need
    a hint, you could start up another instance of Net, set up the same
    parameters but a higher barrier probability, and enter the game seed
    from the original Net window.

  'Ensure unique solution'

    Normally, Net will make sure that the puzzles it presents have only one
    solution. Puzzles with ambiguous sections can be more difficult and more
    subtle, so if you like you can turn off this feature and risk having
    ambiguous puzzles. (Also, finding /all/ the possible solutions can be an
    additional challenge for an advanced player.)

  4.3. Net user preferences
  -------------------------

  The Preferences option on the Net Game window menu will let you configure
  when loops are highlighted as errors. By default, they're always
  highlighted; by changing this option, you can ask for a loop to be
  highlighted only if every tile forming part of the loop is locked. This
  avoids the loop highlighting acting as a spoiler for available deductions
  about squares you haven't even looked at yet.

5. Cube
-------

  This is another one I originally saw as a web game. This one was a Java
  game[2], by Paul Scott. You have a grid of 16 squares, six of which are
  blue; on one square rests a cube. Your move is to use the arrow keys to
  roll the cube through 90 degrees so that it moves to an adjacent square.
  If you roll the cube on to a blue square, the blue square is picked up on
  one face of the cube; if you roll a blue face of the cube on to a non-blue
  square, the blueness is put down again. (In general, whenever you roll the
  cube, the two faces that come into contact swap colours.) Your job is to
  get all six blue squares on to the six faces of the cube at the same time.
  Count your moves and try to do it in as few as possible.

  Unlike the original Java game, my version has an additional feature: once
  you've mastered the game with a cube rolling on a square grid, you can
  change to a triangular grid and roll any of a tetrahedron, an octahedron
  or an icosahedron.

  Note 2

    http://www3.sympatico.ca/paulscott/cube/cube.htm

  5.1. Cube controls
  ------------------

  This game can be played with either the keyboard or the mouse.

  Select-clicking anywhere on the window will move the cube (or other solid)
  towards the mouse pointer.

  The arrow keys can also used to roll the cube on its square grid in the
  four cardinal directions. On the triangular grids, the mapping of arrow
  keys to directions is more approximate. Vertical movement is disallowed
  where it doesn't make sense. The four keys surrounding the arrow keys on
  the numeric keypad (7, 9, 1, 3) can be used for diagonal movement.

  (All the actions described in Section 3.1. Game Window are also available.)

  5.2. Cube parameters
  --------------------

  These parameters are available from the Type -- Custom... option in the
  Game window menu.

  'Type of solid'

    Selects the solid to roll (and hence the shape of the grid):
    tetrahedron, cube, octahedron, or icosahedron.

  'Width / top', 'Height / bottom'

    On a square grid, horizontal and vertical dimensions. On a triangular
    grid, the number of triangles on the top and bottom rows respectively.

6. Fifteen
----------

  The old ones are the best: this is the good old '15-puzzle' with sliding
  tiles, which dates from the 1870s. You have a 4x4 square grid; 15 squares
  contain numbered tiles, and the sixteenth is empty. Your move is to choose
  a tile next to the empty space, and slide it into the space. The aim is to
  end up with the tiles in numerical order, with the space in the bottom
  right (so that the top row reads 1,2,3,4 and the bottom row reads
  13,14,15,space).

  6.1. Fifteen controls
  ---------------------

  This game can be controlled with the mouse or the keyboard.

  A Select-click with the mouse in the row or column containing the empty
  space will move as many tiles as necessary to move the space to the mouse
  pointer.

  By default, the arrow keys will move a tile adjacent to the space in the
  direction indicated (moving the space in the /opposite/ direction).

  Pressing H will make a suggested move. Pressing H enough times will solve
  the game, but it may scramble your progress while doing so.

  (All the actions described in Section 3.1. Game Window are also available.)

  6.2. Fifteen parameters
  -----------------------

  The only options available from the Type -- Custom... option in the Game
  window menu are 'Width' and 'Height', which are self-explanatory. (Once
  you've changed these, it's not a '15-puzzle' any more, of course!)

  6.3. Fifteen user preferences
  -----------------------------

  The Preferences option on the Game window menu will let you configure the
  sense of the arrow keys. With the default setting, Move the tile, the
  arrow key you press indicates the direction that you want a tile to move,
  so that (for example) if you want to move the tile left of the gap
  rightwards into the gap, you'd press Right. With the opposite setting,
  Move the gap, the behaviour of the arrow keys is reversed, and you would
  press Left to move the tile left of the gap into the gap, so that the
  /gap/ ends up one square left of where it was.

7. Sixteen
----------

  Another sliding tile puzzle, visually similar to Fifteen (see Chapter 6.
  Fifteen) but with a different type of move. This time, there is no hole:
  all 16 squares on the grid contain numbered squares. Your move is to shift
  an entire row left or right, or shift an entire column up or down; every
  time you do that, the tile you shift off the grid re-appears at the other
  end of the same row, in the space you just vacated. To win, arrange the
  tiles into numerical order (1,2,3,4 on the top row, 13,14,15,16 on the
  bottom). When you've done that, try playing on different sizes of grid.

  I /might/ have invented this game myself, though only by accident if so
  (and I'm sure other people have independently invented it). I thought I
  was imitating a screensaver I'd seen, but I have a feeling that the
  screensaver might actually have been a Fifteen-type puzzle rather than
  this slightly different kind. So this might be the one thing in my puzzle
  collection which represents creativity on my part rather than just
  engineering.

  7.1. Sixteen controls
  ---------------------

  Select-clicking on an arrow will move the appropriate row or column in the
  direction indicated. Adjust-clicking will move it in the opposite
  direction.

  Alternatively, use the cursor keys to move the position indicator around
  the edge of the grid, and use the return key to move the row/column in the
  direction indicated.

  You can also move the tiles directly. Move the cursor onto a tile, hold
  Ctrl and press an arrow key to move the tile under the cursor and move the
  cursor along with the tile. Or, hold Shift to move only the tile. Pressing
  Enter simulates holding down Ctrl (press Enter again to release), while
  pressing Space simulates holding down Shift.

  (All the actions described in Section 3.1. Game Window are also available.)

  7.2. Sixteen parameters
  -----------------------

  The parameters available from the Type -- Custom... option in the Game
  window menu are:

  'Width' and 'Height'

    Which are self-explanatory.

  Number of shuffling moves

    You can ask for a limited shuffling operation to be performed on the
    grid. By default, Sixteen will shuffle the grid in such a way that any
    arrangement is about as probable as any other. You can override this by
    requesting a precise number of shuffling moves to be performed.
    Typically your aim is then to determine the precise set of shuffling
    moves and invert them exactly, so that you answer (say) a four-move
    shuffle with a four-move solution. Note that the more moves you ask for,
    the more likely it is that solutions shorter than the target length will
    turn out to be possible.

8. Twiddle
----------

  Twiddle is a tile-rearrangement puzzle, visually similar to Sixteen (see
  Chapter 8. Twiddle): you are given a grid of square tiles, each containing
  a number, and your aim is to arrange the numbers into ascending order.

  In basic Twiddle, your move is to rotate a square group of four tiles
  about their common centre. (Orientation is not significant in the basic
  puzzle, although you can select it.) On more advanced settings, you can
  rotate a larger square group of tiles.

  I first saw this type of puzzle in the GameCube game Metroid Prime 2. In
  the Main Gyro Chamber in that game, there is a puzzle you solve to unlock
  a door, which is a special case of Twiddle. I developed this game as a
  generalisation of that puzzle.

  8.1. Twiddle controls
  ---------------------

  To play Twiddle, click the mouse in the centre of the square group you
  wish to rotate. In the basic mode, you rotate a 2x2 square, which means
  you have to click at a corner point where four tiles meet.

  In more advanced modes you might be rotating 3x3 or even more at a time;
  if the size of the square is odd then you simply click in the centre tile
  of the square you want to rotate.

  Clicking with Select rotates the group anticlockwise. Clicking with Adjust
  rotates it clockwise.

  You can also move an outline square around the grid with the cursor keys;
  the square is the size above (2x2 by default, or larger). Pressing the
  Return key or Space bar will rotate the current square anticlockwise or
  clockwise respectively.

  (All the actions described in Section 3.1. Game Window are also available.)

  8.2. Twiddle parameters
  -----------------------

  Twiddle provides several configuration options via the Type -- Custom...
  option in the Game window menu:

  'Width' and 'Height'

    You can configure the width and height of the puzzle grid.

  'Rotating block size'

    You can configure the size of square block that rotates at a time.

  'One number per row'

    You can ask for every square in the grid to be distinguishable (the
    default), or you can ask for a simplified puzzle in which there are
    groups of identical numbers. In the simplified puzzle your aim is just
    to arrange all the 1s into the first row, all the 2s into the second
    row, and so on.

  'Orientation matters'

    You can configure whether the orientation of tiles matters. If you ask
    for an orientable puzzle, each tile will have a triangle drawn in it.
    All the triangles must be pointing upwards to complete the puzzle.

  'Number of shuffling moves'

    You can ask for a limited shuffling operation to be performed on the
    grid. By default, Twiddle will shuffle the grid so much that any
    arrangement is about as probable as any other. You can override this by
    requesting a precise number of shuffling moves to be performed.
    Typically your aim is then to determine the precise set of shuffling
    moves and invert them exactly, so that you answer (say) a four-move
    shuffle with a four-move solution. Note that the more moves you ask for,
    the more likely it is that solutions shorter than the target length will
    turn out to be possible.

9. Rectangles
-------------

  You have a grid of squares, with numbers written in some (but not all) of
  the squares. Your task is to subdivide the grid into rectangles of various
  sizes, such that (a) every rectangle contains exactly one numbered square,
  and (b) the area of each rectangle is equal to the number written in its
  numbered square.

  Credit for this game goes to the Japanese puzzle magazine Nikoli[3]; I've
  also seen a Palm implementation at Puzzle Palace[4]. Unlike Puzzle
  Palace's implementation, my version automatically generates random grids
  of any size you like. The quality of puzzle design is therefore not quite
  as good as hand-crafted puzzles would be, but on the plus side you get an
  inexhaustible supply of puzzles tailored to your own specification.

  Note 3

    https://www.nikoli.co.jp/en/puzzles/shikaku/

  Note 4

    https://web.archive.org/web/20041024001459/http://www.puzzle.gr.jp/puzzl-
    e/sikaku/palm/index.html.en

  9.1. Rectangles controls
  ------------------------

  This game is played with the mouse or cursor keys.

  Select-click any edge to toggle it on or off, or Select-drag to draw an
  entire rectangle (or line) on the grid in one go (removing any existing
  edges within that rectangle). Adjust-dragging will allow you to erase the
  contents of a rectangle without affecting its edges.

  Alternatively, use the cursor keys to move the position indicator around
  the board. Pressing the Return key then allows you to use the cursor keys
  to drag a rectangle out from that position, and pressing the Return key
  again completes the rectangle. Using the Space bar instead of the Return
  key allows you to erase the contents of a rectangle without affecting its
  edges, as above. Pressing Escape cancels a drag.

  When a rectangle of the correct size is completed, it will be shaded.

  (All the actions described in Section 3.1. Game Window are also available.)

  9.2. Rectangles parameters
  --------------------------

  These parameters are available from the Type -- Custom... option in the
  Game window menu.

  'Width', 'Height'

    Size of grid, in squares.

  'Expansion factor'

    This is a mechanism for changing the type of grids generated by the
    program. Some people prefer a grid containing a few large rectangles to
    one containing many small ones. So you can ask Rectangles to essentially
    generate a /smaller/ grid than the size you specified, and then to
    expand it by adding rows and columns.

    The default expansion factor of zero means that Rectangles will simply
    generate a grid of the size you ask for, and do nothing further. If you
    set an expansion factor of (say) 0.5, it means that each dimension of
    the grid will be expanded to half again as big after generation. In
    other words, the initial grid will be 2/3 the size in each dimension,
    and will be expanded to its full size without adding any more rectangles.

    Setting an expansion factor of around 0.5 tends to make the game more
    difficult, and also (in my experience) rewards a less deductive and more
    intuitive playing style. If you set it /too/ high, though, the game
    simply cannot generate more than a few rectangles to cover the entire
    grid, and the game becomes trivial.

