Fairy-Max: an AI for playing user-defined Chess variants

Introduction Setup Pieces Rules Playing Tips Equipment Notes Contact Links to related pages would appear here.

Introduction

Fairy-Max is an AI, also called 'engine', for playing Chess variants. It is free open-source software. It was created for the purpose of empirically evaluating fairy pieces, by allowing those to participate amongst orthodox Chess pieces in Chess games aimed at checkmating an opponent royal piece. It searches ahead like the FIDE-Chess engine from which it was derived (the World's once smallest Chess program, micro-Max) by generating moves and trying them out on the internal board. To know how each piece type moves, micro-Max uses tables with step vectors, and it knows which pieces are leapers and which are sliders. In Fairy-Max, the contents of these tables is not fixed, but is loaded from a configuration file 'fmax.ini' at the start of each game. This allows the user to change the way pieces move (or add pieces of his own design) without changing the program, by altering the contents of the fmax.ini file. Fmax.ini is a normal text file and thus can be modified with any text editor (e.g. Notepad).

Fairy-Max is 'only an AI'. This means it does not provide any graphics to display the position of the game it is playing, but only relays the moves as text. It is primarily designed to be run as an engine inside the WinBoard GUI, (or its Linux counterpart XBoard), and it communicates with this GUI in 'WinBoard protocol'. Although there are several other GUIs that support WinBoard protocol, they will in general not offer as much variant support as WinBoard / XBoard. Most GUIs only support normal Chess, but ChessGUI by Matthias Gemuh also supports 10x8 Capablanca variants. Note that there are also many other WinBoard-compatible engines for playing variants that can be played under WinBoard 4.3 or higher. Some even play variants that Fairy-Max could never implement (e.g. Xiangqi or Crazyhouse).

There are however some dedicated versions of Fairy-Max that are adapted to variants with very unique needs, such as the Shatranj baring rule, or the confinement of pieces to certain board areas in Xiangqi. These versions, known as ShaMax and MaxQi, respectively, are in general only useful for playing a single Chess variant, and are included in the downloads listed at the top of this page. There also is a derivative 'Looking Glass', which plays Alice Chess. There also exists a special research derivative 'Pair-o-Max', which does take into account mating potential in the late end-game.

The board size for each variant is also defined in the fmax.ini file, and is currently limited to 12x8. (Expansion of the number of ranks is on the to-do list.) The default opening position is also defined in there, but can be overruled by loading a position through the User Interface.

Pieces

Alternators are pieces that do not need to hop over anything to change their move type, but do so spontaneously after every step they make. This can be a change in step direction, to provide bend trajectories, but also in capture rights. E.g. a Xiangqi Elephant makes 2 diagonal steps in the same direction, on the first step it is not allowed to do anything (capture or non-capture) but continue, (if the square is empty, of course), on the second step it can both capture and non-capture (but is not allowed to continue any further, even if the square was empty). The Xiangqi Horse is similar, but also changes direction between the first and second step.

The Crooked Bishop is an example of a slider that changes direction on every step. This means both its primary and secondary step allow slider-like continuation. After the second step it switches back to its first, and continues alternating on every subsequent step. For simplicity, Fairy-Max treats every slider that is not a hopper as an alternator. In cases where this is not desired (e.g. for the Rook), it simply 'alternates' between two identical steps.

To provide flexibility in implementation of fairy pieces, the discription of the move type has to be given for each direction separately, rather than for the piece as a whole. This to allow mixed sliders/leapers, such as the Archbishop, which combines the Knight (leaper) and Bishop (slider) moves. For implementation of hoppers and alternators, the move description (direction and type) has to be given twice (primary and secondary move descriptor). The method used for that in the fmax.ini file is to specify which bits should change to make the secondary step and move rights out of the primary. This info can be used to change them in both directions (primary -> secondary and secondary -> primary), which is convenient for alternators. It also means that for 'normal' pieces, where nothing changes as primary and secondary step are the same, the secondary rights and step are all zero, and don't have to be written. Since version 4.8S a 2-bit 'delay count' can be specified with each piece to delay the first toggle between primary and secondary step type by 1, 2 or 3 steps, which allows implementation of limited-range sliders of ranges upto 5.

Fairy-Max makes no assumptions to the symmetry of the piece, meaning that all directions in which the piece moves have to be specified explicitly. E.g. for a Rook we have to list 4 moves, for a Knight 8. However annoying this sometimes might be, this is necessary for allowing implementation of asymmetric pieces such as the Crab or the Shogi Generals. Unfortunately, Fairy-Max is not smart enough yet to realize that the different sides play in opposite direction. Thus asymmetric pieces (such as Pawns) have to be implemented as different piece types for white and black. (E.g. what is a Crab for white, will be a Barc for black, and vice versa.) The following pieces have predefined descriptions in fmax.ini, ready for copying to your own variants.