  'Ensure unique solution'

    Normally, Rectangles will make sure that the puzzles it presents have
    only one solution. Puzzles with ambiguous sections can be more difficult
    and more subtle, so if you like you can turn off this feature and risk
    having ambiguous puzzles. Also, finding /all/ the possible solutions can
    be an additional challenge for an advanced player. Turning off this
    option can also speed up puzzle generation.

10. Netslide
------------

  This game combines the grid generation of Net (see Chapter 4. Net) with
  the movement of Sixteen (see Chapter 7. Sixteen): you have a Net grid, but
  instead of rotating tiles back into place you have to slide them into
  place by moving a whole row at a time.

  As in Sixteen, control is with the mouse or cursor keys. See Section 4.1.
  Net controls.

  The available game parameters have similar meanings to those in Section
  4.2. Net parameters and Section 7.2. Sixteen parameters.

  Netslide was contributed to this collection by Richard Boulton.

11. Pattern
-----------

  You have a grid of squares, which must all be filled in either black or
  white. Beside each row of the grid are listed, in order, the lengths of
  the runs of black squares on that row; above each column are listed, in
  order, the lengths of the runs of black squares in that column. Your aim
  is to fill in the entire grid black or white.

  I first saw this puzzle form around 1995, under the name 'nonograms'. I've
  seen it in various places since then, under different names.

  Normally, puzzles of this type turn out to be a meaningful picture of
  something once you've solved them. However, since this version generates
  the puzzles automatically, they will just look like random groupings of
  squares. (One user has suggested that this is actually a /good/ thing,
  since it prevents you from guessing the colour of squares based on the
  picture, and forces you to use logic instead.) The advantage, though, is
  that you never run out of them.

  11.1. Pattern controls
  ----------------------

  This game is played with the mouse.

  Select-click in a square to colour it black. Adjust-click to colour it
  white. If you make a mistake, you can Ctrl-Select-click, or hold down
  Shift while clicking with either button, to colour the square in the
  default grey (meaning 'undecided') again.

  You can click and drag with Select or Adjust to colour a vertical or
  horizontal line of squares black or white at a time (respectively). If you
  click and drag with Ctrl-Select, or with Shift held down, you can colour a
  whole rectangle of squares grey.

  You can also move around the grid with the cursor keys. Pressing the
  Return key will cycle the current cell through empty, then black, then
  white, then empty, and the Space bar does the same cycle in reverse.

  Moving the cursor while holding Ctrl will colour the moved-over squares
  black. Holding Shift will colour the moved-over squares white, and holding
  both will colour them grey.

  (All the actions described in Section 3.1. Game Window are also available.)

  11.2. Pattern parameters
  ------------------------

  The only options available from Type -- Custom... option in the Game
  window menu are 'Width' and 'Height', which are self-explanatory.

12. Solo
--------

  You have a square grid, which is divided into as many equally sized sub-
  blocks as the grid has rows. Each square must be filled in with a digit
  from 1 to the size of the grid, in such a way that

  * every row contains only one occurrence of each digit,

  * every column contains only one occurrence of each digit, and

  * every block contains only one occurrence of each digit.

  * (optionally, by default off) each of the square's two main diagonals
    contains only one occurrence of each digit.

  You are given some of the numbers as clues; your aim is to place the rest
  of the numbers correctly.

  Under the default settings, the sub-blocks are square or rectangular. The
  default puzzle size is 3x3 (a 9x9 actual grid, divided into nine 3x3
  blocks). You can also select sizes with rectangular blocks instead of
  square ones, such as 2x3 (a 6x6 grid divided into six 3x2 blocks).
  Alternatively, you can select 'jigsaw' mode, in which the sub-blocks are
  arbitrary shapes which differ between individual puzzles.

  Another available mode is 'killer'. In this mode, clues are not given in
  the form of filled-in squares; instead, the grid is divided into 'cages'
  by coloured lines, and for each cage the game tells you what the sum of
  all the digits in that cage should be. Also, no digit may appear more than
  once within a cage, even if the cage crosses the boundaries of existing
  regions.

  If you select a puzzle size which requires more than 9 digits, the
  additional digits will be letters of the alphabet. For example, if you
  select 3x4 then the digits which go in your grid will be 1 to 9, plus 'a',
  'b' and 'c'. This cannot be selected for killer puzzles.

  I first saw this puzzle in Nikoli[5], although it's also been popularised
  by various newspapers under the name 'Sudoku' or 'Su Doku'. Howard Garns
  is considered the inventor of the modern form of the puzzle, and it was
  first published in Dell Pencil Puzzles and Word Games. A more elaborate
  treatment of the history of the puzzle can be found on Wikipedia[6].

  Note 5

    https://www.nikoli.co.jp/en/puzzles/sudoku/

  Note 6

    http://en.wikipedia.org/wiki/Sudoku

  12.1. Solo controls
  -------------------

  To play Solo, simply click the mouse in any empty square and then type a
  digit or letter on the keyboard to fill that square. If you make a
  mistake, click the mouse in the incorrect square and press Space to clear
  it again (or use the Undo feature).

  If you Adjust-click in a square and then type a number, that number will
  be entered in the square as a 'pencil mark'. You can have pencil marks for
  multiple numbers in the same square. Squares containing filled-in numbers
  cannot also contain pencil marks.

  The game pays no attention to pencil marks, so exactly what you use them
  for is up to you: you can use them as reminders that a particular square
  needs to be re-examined once you know more about a particular number, or
  you can use them as lists of the possible numbers in a given square, or
  anything else you feel like.

  To erase a single pencil mark, Adjust-click in the square and type the
  same number again.

  All pencil marks in a square are erased when you Select-click and type a
  number, or when you Select-click and press Space. Adjust-clicking and
  pressing Space will also erase pencil marks.

  Alternatively, use the cursor keys to move the mark around the grid.
  Pressing the Return key toggles the mark (from a normal mark to a pencil
  mark), and typing a number in is entered in the square in the appropriate
  way; typing in a 0 or using the space bar will clear a filled square.

  (All the actions described in Section 3.1. Game Window are also available.)

  12.2. Solo parameters
  ---------------------

  Solo allows you to configure two separate dimensions of the puzzle grid
  using Type -- Custom from the Game window: the number of 'Columns of sub-
  blocks', and the number of 'Rows of sub-blocks', into which the main grid
  is divided. (The size of a block is the inverse of this: for example, if
  you select 2 columns and 3 rows, each actual block will have 3 columns and
  2 rows.)

  If you tick the 'X' checkbox, Solo will apply the optional extra
  constraint that the two main diagonals of the grid also contain one of
  every digit. (This is sometimes known as 'Sudoku-X' in newspapers.) In
  this mode, the squares on the two main diagonals will be shaded slightly
  so that you know it's enabled.

  If you tick the 'Jigsaw' checkbox, Solo will generate randomly shaped sub-
  blocks. In this mode, the actual grid size will be taken to be the product
  of the numbers entered in the 'Columns of sub-blocks' and 'Rows of sub-
  blocks' boxes. There is no reason why you have to enter a number greater
  than 1 in both boxes; Jigsaw mode has no constraint on the grid size, and
  it can even be a prime number if you feel like it.

  If you tick the 'Killer' checkbox, Solo will generate a set of of cages,
  which are randomly shaped and drawn in an outline of a different colour.
  Each of these regions contains a smaller clue which shows the digit sum of
  all the squares in this region.

  You can also configure the type of 'Symmetry' shown in the generated
  puzzles. More symmetry makes the puzzles look prettier but may also make
  them easier, since the symmetry constraints can force more clues than
  necessary to be present. Completely asymmetric puzzles have the freedom to
  contain as few clues as possible.

  Finally, you can configure the 'Difficulty' of the generated puzzles.
  Difficulty levels are judged by the complexity of the techniques of
  deduction required to solve the puzzle: each level requires a mode of
  reasoning which was not necessary in the previous one. In particular, on
  difficulty levels Trivial and Basic there will be a square you can fill in
  with a single number at all times, whereas at Intermediate level and
  beyond you will have to make partial deductions about the /set/ of squares
  a number could be in (or the set of numbers that could be in a square). At
  Unreasonable level, even this is not enough, and you will eventually have
  to make a guess, and then backtrack if it turns out to be wrong.

  Generating difficult puzzles is itself difficult: if you select one of the
  higher difficulty levels, Solo may have to make many attempts at
  generating a puzzle before it finds one hard enough for you. Be prepared
  to wait, especially if you have also configured a large puzzle size.

13. Mines
---------

  You have a grid of covered squares, some of which contain mines, but you
  don't know which. Your job is to uncover every square which does /not/
  contain a mine. If you uncover a square containing a mine, you lose. If
  you uncover a square which does not contain a mine, you are told how many
  mines are contained within the eight surrounding squares.

  This game needs no introduction; popularised by Windows, it is perhaps the
  single best known desktop puzzle game in existence.

  This version of it has an unusual property. By default, it will generate
  its mine positions in such a way as to ensure that you never need to
  /guess/ where a mine is: you will always be able to deduce it somehow. So
  you will never, as can happen in other versions, get to the last four
  squares and discover that there are two mines left but you have no way of
  knowing for sure where they are.

  13.1. Mines controls
  --------------------

  This game is played with the mouse.

  If you Select-click in a covered square, it will be uncovered.

  If you Adjust-click in a covered square, it will place a flag which
  indicates that the square is believed to be a mine. Select-clicking in a
  marked square will not uncover it, for safety. You can Adjust-click again
  to remove a mark placed in error.

  If you Select-click in an /uncovered/ square, it will 'clear around' the
  square. This means: if the square has exactly as many flags surrounding it
  as it should have mines, then all the covered squares next to it which are
  /not/ flagged will be uncovered. So once you think you know the location
  of all the mines around a square, you can use this function as a shortcut
  to avoid having to click on each of the remaining squares one by one.

  If you uncover a square which has /no/ mines in the surrounding eight
  squares, then it is obviously safe to uncover those squares in turn, and
  so on if any of them also has no surrounding mines. This will be done for
  you automatically; so sometimes when you uncover a square, a whole new
  area will open up to be explored.

  You can also use the cursor keys to move around the minefield. Pressing
  the Return key in a covered square uncovers it, and in an uncovered square
  will clear around it (so it acts as the Select button), pressing the Space
  bar in a covered square will place a flag (similarly, it acts as the
  Adjust button).

  All the actions described in Section 3.1. Game Window are also available.

  Even Undo is available, although you might consider it cheating to use it.
  If you step on a mine, the program will only reveal the mine in question
  (unlike most other implementations, which reveal all of them). You can
  then Undo your fatal move and continue playing if you like. The program
  will track the number of times you died (and Undo will not reduce that
  counter), so when you get to the end of the game you know whether or not
  you did it without making any errors.

  (If you really want to know the full layout of the grid, which other
  implementations will show you after you die, you can always use the Solve
  menu option.)

  13.2. Mines parameters
  ----------------------

  The options available from Type -- Custom... option in the Game window
  menu are:

  'Width', 'Height'

    Size of grid in squares.

  'Mines'

    Number of mines in the grid. You can enter this as an absolute mine
    count, or alternatively you can put a % sign on the end in which case
    the game will arrange for that proportion of the squares in the grid to
    be mines.

    Beware of setting the mine count too high. At very high densities, the
    program may spend forever searching for a solvable grid.

  'Ensure solubility'

    When this option is enabled (as it is by default), Mines will ensure
    that the entire grid can be fully deduced starting from the initial open
    space. If you prefer the riskier grids generated by other
    implementations, you can switch off this option.

14. Same Game
-------------

  You have a grid of coloured squares, which you have to clear by
  highlighting contiguous regions of more than one coloured square; the
  larger the region you highlight, the more points you get (and the faster
  you clear the arena).

  If you clear the grid you win. If you end up with nothing but single
  squares (ie., there are no more clickable regions left) you lose.

  Removing a region causes the rest of the grid to shuffle up: blocks that
  are suspended will fall down (first), and then empty columns are filled
  from the right.

  Same Game was contributed to this collection by James Harvey.

  14.1. Same Game controls
  ------------------------

  This game can be played with either the keyboard or the mouse.

  If you Select-click an unselected region, it becomes selected (possibly
  clearing the current selection).

  If you Select-click the selected region, it will be removed (and the rest
  of the grid shuffled immediately).

  If you Adjust-click the selected region, it will be unselected.

  The cursor keys move a cursor around the grid. Pressing the Space or Enter
  keys while the cursor is in an unselected region selects it; pressing
  Space or Enter again removes it as above.

  (All the actions described in Section 3.1. Game Window are also available.)

  14.2. Same Game parameters
  --------------------------

  These parameters are available from Type -- Custom... in the Game window
  menu.

  'Width', 'Height'

    Size of grid in squares.

  'No. of colours'

    Number of different colours used to fill the grid; the more colours, the
    fewer large regions of colour and thus the more difficult it is to
    successfully clear the grid.

  'Scoring system'

    Controls the precise mechanism used for scoring. With the default
    system, (n-2)^2, only regions of three squares or more will score any
    points at all. With the alternative (n-1)^2 system, regions of two
    squares score a point each, and larger regions score relatively more
    points.

  'Ensure solubility'

    If this option is ticked (the default state), generated grids will be
    guaranteed to have at least one solution.

    If you turn it off, the game generator will not try to guarantee soluble
    grids; it will, however, still ensure that there are at least 2 squares
    of each colour on the grid at the start (since a grid with exactly one
    square of a given colour is /definitely/ insoluble). Grids generated
    with this option disabled may contain more large areas of contiguous
    colour, leading to opportunities for higher scores; they can also take
    less time to generate.

15. Flip
--------

  You have a grid of squares, some light and some dark. Your aim is to light
  all the squares up at the same time. You can choose any square and flip
  its state from light to dark or dark to light, but when you do so, other
  squares around it change state as well.

  Each square contains a small diagram showing which other squares change
  when you flip it.

  15.1. Flip controls
  -------------------

  This game can be played with either the keyboard or the mouse.

  Select-click in a square to flip it and its associated squares, or use the
  cursor keys to choose a square and the Space bar or Enter key to flip.

  If you use the Solve function on this game, it will mark some of the
  squares in red. If you click once in every square with a red mark, the
  game should be solved. (If you click in a square /without/ a red mark, a
  red mark will appear in it to indicate that you will need to reverse that
  operation to reach the solution.)

  (All the actions described in Section 3.1. Game Window are also available.)

  15.2. Flip parameters
  ---------------------

  These parameters are available from Type -- Custom... in the Game window
  menu.

  'Width', 'Height'

    Size of grid in squares.

  'Shape type'

    This control determines the shape of the region which is flipped by
    clicking in any given square. The default setting, Crosses,causes every
    square to flip itself and its four immediate neighbours (or three or two
    if it's at an edge or corner). The other setting, Random, causes a
    random shape to be chosen for every square, so the game is different
    every time.

16. Guess
---------

  You have a set of coloured pegs, and have to reproduce a predetermined
  sequence of them (chosen by the computer) within a certain number of
  guesses.

  Each guess gets marked with the number of correctly-coloured pegs in the
  correct places (in black), and also the number of correctly-coloured pegs
  in the wrong places (in white).

  This game is also known (and marketed, by Hasbro, mainly) as a board game
  Mastermind, with 6 colours, 4 pegs per row, and 10 guesses.However, this
  version allows custom settings of number of colours (up to 10), number of
  pegs per row, and number of guesses.

  Guess was contributed to this collection by James Harvey.

  16.1. Guess controls
  --------------------

  This game can be played with either the keyboard or the mouse.

  With the mouse, drag a coloured peg from the tray on the left-hand side to
  its required position in the current guess; pegs may also be dragged from
  current and past guesses to copy them elsewhere. To remove a peg, drag it
  off its current position to somewhere invalid.

  Adjust-clicking in the current guess adds a 'hold' marker; pegs that have
  hold markers will be automatically added to the next guess after marking.

  Alternatively, with the keyboard, the Up and Down cursor keys can be used
  to select a peg colour, the Left and Right keys to select a peg position,
  and the Enter key to place a peg of the selected colour in the chosen
  position. D or Backspace removes a peg, and Space adds a hold marker.

  The number keys can also be used to insert pegs: 1 inserts the top-most
  colour, 2 the second one, and so forth. These also move the peg cursor to
  the right. Pressing L will label the pegs with their numbers.

  Pressing H or ? will fill the current guess with a suggested guess. Using
  this is not recommended for 10 or more pegs as it is slow.

  When the guess is complete, the smaller feedback pegs will be highlighted;
  clicking on these (or moving the peg cursor to them with the arrow keys
  and pressing the Space bar or Enter key) will mark the current guess, copy
  any held pegs to the next guess, and move the 'current guess' marker.

  If you correctly position all the pegs the solution will be displayed
  below; if you run out of guesses (or select Solve...) the solution will
  also be revealed.

  (All the actions described in Section 3.1. Game Window are also available.)

  16.2. Guess parameters
  ----------------------

  These parameters are available from Type -- Custom... in the Game window
  menu. The default game matches the parameters for the board game
  Mastermind.

  'Colours'

    Number of colours the solution is chosen from; from 2 to 10 (more is
    harder).

  'Pegs per guess'

    Number of pegs per guess (more is harder).

  'Guesses'

    Number of guesses you have to find the solution in (fewer is harder).

  'Allow blanks'

    Allows blank pegs to be given as part of a guess (makes it easier,
    because you know that those will never be counted as part of the
    solution). This is turned off by default.

    Note that this doesn't allow blank pegs in the solution; if you really
    wanted that, use one extra colour.

  'Allow duplicates'

    Allows the solution (and the guesses) to contain colours more than once;
    this increases the search space (making things harder), and is turned on
    by default.

  16.3. Guess user preferences
  ----------------------------

  The Preferences option on the Game window menu will let you configure
  whether pegs are labelled with their numbers. Unlike the L key, this will
  persist between games.

17. Pegs
--------

  A number of pegs are placed in holes on a board. You can remove a peg by
  jumping an adjacent peg over it (horizontally or vertically) to a vacant
  hole on the other side. Your aim is to remove all but one of the pegs
  initially present.

  This game, best known as 'Peg Solitaire', is possibly one of the oldest
  puzzle games still commonly known.

  17.1. Pegs controls
  -------------------

  To move a peg, drag it with the mouse from its current position to its
  final position. If the final position is exactly two holes away from the
  initial position, is currently unoccupied by a peg, and there is a peg in
  the intervening square, the move will be permitted and the intervening peg
  will be removed.

  Vacant spaces which you can move a peg into are marked with holes. A space
  with no peg and no hole is not available for moving at all: it is an
  obstacle which you must work around.

  You can also use the cursor keys to move a position indicator around the
  board. Pressing the Return key while over a peg, followed by a cursor key,
  will jump the peg in that direction (if that is a legal move).

  (All the actions described in Section 3.1. Game Window are also available.)

  17.2. Pegs parameters
  ---------------------

  These parameters are available from Type -- Custom... in the Game window
  menu.

  'Width', 'Height'

    Size of grid in holes.

  'Board type'

    Controls whether you are given a board of a standard shape or a randomly
    generated shape. The two standard shapes currently supported are Cross
    (in various sizes) and Octagon;. The 7x7 Cross is the traditional
    English board layout. The Octagon is the traditional French one.
    Selecting Random; will give you a different board shape every time (but
    always one that is known to have a solution).

18. Dominosa
------------

  A normal set of dominoes -- that is, one instance of every (unordered)
  pair of numbers from 0 to 6 -- has been arranged irregularly into a
  rectangle; then the number in each square has been written down and the
  dominoes themselves removed. Your task is to reconstruct the pattern by
  arranging the set of dominoes to match the provided array of numbers.

  This puzzle is widely credited to O. S. Adler, and takes part of its name
  from those initials.

  18.1. Dominosa controls
  -----------------------

  Select-clicking between any two adjacent numbers places a domino covering
  them, or removes one if it is already present. Trying to place a domino
  which overlaps existing dominoes will remove the ones it overlaps.

  Adjust-clicking between two adjacent numbers draws a line between them,
  which you can use to remind yourself that you know those two numbers are
  /not/ covered by a single domino. Adjust-clicking again removes the line.

  You can also use the cursor keys to move a cursor around the grid. When
  the cursor is half way between two adjacent numbers, pressing the Return
  key will place a domino covering those numbers, or pressing the Space bar
  will lay a line between the two squares. Repeating either action removes
  the domino or line.

  Pressing a number key will highlight all occurrences of that number.
  Pressing that number again will clear the highlighting. Up to two
  different numbers can be highlighted at any given time.

  (All the actions described in Section 3.1. Game Window are also available.)

  18.2. Dominosa parameters
  -------------------------

  These parameters are available from Type -- Custom... in the Game window
  menu.

  'Maximum number on dominoes'

    Controls the size of the puzzle, by controlling the size of the set of
    dominoes used to make it. Dominoes with numbers going up to N will give
    rise to an (N + 2) x (N + 1) rectangle; so, in particular, the default
    value of 6 gives an 8x7 grid.

  'Ensure unique solution'

    Normally, Dominosa will make sure that the puzzles it presents have only
    one solution. Puzzles with ambiguous sections can be more difficult and
    sometimes more subtle, so if you like you can turn off this feature.
    Also, finding /all/ the possible solutions can be an additional
    challenge for an advanced player. Turning off this option can also speed
    up puzzle generation.

19. Untangle
------------

  You are given a number of points, some of which have lines drawn between
  them. You can move the points about arbitrarily; your aim is to position
  the points so that no line crosses another.

  I originally saw this in the form of a Flash game called Planarity[7],
  written by John Tantalo.

  Note 7

    http://planarity.net

  19.1. Untangle controls
  -----------------------

  To move a point, drag it into a new position with Select.

  The cursor keys may also be used to navigate amongst the points. Pressing
  the Enter key will toggle dragging the currently-highlighted point.
  Pressing Tab or Space will cycle through all the points.

  (All the actions described in Section 3.1. Game Window are also available.)

  19.2. Untangle parameters
  -------------------------

  There is only one parameter available from Type -- Custom... in the Game
  window menu.

  'Number of points'

    Controls the size of the puzzle, by specifying the number of points in
    the generated graph.

20. Black Box
-------------

  A number of balls are hidden in a rectangular arena. You have to deduce
  the positions of the balls by firing lasers positioned at the edges of the
  arena and observing how their beams are deflected.

  Beams will travel straight from their origin until they hit the opposite
  side of the arena (at which point they emerge), unless affected by balls
  in one of the following ways:

  * A beam that hits a ball head-on is absorbed and will never re- emerge.
    This includes beams that meet a ball on the first rank of the arena.

  * A beam with a ball in its front-left square and no ball ahead of it gets
    deflected 90 degrees to the right.

  * A beam with a ball in its front-right square and no ball ahead of it
    gets similarly deflected to the left.

  * A beam that would re-emerge from its entry location is considered to be
    'reflected'.

  * A beam which would get deflected before entering the arena by a ball to
    the front-left or front-right of its entry point is also considered to
    be 'reflected'.

  Beams that are reflected appear as a 'R'; beams that hit balls head-on
  appear as 'H'. Otherwise, a number appears at the firing point and the
  location where the beam emerges (this number is unique to that shot).

  You can place guesses as to the location of the balls, based on the entry
  and exit patterns of the beams; once you have placed enough balls a button
  appears enabling you to have your guesses checked.

  Here is a diagram showing how the positions of balls can create each of
  the beam behaviours shown above:

     1RHR----
    |..O.O...|
    2........3
    |........|
    |........|
    3........|
    |......O.|
    H........|
    |.....O..|
     12-RR---

  As shown, it is possible for a beam to receive multiple reflections before
  re-emerging (see turn 3). Similarly, a beam may be reflected (possibly
  more than once) before receiving a hit (the 'H' on the left side of the
  example).

  Note that any layout with more than 4 balls may have a non-unique
  solution. The following diagram illustrates this; if you know the board
  contains 5 balls, it is impossible to determine where the fifth ball is
  (possible positions marked with an x):

     --------
    |........|
    |........|
    |..O..O..|
    |...xx...|
    |...xx...|
    |..O..O..|
    |........|
    |........|
     --------

  For this reason, when you have your guesses checked, the game will check
  that your solution /produces the same results/ as the computer's, rather
  than that your solution is identical to the computer's. So in the above
  example, you could put the fifth ball at /any/ of the locations marked
  with an x, and you would still win.