Table 3: Pieces that are pre-defined in the fmax.ini that comes with the download

Rules

This section gives an indication of how the Chess variants are defined in the fmax.ini file. After a line "Game: " to indicate that a new variant description follows, we have lines that specify the board size, and the back rank of the array for white and black. (Fairy-Max always assumed a close rank of Pawns directly in front of that.) A name can be prefixed with "fairy/", meaning that it is not supported by WinBoard, and has to be played as the catch-all vatiant "fairy". An engine option (appearing in the GUI's Engine Settings dialog) will in this case determine what variant is really played (identified by the part of the name after the slash).

The variant name can be followed by a # and extra information. In such a case Fairy-Max will send a 'setup' command to the GUI to inform it of the default opening position for the variant, possibly accompanied by the info behind the #, which designates the one-letter piece ID with which the GUI should represent the various pieces. After the three initial lines follow the descriptions of all participating pieces, one line per piece.

For each piece, we have to give the letter by which it is represented when feeding a position to the engine, and the value the engine should attach to it in play. (So beware, Fairy-Max is not smart enough to figure this out by itself. If you tell it an immensely powerful piece like a Queen is only worth half a Pawn, it will happily trade it for one!) Then follows on the same line a list of move descriptors, one for each direction the piece moves in. A move description consists of an elementary board step, and a number that specifies the move attributes, i.e. what the piece is allowed to do in this direction (if it can capture and/or non-capture, if it is leaper or slider, if it can hop over an obstacle or not, if it respects the board edges or wraps around.) The exact way the step vectors and attributes are encoded is specified in comment lines contained in the fmax.ini, as it might still be subject to change in future versions.

The first two pieces specified are by definition the white and black Pawns. That means that Fairy-Max will let them promote if they reach the last rank. Fairy-Max does not think about under-promotion, but always promotes to 'Queen', i.e. the 7th piece specified, however that may move. Except when piece number 7 only appears in the initial setup for white, and a piece with number 9 occurs in the initial setup for black, in which case black would always promote to piece 9 (to allow variants with different armies).

Any piece with a negative value is considered royal. Fairy-Max makes a difference between value -1 and other negative values, though: -1 indicates absolute royalty, i.e. loss of any piece of this type means loss of the game. Other values indicate only loss of the last royal piece result in loss of the game, and any spare ones should be treated as if they are worth the absolute value of the given number. This allows for variants with 'delocalized royalty', such as Spartan Chess.

Notes

There are many things that Fairy-Max cannot do; it was never meant to be a general game-playing system like Zillions of Games. Amongst the impossibilities are piece drops, (as in Crazyhouse or Shogi), pieces for which the moves are dependent on their location on the board (as in Xiangqi, but MaxQi handles this particular case). and pieces whose moving exerts side effects on other squares that their old and new location (with the exception of castling and e.p capture). For some pieces the move is simply too complicated to fit within the Fairy-Max move-type classification system (e.g. Rose, Griffon).

Only boards with 8 ranks are currently implemented.

The current version of Fairy-Max implements e.p. capture in such a way that it also correctly handles Berolina Pawns, and allows castling with any corner piece.

Playing Tips

Positional evaluation in Fairy-Max is fairly primitive: a piece either is drawn towards the center, or not. If you want a piece to be drawn to the center, you should indicate it by a lower-case letter. Pawns do get a large positional bonus when they reach 6th or 7th rank, of 64 and 128 units ('centiPawn' in FIDE Chess), respectively. This assumes that the piece to which the Pawns will promote is of decisive value, so the opponent will have to sacrifice a piece to stop the promotion. Unfortunately, this does not work very well in Shatranj and Courier Chess, where Pawns promote to the worthless Ferz. Recent versions of Faiy-Max divide this bonus by 8 in cases where a "Queen" (the piece to which Pawns promote by default) is worth less than 250 centi-Pawn, to repair this problem. Nevertheless, the strategy of Fairy-Max is tailored too much to normal Chess, with normal Pawns to be a really good Shatranj or Courier player.

Equipment

Fairy-Max runs on a PC under any Windows operating system. The memory demand is determined by the size of its hash table, which by default is 96M. It would still run with a smaller hash table, but the larger the hash table, the better its play. Running with a hash table smaller than 6MB is not recommended. Recently also versions that can run on other platforms, such as Linux, were released.

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