  Acornsoft published versions of Black Box for the Acorn Atom and the
  BBC Microcomputer in the early 1980s. The puzzle was contributed to this
  collection by James Harvey.

  20.1. Black Box controls
  ------------------------

  To fire a laser beam, Select-click in a square around the edge of the
  arena. The results will be displayed immediately. Clicking or holding
  Select on one of these squares will highlight the current go (or a
  previous go) to confirm the exit point for that laser, if applicable.

  To guess the location of a ball, Select-click within the arena and a black
  circle will appear marking the guess; click again to remove the guessed
  ball.

  Locations in the arena may be locked against modification by Adjust-
  clicking; whole rows and columns may be similarly locked by Adjust-
  clicking in the laser square above/below that column, or to the left/right
  of that row.

  The cursor keys may also be used to move around the grid. Pressing the
  Enter key will fire a laser or add a new ball-location guess, and pressing
  Space will lock a cell, row, or column.

  When an appropriate number of balls have been guessed, a button will
  appear at the top-left corner of the grid; clicking that (with mouse or
  cursor) will check your guesses.

  If you click the 'check' button and your guesses are not correct, the game
  will show you the minimum information necessary to demonstrate this to
  you, so you can try again. If your ball positions are not consistent with
  the beam paths you already know about, one beam path will be circled to
  indicate that it proves you wrong. If your positions match all the
  existing beam paths but are still wrong, one new beam path will be
  revealed (written in red) which is not consistent with your current
  guesses.

  If you decide to give up completely, you can select Solve to reveal the
  actual ball positions. At this point, correctly-placed balls will be
  displayed as filled black circles, incorrectly-placed balls as filled
  black circles with red crosses, and missing balls as filled red circles.
  In addition, a red circle marks any laser you had already fired which is
  not consistent with your ball layout (just as when you press the 'check'
  button), and red text marks any laser you /could/ have fired in order to
  distinguish your ball layout from the correct one.

  (All the actions described in Section 3.1. Game Window are also available.)

  20.2. Black Box parameters
  --------------------------

  These parameters are available from Type -- Custom... in the Game window
  menu.

  'Width', 'Height'

    Size of grid in squares. There are 2 x 'Width' x 'Height' lasers per
    grid, two per row and two per column.

  'No. of balls'

    Number of balls to place in the grid. This can be a single number, or a
    range (separated with a hyphen, like "2-6"), and determines the number
    of balls to place on the grid. The 'reveal' button is only enabled if
    you have guessed an appropriate number of balls; a guess using a
    different number to the original solution is still acceptable, if all
    the beam inputs and outputs match.

21. Slant
---------

  You have a grid of squares. Your aim is to draw a diagonal line through
  each square, and choose which way each line slants so that the following
  conditions are met:

  * The diagonal lines never form a loop.

  * Any point with a circled number has precisely that many lines meeting at
    it. (Thus, a 4 is the centre of a cross shape, whereas a zero is the
    centre of a diamond shape -- or rather, a partial diamond shape, because
    a zero can never appear in the middle of the grid because that would
    immediately cause a loop.)

  Credit for this puzzle goes to Nikoli[8].

  Note 8

    https://www.nikoli.co.jp/ja/puzzles/gokigen_naname/ (in Japanese)

  21.1. Slant controls
  --------------------

  Select-clicking in a blank square will place a "\" in it (a line leaning
  to the left, ie. running from the top left of the square to the bottom
  right). Adjust-clicking in a blank square will place a "/" in it (leaning
  to the right, running from top right to bottom left).

  Continuing to click either button will cycle between the three possible
  square contents. Thus, if you Select-click repeatedly in a blank square it
  will change from blank to "\" to "/" back to blank, and if you Adjust-
  click repeatedly the square will change from blank to "/" to "\" back to
  blank. (Therefore, you can play the game entirely with one button if you
  need to.)

  You can also use the cursor keys to move around the grid. Pressing the
  Return or Space keys will place a "\" or a "/", respectively, and will
  then cycle them as above. You can also press / or \ to place a "/" or "\",
  respectively, independent of what is already in the cursor square.
  Backspace removes any line from the cursor square.

  (All the actions described in Section 3.1. Game Window are also available.)

  21.2. Slant parameters
  ----------------------

  These parameters are available from Type -- Custom... in the Game window
  menu.

  'Width', 'Height'

    Size of grid in squares.

  'Difficulty'

    Controls the difficulty of the generated puzzle. At Hard level, you are
    required to do deductions based on knowledge of /relationships/ between
    squares rather than always being able to deduce the exact contents of
    one square at a time. (For example, you might know that two squares
    slant in the same direction, even if you don't yet know what that
    direction is, and this might enable you to deduce something about still
    other squares.) Even at Hard level, guesswork and backtracking should
    never be necessary.

  21.3. Slant user preferences
  ----------------------------

  The Preferences option on the Game window menu will let you configure
  which way round the mouse buttons work (where 'Left' and 'Right' refer to
  Select and Adjust respectively).

22. Light Up
------------

  You have a grid of squares. Some are filled in black; some of the black
  squares are numbered. Your aim is to 'light up' all the empty squares by
  placing light bulbs in some of them.

  Each light bulb illuminates the square it is on, plus all squares in line
  with it horizontally or vertically unless a black square is blocking the
  way.

  To win the game, you must satisfy the following conditions:

  * All non-black squares are lit.

  * No light is lit by another light.

  * All numbered black squares have exactly that number of lights adjacent
    to them (in the four squares above, below, and to the side).

  Non-numbered black squares may have any number of lights adjacent to them.

  Credit for this puzzle goes to Nikoli[9].

  Light Up was contributed to this collection by James Harvey.

  Note 9

    https://www.nikoli.co.jp/en/puzzles/akari/

  22.1. Light Up controls
  -----------------------

  Select-clicking in a non-black square will toggle the presence of a light
  in that square. Adjust-clicking in a non-black square toggles a mark there
  to aid solving; it can be used to highlight squares that cannot be lit,
  for example.

  You may not place a light in a marked square, nor place a mark in a lit
  square.

  The game will highlight obvious errors in red. Lights lit by other lights
  are highlighted in this way, as are numbered squares which do not (or
  cannot) have the right number of lights next to them.

  Thus, the grid is solved when all non-black squares have yellow highlights
  and there are no red lights.

  (All the actions described in Section 3.1. Game Window are also available.)

  22.2. Light Up parameters
  -------------------------

  These parameters are available from Type -- Custom... in the Game window
  menu.

  'Width', 'Height'

    Size of grid in squares.

  '%age of black squares'

    Rough percentage of black squares in the grid.

    This is a hint rather than an instruction. If the grid generator is
    unable to generate a puzzle to this precise specification, it will
    increase the proportion of black squares until it can.

  'Symmetry'

    Allows you to specify the required symmetry of the black squares in the
    grid. (This does not affect the difficulty of the puzzles noticeably.)

  'Difficulty'

    Easy means that the puzzles should be soluble without backtracking or
    guessing, Hard means that some guesses will probably be necessary.

  22.3. Light Up user preferences
  -------------------------------

  The Preferences option on the Game window menu will let you configure
  whether 'this is not a light' marks are shown when the square is also lit.

23. Map
-------

  You are given a map consisting of a number of regions. Your task is to
  colour each region with one of four colours, in such a way that no two
  regions sharing a boundary have the same colour. You are provided with
  some regions already coloured, sufficient to make the remainder of the
  solution unique, and these cannot be changed.

  Only regions which share a length of border are required to be different
  colours. Two regions which meet at only one /point/ (ie. are diagonally
  separated) may be the same colour.

  I believe this puzzle is original; I've never seen an implementation of it
  anywhere else. The concept of a four-colouring puzzle was suggested by
  Alexandra Lanes; credit must also go to Nikoli and to Verity Allan for
  inspiring the train of thought that led to me realising Alex's suggestion
  was a viable puzzle. Thanks also to Gareth Taylor for many detailed
  suggestions.

  23.1. Map controls
  ------------------

  To colour a region, click Select on an existing region of the desired
  colour and drag that colour into the new region.

  (The program will always ensure the starting puzzle has at least one
  region of each colour, so that this is always possible!)

  If you need to clear a region, you can drag from an empty region, or from
  the puzzle boundary if there are no empty regions left.

  Dragging a colour using Adjust will stipple the region in that colour,
  which you can use as a note to yourself that you think the region /might/
  be that colour. A region can contain stipples in multiple colours at once.
  (This is often useful at the harder difficulty levels.)

  You can also use the cursor keys to move around the map: the colour of the
  cursor indicates the position of the colour you would drag (which is not
  obvious if you're on a region's boundary, since it depends on the
  direction from which you approached the boundary). Pressing the Return key
  starts a drag of that colour, as above, which you control with the cursor
  keys; pressing the Return key again finishes the drag. The Space bar can
  be used similarly to create a stippled region. Double-pressing the Return
  key (without moving the cursor) will clear the region, as a drag from an
  empty region does: this is useful with the cursor mode if you have filled
  the entire map in but need to correct the layout.

  If you press L during play, the game will toggle display of a number in
  each region of the map. This is useful if you want to discuss a particular
  puzzle instance with a friend -- having an unambiguous name for each
  region is much easier than trying to refer to them all by names such as
  'the one down and right of the brown one on the top border'.

  (All the actions described in Section 3.1. Game Window are also available.)

  23.2. Map parameters
  --------------------

  These parameters are available from Type -- Custom... in the Game window
  menu.

  'Width', 'Height'

    Size of grid in squares.

  'Regions'

    Number of regions in the generated map.

  'Difficulty'

    In Easy mode, there should always be at least one region whose colour
    can be determined trivially. In Normal and Hard modes, you will have to
    use increasingly complex logic to deduce the colour of some regions.
    However, it will always be possible without having to guess or backtrack.

    In Unreasonable mode, the program will feel free to generate puzzles
    which are as hard as it can possibly make them: the only constraint is
    that they should still have a unique solution. Solving Unreasonable
    puzzles may require guessing and backtracking.

  23.3. Map user preferences
  --------------------------

  The Preferences option on the Game window menu will let you configure the
  style of the victory flash and also whether the regions start out labelled
  with numbers.

24. Loopy
---------

  You are given a grid of dots, marked with yellow lines to indicate which
  dots you are allowed to connect directly together. Your aim is to use some
  subset of those yellow lines to draw a single unbroken loop from dot to
  dot within the grid.

  Some of the spaces between the lines contain numbers. These numbers
  indicate how many of the lines around that space form part of the loop.
  The loop you draw must correctly satisfy all of these clues to be
  considered a correct solution.

  In the default mode, the dots are arranged in a grid of squares; however,
  you can also play on triangular or hexagonal grids, or even more exotic
  ones.

  Credit for the basic puzzle idea goes to Nikoli[10].

  Loopy was originally contributed to this collection by Mike Pinna, and
  subsequently enhanced to handle various types of non-square grid by
  Lambros Lambrou.

  Note 10

    https://www.nikoli.co.jp/en/puzzles/slitherlink/

  24.1. Loopy controls
  --------------------

  Click Select on a yellow line to turn it black, indicating that you think
  it is part of the loop. Click again to turn the line yellow again (meaning
  you aren't sure yet).

  If you are sure that a particular line segment is /not/ part of the loop,
  you can click Adjust to remove it completely. Again, clicking a second
  time will turn the line back to yellow.

  (All the actions described in Section 3.1. Game Window are also available.)

  24.2. Loopy parameters
  ----------------------

  These parameters are available from Type -- Custom... in the Game window
  menu.

  'Width', 'Height'

    Size of grid, measured in number of regions across and down. For square
    grids, it's clear how this is counted; for other types of grid you may
    have to think a bit to see how the dimensions are measured.

  'Grid type'

    Allows you to choose between a selection of types of tiling. Some have
    all the faces the same but may have multiple different types of vertex
    (eg. the Cairo or Kites mode); others have all the vertices the same but
    may have different types of face (eg. the Great Hexagonal). The square,
    triangular and honeycomb grids are fully regular, and have all their
    vertices /and/ faces the same; this makes them the least confusing to
    play.

  'Difficulty'

    Controls the difficulty of the generated puzzle.

  24.3. Loopy user preferences
  ----------------------------

  The Preferences option on the Game window menu will let you configure the
  following things:

  'Draw excluded grid lines faintly'. This is on by default: when a line of
  the grid has been explicitly excluded from the solution by Adjust-clicking
  it, the line is still drawn, just in a faint grey colour. If you turn this
  option off, excluded lines are not drawn at all.

  'Auto-follow unique paths of edges'. This is off by default. When it's on,
  clicking to change the status of a single grid line will potentially
  propagate the change along multiple lines, if one or both ends of the line
  you clicked connect to only one other line. (The idea is that if two lines
  meet at a vertex and no other lines do at all, then those lines are either
  both part of the loop or neither, so there's no reason you should have to
  click separately to toggle each one.)

  In the mode Based on grid only, the effects of a click will only propagate
  across vertices that have degree 2 in the underlying grid. For example, in
  the square grid, the effect will /only/ occur at the four grid corners.

  In the mode Based on grid and game state, the propagation will also take
  account of edges you've already excluded from the solution, so that it
  will do even more work for you.

25. Inertia
-----------

  You are a small green ball sitting in a grid full of obstacles. Your aim
  is to collect all the gems without running into any mines.

  You can move the ball in any orthogonal /or diagonal/ direction. Once the
  ball starts moving, it will continue until something stops it. A wall
  directly in its path will stop it (but if it is moving diagonally, it will
  move through a diagonal gap between two other walls without stopping).
  Also, some of the squares are 'stops'; when the ball moves on to a stop,
  it will stop moving no matter what direction it was going in. Gems do
  /not/ stop the ball; it picks them up and keeps on going.

  Running into a mine is fatal. Even if you picked up the last gem in the
  same move which then hit a mine, the game will count you as dead rather
  than victorious.

  This game was originally implemented for Windows by Ben Olmstead[11], who
  was kind enough to release his source code on request so that it could be
  re-implemented for this collection.

  Note 11

    http://xn13.com/

  25.1. Inertia controls
  ----------------------

  You can move the ball in any of the eight directions using the numeric
  keypad. Alternatively, if you click Select on the grid, the ball will
  begin a move in the general direction of where you clicked.

  If you use the Solve function on this game, the program will compute a
  path through the grid which collects all the remaining gems and returns to
  the current position. A hint arrow will appear on the ball indicating the
  direction in which you should move to begin on this path. If you then move
  in that direction, the arrow will update to indicate the next direction on
  the path. You can also press Space to automatically move in the direction
  of the hint arrow. If you move in a different direction from the one shown
  by the arrow, arrows will be shown only if the puzzle is still solvable.

  All the actions described in Section 3.1. Game Window are also available.
  In particular, if you do run into a mine and die, you can use the Undo
  function and resume playing from before the fatal move. The game will keep
  track of the number of times you have done this.

  25.2. Inertia parameters
  ------------------------

  These parameters are available from Type -- Custom... in the Game window
  menu.

  'Width', 'Height'

    Size of grid in squares.

26. Tents
---------

  You have a grid of squares, some of which contain trees. Your aim is to
  place tents in some of the remaining squares, in such a way that the
  following conditions are met:

  * There are exactly as many tents as trees.

  * The tents and trees can be matched up in such a way that each tent is
    directly adjacent (horizontally or vertically, but not diagonally) to
    its own tree. However, a tent may be adjacent to other trees as well as
    its own.

  * No two tents are adjacent horizontally, vertically /or diagonally/.

  * The number of tents in each row, and in each column, matches the numbers
    given round the sides of the grid.

  This puzzle can be found in several places on the Internet, and was
  brought to my attention by e-mail. I don't know who I should credit for
  inventing it.

  26.1. Tents controls
  --------------------

  Select-clicking in a blank square will place a tent in it. Adjust-clicking
  in a blank square will colour it green, indicating that you are sure it
  /isn't/ a tent. Clicking either button in an occupied square will clear it.

  If you /drag/ with Adjust along a row or column, every blank square in the
  region you cover will be turned green, and no other squares will be
  affected. (This is useful for clearing the remainder of a row once you
  have placed all its tents.)

  You can also use the cursor keys to move around the grid. Pressing the
  Return key over an empty square will place a tent, and pressing the Space
  bar over an empty square will colour it green; either key will clear an
  occupied square. Holding Shift and pressing the cursor keys will colour
  empty squares green. Holding Ctrl and pressing the cursor keys will colour
  green both empty squares and squares with tents.

  (All the actions described in Section 3.1. Game Window are also available.)

  26.2. Tents parameters
  ----------------------

  These parameters are available from Type -- Custom... in the Game window
  menu.

  'Width', 'Height'

    Size of grid in squares.

  'Difficulty'

    Controls the difficulty of the generated puzzle. More difficult puzzles
    require more complex deductions, but at present none of the available
    difficulty levels requires guesswork or backtracking.

27. Bridges
-----------

  You have a set of islands distributed across the playing area. Each island
  contains a number. Your aim is to connect the islands together with
  bridges, in such a way that:

  * Bridges run horizontally or vertically.

  * The number of bridges terminating at any island is equal to the number
    written in that island.

  * Two bridges may run in parallel between the same two islands, but no
    more than two may do so.

  * No bridge crosses another bridge.

  * All the islands are connected together.

  There are some configurable alternative modes, which involve changing the
  parallel-bridge limit to something other than 2, and introducing the
  additional constraint that no sequence of bridges may form a loop from one
  island back to the same island. The rules stated above are the default
  ones.

  Credit for this puzzle goes to Nikoli[12].

  Bridges was contributed to this collection by James Harvey.

  Note 12

    https://www.nikoli.co.jp/en/puzzles/Hashiwokakero/

  27.1. Bridges controls
  ----------------------

  To place a bridge between two islands, click the mouse down on one island
  and drag it towards the other. You do not need to drag all the way to the
  other island; you only need to move the mouse far enough for the intended
  bridge direction to be unambiguous. (So you can keep the mouse near the
  starting island and conveniently throw bridges out from it in many
  directions.)

  Doing this again when a bridge is already present will add another
  parallel bridge. If there are already as many bridges between the two
  islands as permitted by the current game rules (ie. two by default), the
  same dragging action will remove all of them.

  If you want to remind yourself that two islands definitely /do not/ have a
  bridge between them, you can Adjust-drag between them in the same way to
  draw a 'non-bridge' marker.

  If you think you have finished with an island (ie. you have placed all its
  bridges and are confident that they are in the right places), you can mark
  the island as finished by Select-clicking on it. This will highlight it
  and all the bridges connected to it, and you will be prevented from
  accidentally modifying any of those bridges in future. Select-clicking
  again on a highlighted island will unmark it and restore your ability to
  modify it.

  You can also use the cursor keys to move around the grid: if possible the
  cursor will always move orthogonally, otherwise it will move towards the
  nearest island to the indicated direction. Holding Ctrl and pressing a
  cursor key will lay a bridge in that direction (if available); Shift and a
  cursor key will lay a 'non-bridge' marker. Pressing the Return key
  followed by a cursor key will also lay a bridge in that direction.

  You can mark an island as finished by pressing the Space bar or by
  pressing the Return key twice.

  By pressing a number key, you can jump to the nearest island with that
  number. Letters A through F count as 10 through 15 and 0 as 16.

  The G key will draw a grey line between each pair of islands that could be
  connected with a bridge or non-bridge but are currently not.

  Violations of the puzzle rules will be marked in red:

  * An island with too many bridges will be highlighted in red.

  * An island with too few bridges will be highlighted in red if it is
    definitely an error (as opposed to merely not being finished yet): if
    adding enough bridges would involve having to cross another bridge or
    remove a non-bridge marker, or if the island has been highlighted as
    complete.

  * A group of islands and bridges may be highlighted in red if it is a
    closed subset of the puzzle with no way to connect it to the rest of the
    islands. For example, if you directly connect two 1s together with a
    bridge and they are not the only two islands on the grid, they will
    light up red to indicate that such a group cannot be contained in any
    valid solution.

  * If you have selected the (non-default) option to disallow loops in the
    solution, a group of bridges which forms a loop will be highlighted.

  (All the actions described in Section 3.1. Game Window are also available.)

  27.2. Bridges parameters
  ------------------------

  These parameters are available from Type -- Custom... in the Game window
  menu.

  'Width', 'Height'

    Size of grid in squares.

  'Difficulty'

    Difficulty level of puzzle.

  'Allow loops'

    This is set by default. If cleared, puzzles will be generated in such a
    way that they are always soluble without creating a loop, and solutions
    which do involve a loop will be disallowed.

  'Max. bridges per direction_'

    Maximum number of bridges in any particular direction. The default is 2,
    but you can change it to 1, 3 or 4. In general, fewer is easier.

  '%age of island squares'

    Gives a rough percentage of islands the generator will try and lay
    before finishing the puzzle. Certain layouts will not manage to lay
    enough islands; this is an upper bound.

  'Expansion factor (%age)'

    The grid generator works by picking an existing island at random (after
    first creating an initial island somewhere). It then decides on a
    direction (at random), and then works out how far it could extend before
    creating another island. This parameter determines how likely it is to
    extend as far as it can, rather than choosing somewhere closer.

    High expansion factors usually mean easier puzzles with fewer possible
    islands; low expansion factors can create lots of tightly-packed islands.

  27.3. Bridges user preferences
  ------------------------------

  The Preferences option on the Game window menu will let you configure
  whether possible bridge locations are shown. Unlike the G key, this will
  persist between games.

28. Unequal
-----------

  You have a square grid; each square may contain a digit from 1 to the size
  of the grid, and some squares have clue signs between them. Your aim is to
  fully populate the grid with numbers such that:

  * Each row contains only one occurrence of each digit,

  * Each column contains only one occurrence of each digit, and

  * All the clue signs are satisfied.

  There are two modes for this game, 'Unequal' and 'Adjacent'.

  In 'Unequal' mode, the clue signs are greater-than symbols indicating one
  square's value is greater than its neighbour's. In this mode not all clues
  may be visible, particularly at higher difficulty levels.

  In 'Adjacent' mode, the clue signs are bars indicating one square's value
  is numerically adjacent (ie. one higher or one lower) than its neighbour.
  In this mode all clues are always visible: absence of a bar thus means
  that a square's value is definitely not numerically adjacent to that
  neighbour's.

  In 'Trivial' difficulty level (available via the Type -- Custom game type
  selector), there are no greater-than signs in 'Unequal' mode; the puzzle
  is to solve the Latin square only.

  At the time of writing, the 'Unequal' mode of this puzzle is appearing in
  the Guardian weekly under the name 'Futoshiki'.

  Unequal was contributed to this collection by James Harvey.

  28.1. Unequal controls
  ----------------------

  Unequal shares much of its control system with Solo (see Chapter 12. Solo).

  To play Unequal, simply click the mouse in any empty square and then type
  a digit or letter on the keyboard to fill that square. If you make a
  mistake, click the mouse in the incorrect square and press Space to clear
  it again (or use the Undo feature).

  If you Adjust-click in a square and then type a number, that number will
  be entered in the square as a 'pencil mark'. You can have pencil marks for
  multiple numbers in the same square. Squares containing filled-in numbers
  cannot also contain pencil marks.

  The game pays no attention to pencil marks, so exactly what you use them
  for is up to you: you can use them as reminders that a particular square
  needs to be re-examined once you know more about a particular number, or
  you can use them as lists of the possible numbers in a given square, or
  anything else you feel like.

  To erase a single pencil mark, Adjust-click in the square and type the
  same number again.

  All pencil marks in a square are erased when you Select-click and type a
  number, or when you Select-click and press Space. Adjust-clicking and
  pressing Space> will also erase pencil marks.

  As for Solo, the cursor keys can be used in conjunction with the digit
  keys to set numbers or pencil marks. You can also use the M key to auto-
  fill every numeric hint, ready for removal as required, or the H key to do
  the same but also to remove all obvious hints.

  Alternatively, use the cursor keys to move the mark around the grid.
  Pressing the Return key toggles the mark (from a normal mark to a pencil
  mark), and typing a number in is entered in the square in the appropriate
  way; typing in a 0 or using the Space bar will clear a filled square.

  Select-clicking a clue will mark it as done (grey it out), or unmark it if
  it is already marked. Holding Ctrl or Shift and pressing an arrow key
  likewise marks any clue adjacent to the cursor in the given direction.

  (All the actions described in Section 3.1. Game Window are also available.)

  28.2. Unequal parameters
  ------------------------

  These parameters are available from Type -- Custom... in the Game window
  menu.

  'Mode'

    Mode of the puzzle (Unequal or Adjacent)

  'Size (s*s)'

    Size of grid.

  'Difficulty'

    Controls the difficulty of the generated puzzle. At Trivial level, there
    are no greater-than signs; the puzzle is to solve the Latin square only.
    At Recursive level (only available via the Type -- Custom... game type
    selector) backtracking will be required, but the solution should still
    be unique. The levels in between require increasingly complex reasoning
    to avoid having to backtrack.

29. Galaxies
------------

  You have a rectangular grid containing a number of dots. Your aim is to
  partition the rectangle into connected regions of squares, in such a way
  that every region is 180-degree rotationally symmetric, and contains
  exactly one dot which is located at its centre of symmetry.

  To enter your solution, you draw lines along the grid edges to mark the
  boundaries of the regions. The puzzle is complete when the marked lines on
  the grid are precisely those that separate two squares belonging to
  different regions.

  This puzzle was invented by Nikoli[13], under the name 'Tentai Show'; its
  name is commonly translated into English as 'Spiral Galaxies'.

  Galaxies was contributed to this collection by James Harvey.

  Note 13

    https://www.nikoli.co.jp/en/puzzles/tentai_show/

  29.1. Galaxies controls
  -----------------------

  Select-click on any grid line to draw an edge if there isn't one already,
  or to remove one if there is. When you create a valid region (one which is
  closed, contains exactly one dot, is 180-degree symmetric about that dot,
  and contains no extraneous edges between two of its own squares), it will
  be highlighted automatically; so your aim is to have the whole grid
  highlighted in that way.

  During solving, you might know that a particular grid square belongs to a
  specific dot, but not be sure of where the edges go and which other
  squares are connected to the dot. In order to mark this so you don't
  forget, you can Adjust-drag on the dot, which will create an arrow marker
  pointing at the dot. Drop that in a square of your choice and it will
  remind you which dot it's associated with. You can also Adjust-drag
  existing arrows to pick them up and move them, or destroy them by dropping
  them off the edge of the grid. (Also, if you're not sure which dot an
  arrow is pointing at, you can pick it up and move it around to make it
  clearer. It will swivel constantly as you drag it, to stay pointed at its
  parent dot.)

  You can also use the cursor keys to move around the grid squares and
  lines. Pressing the Return key when over a grid line will draw or clear
  its edge, as above. Pressing the Return key when over a dot will pick up
  an arrow, to be dropped the next time the return key is pressed; this can
  also be used to move existing arrows around, removing them by dropping
  them on a dot or another arrow.

  (All the actions described in Section 3.1. Game Window are also available.)

  29.2. Galaxies parameters
  -------------------------

  These parameters are available from Type -- Custom... in the Game window
  menu.

  'Width', 'Height'

    Size of grid in squares.

  'Difficulty'

    Controls the difficulty of the generated puzzle. More difficult puzzles
    require more complex deductions, and the Unreasonable difficulty level
    may require backtracking.

30. Filling
-----------

  You have a grid of squares, some of which contain digits, and the rest of
  which are empty. Your job is to fill in digits in the empty squares, in
  such a way that each connected region of squares all containing the same
  digit has an area equal to that digit.

  ('Connected region', for the purposes of this game, does not count
  diagonally separated squares as adjacent.)

  For example, it follows that no square can contain a zero, and that two
  adjacent squares can not both contain a one. No region has an area greater
  than 9 (because then its area would not be a single digit).

  Credit for this puzzle goes to Nikoli[14].

  Filling was contributed to this collection by Jonas Koelker.

  Note 14

    https://www.nikoli.co.jp/en/puzzles/fillomino/

  30.1. Filling controls
  ----------------------

  To play Filling, simply click the mouse in any empty square and then type
  a digit on the keyboard to fill that square. By dragging the mouse, you
  can select multiple squares to fill with a single keypress. If you make a
  mistake, click the mouse in the incorrect square and press 0, Space,
  Backspace or Enter to clear it again (or use the Undo feature).

  You can also move around the grid with the cursor keys; typing a digit
  will fill the square containing the cursor with that number; typing 0 will
  clear it. You can also select multiple squares for numbering or clearing
  with the Return and arrow keys, before typing a digit to fill or clear the
  highlighted squares (as above). The Space bar adds and removes single
  squares to and from the selection. Escape removes all squares from the
  selection.

  (All the actions described in Section 3.1. Game Window are also available.)

  30.2. Filling parameters
  ------------------------

  Filling allows you to configure the number of rows and columns of the
  grid, through Type -- Custom... in the Game window menu.

31. Keen
--------

  You have a square grid; each square may contain a digit from 1 to the size
  of the grid. The grid is divided into blocks of varying shape and size,
  with arithmetic clues written in them. Your aim is to fully populate the
  grid with digits such that:

  * Each row contains only one occurrence of each digit

  * Each column contains only one occurrence of each digit

  * The digits in each block can be combined to form the number stated in
    the clue, using the arithmetic operation given in the clue. That is:

    + An addition clue means that the sum of the digits in the block must be
      the given number. For example, '15+' means the contents of the block
      adds up to fifteen.

    + A multiplication clue (eg. '60*'), similarly, means that the product
      of the digits in the block must be the given number.

    + A subtraction clue will always be written in a block of size two, and
      it means that one of the digits in the block is greater than the other
      by the given amount. For example, '2-' means that one of the digits in
      the block is 2 more than the other, or equivalently that one digit
      minus the other one is 2. The two digits could be either way round,
      though.

    + A division clue (eg. '3/'), similarly, is always in a block of size
      two and means that one digit divided by the other is equal to the
      given amount.

    Note that a block may contain the same digit more than once (provided
    the identical ones are not in the same row and column). This rule is
    precisely the opposite of the rule in Solo (see Chapter 12. Solo)'s
    'Killer' mode (see Chapter 12. Solo).

  This puzzle appears in the Times under the name 'KenKen'.

  31.1. Keen controls
  -------------------

  Keen shares much of its control system with Solo (see Chapter 12. Solo)
  (and Unequal (see Chapter 28. Unequal)).

  To play Keen, simply click the mouse in any empty square and then type a
  digit on the keyboard to fill that square. If you make a mistake, click
  the mouse in the incorrect square and press Space to clear it again (or
  use the Undo feature).

  If you Adjust-click in a square and then type a number, that number will
  be entered in the square as a 'pencil mark'. You can have pencil marks for
  multiple numbers in the same square. Squares containing filled-in numbers
  cannot also contain pencil marks.

  The game pays no attention to pencil marks, so exactly what you use them
  for is up to you: you can use them as reminders that a particular square
  needs to be re-examined once you know more about a particular number, or
  you can use them as lists of the possible numbers in a given square, or
  anything else you feel like.

  To erase a single pencil mark, Adjust-click in the square and type the
  same number again.

  All pencil marks in a square are erased when you Select-click and type a
  number, or when you Select-click and press Space. Adjust-clicking and
  pressing space will also erase pencil marks.

  As for Solo (see Chapter 12. Solo), the cursor keys can be used in
  conjunction with the digit keys to set numbers or pencil marks. Use the
  cursor keys to move a highlight around the grid, and type a digit to enter
  it in the highlighted square. Pressing Return toggles the highlight into a
  mode in which you can enter or remove pencil marks.

  Pressing M will fill in a full set of pencil marks in every square that
  does not have a main digit in it.

  (All the actions described in Section 3.1. Game Window are also available.)

  31.2. Keen parameters
  ---------------------

  These parameters are available from Type -- Custom... in the Game window
  menu.

  'Grid size'

    Specifies the size of the grid. Lower limit is 3; upper limit is 9
    (because the user interface would become more difficult with 'digits'
    bigger than 9!).

  'Difficulty'

    Controls the difficulty of the generated puzzle. At Unreasonable level,
    some backtracking will be required, but the solution should still be
    unique. The remaining levels require increasingly complex reasoning to
    avoid having to backtrack.

  'Multiplication only'

    If this is enabled, all boxes will be multiplication boxes. With this
    rule, the puzzle is known as 'Inshi No Heya'.

32. Towers
----------

  You have a square grid. On each square of the grid you can build a tower,
  with its height ranging from 1 to the size of the grid. Around the edge of
  the grid are some numeric clues.

  Your task is to build a tower on every square, in such a way that:

  * Each row contains every possible height of tower once

  * Each column contains every possible height of tower once

  * Each numeric clue describes the number of towers that can be seen if you
    look into the square from that direction, assuming that shorter towers
    are hidden behind taller ones. For example, in a 5x5 grid, a clue marked
    '5' indicates that the five tower heights must appear in increasing
    order (otherwise you would not be able to see all five towers), whereas
    a clue marked '1' indicates that the tallest tower (the one marked 5)
    must come first.

  In harder or larger puzzles, some towers will be specified for you as well
  as the clues round the edge, and some edge clues may be missing.

  This puzzle appears on the web under various names, particularly
  'Skyscrapers', but I don't know who first invented it.

  32.1. Towers controls
  ---------------------

  Towers shares much of its control system with Solo (see Chapter 12. Solo),
  Unequal (see Chapter 28. Unequal) and Keen (see Chapter 31. Keen).

  To play Towers, simply click the mouse in any empty square and then type a
  digit on the keyboard to fill that square with a tower of the given
  height. If you make a mistake, click the mouse in the incorrect square and
  press Space to clear it again (or use the Undo feature).

  If you Adjust-click in a square and then type a number, that number will
  be entered in the square as a 'pencil mark'. You can have pencil marks for
  multiple numbers in the same square. A square containing a tower cannot
  also contain pencil marks.

  The game pays no attention to pencil marks, so exactly what you use them
  for is up to you: you can use them as reminders that a particular square
  needs to be re-examined once you know more about a particular number, or
  you can use them as lists of the possible numbers in a given square, or
  anything else you feel like.

  To erase a single pencil mark, Adjust-click in the square and type the
  same number again.

  All pencil marks in a square are erased when you Select-click and type a
  number, or when you Select-click and press Space. Adjust-clicking and
  pressing Space will also erase pencil marks.

  As for Solo (see Chapter 12. Solo), the cursor keys can be used in
  conjunction with the digit keys to set numbers or pencil marks. Use the
  cursor keys to move a highlight around the grid, and type a digit to enter
  it in the highlighted square. Pressing Return toggles the highlight into a
  mode in which you can enter or remove pencil marks.

  Pressing M will fill in a full set of pencil marks in every square that
  does not have a main digit in it.

  Select-clicking a clue will mark it as done (grey it out), or unmark it if
  it is already marked. Holding Ctrl or Shift and pressing an arrow key
  likewise marks any clue in the given direction.

  (All the actions described in Section 3.1. Game Window are also available.)

  32.2. Towers parameters
  -----------------------

  These parameters are available from Type -- Custom... in the Game window
  menu.

  'Grid size'

    Specifies the size of the grid. Lower limit is 3; upper limit is 9
    (because the user interface would become more difficult with 'digits'
    bigger than 9!).

  'Difficulty'

    Controls the difficulty of the generated puzzle. At Unreasonable level,
    some backtracking will be required, but the solution should still be
    unique. The remaining levels require increasingly complex reasoning to
    avoid having to backtrack.

  32.3. Towers user preferences
  -----------------------------

  The Preferences option on the Game window menu will let you configure the
  style of the game display. If you don't like the three-dimensional mode,
  selecting 2D will switch to a simpler display style in which towers are
  shown by just writing their height in the square.

33. Singles
-----------

  You have a grid of white squares, all of which contain numbers. Your task
  is to colour some of the squares black (removing the number) so as to
  satisfy all of the following conditions:

  * No number occurs more than once in any row or column.

  * No black square is horizontally or vertically adjacent to any other
    black square.

  * The remaining white squares must all form one contiguous region
    (connected by edges, not just touching at corners).

  Credit for this puzzle goes to Nikoli[15] who call it Hitori.

  Singles was contributed to this collection by James Harvey.

  Note 15

    https://www.nikoli.co.jp/en/puzzles/hitori/

  33.1. Singles controls
  ----------------------

  Select-clicking on an empty square will colour it black; Select-clicking
  again will restore the number. Adjust-clicking will add a circle (useful
  for indicating that a cell is definitely not black). Clicking outside the
  grid will toggle whether black squares completely hide the numbers on
  them, or display them in dark grey.

  You can also use the cursor keys to move around the grid. Pressing the
  Return or Space keys will turn a square black or add a circle
  respectively, and pressing the key again will restore the number or remove
  the circle.

  (All the actions described in Section 3.1. Game Window are also available.)

  33.2. Singles parameters
  ------------------------

  These parameters are available from Type -- Custom... in the Game window
  menu.

  'Width', 'Height'

    Size of grid in squares.

  'Difficulty'

    Controls the difficulty of the generated puzzle.

  33.3. Singles user preferences
  ------------------------------

  The Preferences option on the Game window menu will let you configure
  whether numbers on black squares are visible. Unlike clicking outside the
  grid, this will persist between games.

34. Magnets
-----------

  A rectangular grid has been filled with a mixture of magnets (that is,
  dominoes with one positive end and one negative end) and blank dominoes
  (that is, dominoes with two neutral poles). These dominoes are initially
  only seen in silhouette. Around the grid are placed a number of clues
  indicating the number of positive and negative poles contained in certain
  columns and rows.

  Your aim is to correctly place the magnets and blank dominoes such that
  all the clues are satisfied, with the additional constraint that no two
  similar magnetic poles may be orthogonally adjacent (since they repel).
  Neutral poles do not repel, and can be adjacent to any other pole.

  Credit for this puzzle goes to Janko[16].

  Magnets was contributed to this collection by James Harvey.

  Note 16

    http://www.janko.at/Raetsel/Magnete/index.htm

  34.1. Magnets controls
  ----------------------

  Select-clicking on an empty square places a magnet at that position with
  the positive pole on the square and the negative pole on the other half of
  the magnet; Select-clicking again reverses the polarity, and a third click
  removes the magnet.

  Adjust-clicking on an empty square places a blank domino there. Adjust-
  clicking again places two question marks on the domino, signifying 'this
  cannot be blank' (which can be useful to note deductions while solving),
  and Adjust-clicking again empties the domino.

  Select-clicking a clue will mark it as done (grey it out), or unmark it if
  it is already marked.

  You can also use the cursor keys to move a cursor around the grid.
  Pressing the Return key will lay a domino with a positive pole at that
  position; pressing again reverses the polarity and then removes the
  domino, as with Select-clicking. Using the Space bar allows placement of
  blank dominoes and cannot-be-blank hints, as for Adjust-clicking.

  (All the actions described in Section 3.1. Game Window are also available.)

  34.2. Magnets parameters
  ------------------------

  These parameters are available from Type -- Custom... in the Game window
  menu.

  'Width', 'Height'

    Size of grid in squares. There will be half 'Width' x 'Height' dominoes
    in the grid: if this number is odd then one square will be blank.

    (Grids with at least one odd dimension tend to be easier to solve.)

  'Difficulty'

    Controls the difficulty of the generated puzzle. At Tricky level, you
    are required to make more deductions about empty dominoes and row/column
    counts.

  'Strip clues'

    If true, some of the clues around the grid are removed at generation
    time, making the puzzle more difficult.

35. Signpost
------------

  You have a grid of squares; each square (except the last one) contains an
  arrow, and some squares also contain numbers. Your job is to connect the
  squares to form a continuous list of numbers starting at 1 and linked in
  the direction of the arrows -- so the arrow inside the square with the
  number 1 will point to the square containing the number 2, which will
  point to the square containing the number 3, etc. Each square can be any
  distance away from the previous one, as long as it is somewhere in the
  direction of the arrow.

  By convention the first and last numbers are shown; one or more interim
  numbers may also appear at the beginning.

  Credit for this puzzle goes to Janko[17], who call it 'Pfeilpfad' ('arrow
  path').

  Signpost was contributed to this collection by James Harvey.

  Note 17

    http://janko.at/Raetsel/Pfeilpfad/index.htm

  35.1. Signpost controls
  -----------------------

  To play Signpost, you connect squares together by dragging from one square
  to another, indicating that they are adjacent in the sequence. Drag with
  Select from a square to its successor, or with Adjust from a square to its
  predecessor.

  If you connect together two squares in this way and one of them has a
  number in it, the appropriate number will appear in the other square. If
  you connect two non-numbered squares, they will be assigned temporary
  algebraic labels: on the first occasion, they will be labelled 'a' and
  'a+1', and then 'b' and 'b+1', and so on. Connecting more squares on to
  the ends of such a chain will cause them all to be labelled with the same
  letter.

  When you Select-click or Adjust-click in a square, the legal squares to
  connect it to will be shown.

  The arrow in each square starts off black, and goes grey once you connect
  the square to its successor. Also, each square which needs a predecessor
  has a small dot in the bottom left corner, which vanishes once you link a
  square to it. So your aim is always to connect a square with a black arrow
  to a square with a dot.

  To remove any links for a particular square (both incoming and outgoing),
  Select-drag it off the grid. To remove a whole chain, Adjust-drag any
  square in the chain off the grid.

  You can also use the cursor keys to move around the grid squares and
  lines. Pressing the Return key when over a square starts a link operation,
  and pressing the Return key again over a square will finish the link, if
  allowable. Pressing the Space bar over a square will show the other
  squares pointing to it, and allow you to form a backward link, and
  pressing the Space bar again cancels this.

  (All the actions described in Section 3.1. Game Window are also available.)

  35.2. Signpost parameters
  -------------------------

  These parameters are available from Type -- Custom... in the Game window
  menu.

  'Width', 'Height'

    Size of grid in squares.

  'Force start/end to corners'

    If true, the start and end squares are always placed in opposite corners
    (the start at the top left, and the end at the bottom right). If false
    the start and end squares are placed randomly (although always both
    shown).

  35.3. Signpost user preferences
  -------------------------------

  The Preferences option on the Game window menu will let you configure the
  style of the victory effect.

36. Range
---------

  You have a grid of squares; some squares contain numbers. Your job is to
  colour some of the squares black, such that several criteria are satisfied:

  * no square with a number is coloured black.

  * no two black squares are adjacent (horizontally or vertically).

  * for any two white squares, there is a path between them using only white
    squares.

  * for each square with a number, that number denotes the total number of
    white squares reachable from that square going in a straight line in any
    horizontal or vertical direction until hitting a wall or a black square;
    the square with the number is included in the total (once).

  For instance, a square containing the number one must have four black
  squares as its neighbours by the last criterion; but then it's impossible
  for it to be connected to any outside white square, which violates the
  second to last criterion. So no square will contain the number one.

  Credit for this puzzle goes to Nikoli, who have variously called it
  'Kurodoko', 'Kuromasu' or 'Where is Black Cells'[18].

  Range was contributed to this collection by Jonas Koelker.

  Note 18

    https://www.nikoli.co.jp/en/puzzles/kurodoko/

  36.1. Range controls
  --------------------

  Click with Select to paint a square black, or with Adjust to mark a square
  with a dot to indicate that you are sure it should /not/ be painted black.
  Repeated clicking with either button will cycle the square through the
  three possible states (filled, dotted or empty) in opposite directions.

  You can also use the cursor keys to move around the grid squares. Pressing
  Return does the same as clicking with Select, while pressing Space does
  the same as a right button click. Moving with the cursor keys while
  holding Shift will place dots in all squares that are moved through.

  (All the actions described in Section 3.1. Game Window are also available.)

  36.2. Range parameters
  ----------------------

  These parameters are available from Type -- Custom... in the Game window
  menu.

  'Width', 'Height'

    Size of grid in squares.

  36.3. Range user preferences
  ----------------------------

  The Preferences option on the Game window menu will let you configure
  which way round the mouse buttons work (where 'Left' and 'Right' refer to
  Select and Adjust respectively).

37. Pearl
---------

  You have a grid of squares. Your job is to draw lines between the centres
  of horizontally or vertically adjacent squares, so that the lines form a
  single closed loop. In the resulting grid, some of the squares that the
  loop passes through will contain corners, and some will be straight
  horizontal or vertical lines. (And some squares can be completely empty --
  the loop doesn't have to pass through every square.)

  Some of the squares contain black and white circles, which are clues that
  the loop must satisfy.

  A black circle in a square indicates that that square is a corner, but
  neither of the squares adjacent to it in the loop is also a corner.

  A white circle indicates that the square is a straight edge, but /at least
  one/ of the squares adjacent to it in the loop is a corner.

  (In both cases, the clue only constrains the two squares adjacent /in the
  loop/, that is, the squares that the loop passes into after leaving the
  clue square. The squares that are only adjacent /in the grid/ are not
  constrained.)

  Credit for this puzzle goes to Nikoli, who call it 'Masyu'.[19]

  Thanks to James Harvey for assistance with the implementation.

  Note 19

    https://www.nikoli.co.jp/en/puzzles/masyu/

  37.1. Pearl controls
  --------------------

  Click with Select on a grid edge to draw a segment of the loop through
  that edge, or to remove a segment once it is drawn.

  Drag with Select through a series of squares to draw more than one segment
  of the loop in one go. Alternatively, drag over an existing part of the
  loop to undraw it, or to undraw part of it and then go in a different
  direction.

  Click with Adjust on a grid edge to mark it with a cross, indicating that
  you are sure the loop does not go through that edge. (For instance, if you
  have decided which of the squares adjacent to a white clue has to be a
  corner, but don't yet know which way the corner turns, you might mark the
  one way it /can't/ go with a cross.)

  Alternatively, use the cursor keys to move the cursor. Use the Enter key
  to begin and end keyboard 'drag' operations. Use the Space, Escape or
  Backspace keys to cancel the drag. Or, hold Ctrl while dragging with the
  cursor keys to toggle segments as you move between squares.

  Pressing Ctrl-Shift-arrowkey or Shift-arrowkey simulates a left or Adjust
  click, respectively, on the edge in the direction of the key.

  (All the actions described in Section 3.1. Game Window are also available.)

  37.2. Pearl parameters
  ----------------------

  These parameters are available from Type -- Custom... in the Game window
  menu.

  'Width', 'Height'

    Size of grid in squares.

  'Difficulty'

    Controls the difficulty of the generated puzzle.

  'Allow unsoluble_'

    If this is set, then the game will be generated in the simplest way:
    every clue square that can possibly be provided will be shown, and the
    generator will not check whether the puzzle can be uniquely solved.

    This speeds up game generation, and allows much larger grids to be
    played. At least one possible solution will still always exist, but
    there's no guarantee that it will be unique, or that it will be possible
    to deduce it step by step.

  37.3. Pearl user preferences
  ----------------------------

  The Preferences option on the Game window menu will let you configure the
  style of the game display. Traditional is the default mode, in which the
  loop runs between centres of grid squares, and each clue occupies a
  square. Loopy-style is an alternative mode that looks more like Loopy (see
  Chapter 24. Loopy), in which the loop runs between grid /vertices/, and
  the clues also occupy vertices.

38. Undead
----------

  You are given a grid of squares, some of which contain diagonal mirrors.
  Every square which is not a mirror must be filled with one of three types
  of undead monster: a ghost, a vampire, or a zombie.

  Vampires can be seen directly, but are invisible when reflected in
  mirrors. Ghosts are the opposite way round: they can be seen in mirrors,
  but are invisible when looked at directly. Zombies are visible by any
  means.

  You are also told the total number of each type of monster in the grid.
  Also around the edge of the grid are written numbers, which indicate how
  many monsters can be seen if you look into the grid along a row or column
  starting from that position. (The diagonal mirrors are reflective on both
  sides. If your reflected line of sight crosses the same monster more than
  once, the number will count it each time it is visible, not just once.)

  This puzzle type was invented by David Millar, under the name 'Haunted
  Mirror Maze'[20] (see the footnote for more details).

  Undead was contributed to this collection by Steffen Bauer.

  Note 20

    http://www.janko.at/Raetsel/Spukschloss/index.htm

  38.1. Undead controls
  ---------------------

  Undead has a similar control system to Solo (see Chapter 12. Solo),
  Unequal (see Chapter 28. Unequal) and Keen (see Chapter 31. Keen).

  To play Undead, click the mouse in any empty square and then type a letter
  or number on the keyboard indicating the type of monster: G or 1 for a
  ghost, V or 2 for a vampire, or Z or 3 for a zombie. If you make a
  mistake, click the mouse in the incorrect square and press Space to clear
  it again (or use the Undo feature).

  If you Adjust-click in a square and then type a letter or number, the
  corresponding monster will be shown in reduced size in that square, as a
  'pencil mark'. You can have pencil marks for multiple monsters in the same
  square. A square containing a full-size monster cannot also contain pencil
  marks.

  The game pays no attention to pencil marks, so exactly what you use them
  for is up to you: you can use them as reminders that a particular square
  needs to be re-examined once you know more about a particular monster, or
  you can use them as lists of the possible monster in a given square, or
  anything else you feel like.

  To erase a single pencil mark, Adjust-click in the square and type the
  same letter or number again.

  All pencil marks in a square are erased when you Select-click and type a
  monster letter, or when you Select-click and press Space. Adjust-clicking
  and pressing Space will also erase pencil marks.

  As for Solo (see Chapter 12. Solo), the cursor keys can be used in
  conjunction with the letter keys to place monsters or pencil marks. Use
  the cursor keys to move a highlight around the grid, and type a monster
  letter or number to enter it in the highlighted square. Pressing Return
  toggles the highlight into a mode in which you can enter or remove pencil
  marks.

  If you prefer plain letters of the alphabet to cute monster pictures, you
  can press A to toggle between showing the monsters as monsters or showing
  them as letters.

  Select-clicking a clue will mark it as done (grey it out), or unmark it if
  it is already marked.

  (All the actions described in Section 3.1. Game Window are also available.)

  38.2. Undead parameters
  -----------------------

  These parameters are available from Type -- Custom... in the Game window
  menu.

  'Width', 'Height'

    Size of grid in squares.

  'Difficulty'

    Controls the difficulty of the generated puzzle

  38.3. Undead user preferences
  -----------------------------

  The Preferences option on the Game window menu will let you configure
  whether Undead uses letters or pictures to represent monsters.

39. Unruly
----------

  You are given a grid of squares, which you must colour either black or
  white. Some squares are provided as clues; the rest are left for you to
  fill in. Each row and column must contain the same number of black and
  white squares, and no row or column may contain three consecutive squares
  of the same colour.

  This puzzle type was invented by Adolfo Zanellati, under the name 'Tohu wa
  Vohu'[21].

  Unruly was contributed to this collection by Lennard Sprong.

  Note 21

    http://www.janko.at/Raetsel/Tohu-Wa-Vohu/index.htm

  39.1. Unruly controls
  ---------------------

  To play Unruly, click the mouse in a square to change its colour. Select-
  clicking an empty square will turn it black, and Adjust-clicking will turn
  it white. Keep clicking the same button to cycle through the three
  possible states for the square. If you Ctrl-Select-click in a square it
  will be reset to empty.

  You can also use the cursor keys to move around the grid. Pressing the
  Return or Space keys will turn an empty square black or white respectively
  (and then cycle the colours in the same way as the mouse buttons), and
  pressing Backspace> will reset a square to empty.

  (All the actions described in Section 3.1. Game Window are also available.)

  39.2. Unruly parameters
  -----------------------

  These parameters are available from Type -- Custom... in the Game window
  menu.

  'Width', 'Height'

    Size of grid in squares. (Note that the rules of the game require both
    the width and height to be even numbers.)

  'Difficulty'

    Controls the difficulty of the generated puzzle.

  'Unique rows and columns'

    If enabled, no two rows are permitted to have exactly the same pattern,
    and likewise columns. (A row and a column can match, though.)

40. Flood
---------

  You are given a grid of squares, coloured at random in multiple colours.
  In each move, you can flood-fill the top left square in a colour of your
  choice (ie. every square reachable from the starting square by an
  orthogonally connected path of squares all the same colour will be filled
  in the new colour). As you do this, more and more of the grid becomes
  connected to the starting square.

  Your aim is to make the whole grid the same colour, in as few moves as
  possible. The game will set a limit on the number of moves, based on
  running its own internal solver. You win if you can make the whole grid
  the same colour in that many moves or fewer.

  I saw this game (with a fixed grid size, fixed number of colours, and
  fixed move limit) at http://floodit.appspot.com (no longer accessible).

  40.1. Flood controls
  --------------------

  To play Flood, click the mouse in a square. The top left corner and
  everything connected to it will be flood-filled with the colour of the
  square you clicked. Clicking a square the same colour as the top left
  corner has no effect, and therefore does not count as a move.

  You can also use the cursor keys to move a cursor (outline black square)
  around the grid. Pressing the Return key will fill the top left corner in
  the colour of the square under the cursor.

  (All the actions described in Section 3.1. Game Window are also available.)

  40.2. Flood parameters
  ----------------------

  These parameters are available from Type -- Custom... in the Game window
  menu.

  'Width', 'Height'

    Size of the grid, in squares.

  'Colours'

    Number of colours used to fill the grid. Must be at least 3 (with two
    colours there would only be one legal move at any stage, hence no choice
    to make at all), and at most 10.

  'Extra moves permitted'

    Controls the difficulty of the puzzle, by increasing the move limit. In
    each new grid, Flood will run an internal solver to generate its own
    solution, and then the value in this field will be added to the length
    of Flood's solution to generate the game's move limit. So a value of 0
    requires you to be just as efficient as Flood's automated solver, and a
    larger value makes it easier.

    (Note that Flood's internal solver will not necessarily find the
    shortest possible solution, though I believe it's pretty close. For a
    real challenge, set this value to 0 and then try to solve a grid in
    /strictly fewer/ moves than the limit you're given!)

41. Tracks
----------

  You are given a grid of squares, some of which are filled with train
  tracks. You need to complete the track from A to B so that the rows and
  columns contain the same number of track segments as are indicated in the
  clues to the top and right of the grid.

  There are only straight and 90 degree curved rails, and the track may not
  cross itself.

  Tracks was contributed to this collection by James Harvey.

  41.1. Tracks controls
  ---------------------

  Select-clicking on an edge between two squares adds a track segment
  between the two squares. Adjust-clicking on an edge adds a cross on the
  edge, indicating no track is possible there.

  Select-clicking in a square adds a colour indicator showing that you know
  the square must contain a track, even if you don't know which edges it
  crosses yet. Adjust-clicking in a square adds a cross indicating it
  contains no track segment.

  Select- or Adjust-dragging between squares allows you to lay a straight
  line of is-track or is-not-track indicators, useful for filling in rows or
  columns to match the clue.

  (All the actions described in Section 3.1. Game Window are also available.)

  41.2. Tracks parameters
  -----------------------

  These parameters are available from Type -- Custom... in the Game window
  menu.

  'Width', 'Height'

    Size of the grid, in squares.

  'Difficulty'

    Controls the difficulty of the generated puzzle: at Tricky level, you
    are required to make more deductions regarding disregarding moves that
    would lead to impossible crossings later.

  'Disallow consecutive 1 clues'

    Controls whether the Tracks game generation permits two adjacent rows or
    columns to have a 1 clue, or permits the row or column of the track's
    endpoint to have a 1 clue. By default this is not permitted, to avoid
    long straight boring segments of track and make the games more twiddly
    and interesting. If you want to restore the possibility, turn this
    option off.

42. Palisade
------------

  You're given a grid of squares, some of which contain numbers. Your goal
  is to subdivide the grid into contiguous regions, all of the same (given)
  size, such that each square containing a number is adjacent to exactly
  that many edges (including those between the inside and the outside of the
  grid).

  Credit for this puzzle goes to Nikoli, who call it 'Five Cells'[22].

  Palisade was contributed to this collection by Jonas Koelker.

  Note 22

    https://www.nikoli.co.jp/en/puzzles/five_cells/

  42.1. Palisade controls
  -----------------------

  Select-click to place an edge. Adjust-click to indicate 'no edge'.

  Alternatively, the arrow keys will move a keyboard cursor. Depending on
  the 'Cursor mode' preference (see Section 42.3. Palisade user
  preferences), the cursor will either navigate among the grid squares, or
  along their borders. In Full-grid mode, hold Ctrl while pressing an arrow
  key to place an edge, and press Shift-arrowkey to switch off an edge. In
  Half-grid mode, press Enter to place an edge, and Space to switch off an
  edge. In either mode, you can repeat an action to perform its inverse.

  (All the actions described in Section 3.1. Game Window are also available.)

  42.2. Palisade parameters
  -------------------------

  These parameters are available from Type -- Custom... in the Game window
  menu.

  'Width', 'Height'

    Size of grid in squares.

  Region size

    The size of the regions into which the grid must be subdivided.

  42.3. Palisade user preferences
  -------------------------------

  The Preferences option on the Game window menu will let you configure the
  behaviour of the cursor keys to either navigate among full grid squares,
  or along the borders of the grid squares.

43. Mosaic
----------

  You are given a grid of squares, which you must colour either black or
  white.

  Some squares contain clue numbers. Each clue tells you the number of black
  squares in the 3x3 region surrounding the clue -- /including/ the clue
  square itself.

  This game is variously known in other locations as: ArtMosaico, Count and
  Darken, Cuenta Y Sombrea, Fill-a-Pix, Fill-In, Komsu Karala, Magipic,
  Majipiku, Mosaico, Mosaik, Mozaiek, Nampre Puzzle, Nurie-Puzzle, Oekaki-
  Pix, Voisimage.

  Mosaic was contributed to this collection by Didi Kohen. Colour design by
  Michal Shomer. The implementation is loosely based on
  https://github.com/mordechaim/Mosaic.

  43.1. Mosaic controls
  ---------------------

  To play Mosaic, click the mouse in a square to change its colour. Select-
  clicking an empty square will turn it black, and Adjust-clicking will turn
  it white. Keep clicking the same button to cycle through the three
  possible states for the square.

  If you hold down the mouse button and drag, you can colour multiple cells
  in a single action.

  You can also use the cursor keys to move around the grid. Pressing the
  Return or Space keys will turn an empty square black or white respectively
  (and then cycle the colours in the same way as the mouse buttons), and
  pressing Backspace will reset a square to empty.

  (All the actions described in Section 3.1. Game Window are also available.)

  43.2. Mosaic parameters
  -----------------------

  These parameters are available from Type -- Custom... in the Game window
  menu.

  'Width', 'Height'

    Size of grid in squares.

  'Aggressive generation'

    With this option set, the game generator will try harder to eliminate
    unnecessary clues on the board. This slows down generation, so it's not
    recommended for boards larger than, say, 30x30.

44. Known Issues
----------------

  The RISC OS port of Puzzles is a work in progress, and there are a few
  things which have either not yet been implemented, are incomplete, or
  contain bugs and mis-features.

  The following is a non-exhaustive list of things which are known to need
  attention.

  * The game windows, toolbars and drawing canvases may not be perfectly
    aligned in EX0 EY0 modes.

  * Graphics can sometimes render poorly, with artefacts on corners.

  * The game windows can not be resized from their defaults.

  * The printing of games is not supported.

  Before reporting bugs or suggesting new features, it is a good idea to
  check the project's issues list on GitHub [https://github.com/steve-
  fryatt/puzzles/issues] (this will probably need Iris or a non-RISC OS
  browser to work correctly).

  The intention is to address many of these items in future releases. The
  implementation of printing will depend on the level of interest shown in
  the RISC OS port.

45. Version History
-------------------

  This is a complete list of all the changes made to the publicly released
  versions of Puzzles on the RISC OS platform.

  45.1. Version 1.00 (24 October 2025)
  ------------------------------------

  First RISC OS release, based on commit d928126c from the upstream
  repository.

Updates and Contacting Me
-------------------------

  If you have any comments about the RISC OS port of Puzzles, or would like
  to report any bugs that you find, you can email me at the address below.
  Please contact me in the first instance for issues encountered on RISC OS,
  so that it can be confirmed that they are not due to the front end.

  Updates to the RISC OS port of Puzzles and more software for RISC OS
  computers can be found on my website at http://www.stevefryatt.org.uk/risc-
  os.

  Stephen Fryatt
  email: info@stevefryatt.org.uk
