#chess #chess-board #chess-engine #ataxx #board-game #movegen

bin+lib monster_chess

A fairy chess movegen library that can be easily extended to new chess-adjacent games

24 releases

0.0.24 Dec 12, 2023
0.0.23 Dec 11, 2023
0.0.18 Mar 29, 2023

#25 in Games

Used in monster_ugi

MIT license




monster-chess is a fairy chess move generation library written in Rust. It was created as part of the Chesstastic project (which aims to allow for users to easily create chess variants, play against others in those chess variants and analyze games of said variants), or more specifically, the Ampersand chess engine. The library is meant to handle move generation and move validation logic that happens in chess variants and chess-adjacent games.


monster-chess is a library that can be installed as a cargo crate. You can install it as follows:

cargo add monster_chess

Then, you can import the library as follows:

use monster_chess::{games::{chess::Chess, ataxx::Ataxx}, board::game::NORMAL_MODE};

You can create a game of your choosing as follows:

let game = Chess::create();
// OR
let game = Ataxx::create();
// OR
let game = MyCustomGame::create();

You can load positions either from the default FEN, or from a FEN of your own choosing.

let board = game.default();
// OR
let board = game.from_fen("r1bqkb1r/pppp1ppp/2n2n2/4p2Q/2B1P3/8/PPPP1PPP/RNB1K1NR w KQkq - 0 1");

You can generate legal moves as follows (you may want to generate somewhat differently for pseudolegal move generation of some games.)

// `NORMAL_MODE` is the normal move generation of any game, other move generations generate specialized moves.
let moves = board.generate_legal_moves(NORMAL_MODE);

You can create a Zobrist hash of a position as follows:

let hash = game.zobrist.compute(board);


When we say that we're compatible with a given game, that doesn't mean we'll necessarily be providing out-of-the-box support for it, but that the game will be implementable given the framework that monster-chess provides.

Types of games that we're aiming to be compatible with:

Types of games we may aim to be compatible with:

Types of games we're not aiming to be compatible with:

However, the following games will be supported out of the box:

If you're wondering if a given game or chess variant is compatible with chess, imagine starting with the base game of chess, and see if you can do any of the following to get to your variant.

  • Can I increase/decrease the board size?
  • Can I remove existing pieces, or add new pieces with new types of movement?
  • Can I change the capture style of pieces (eg. swapping positions with the piece instead of capturing it.)
  • Can I change the movement restrictions? (eg. being allowed to move into check)
  • Can I change the win conditions? (eg. being able to win by capturing all pieces.)

If so, monster-chess most likely will be able to be compatible with said variant, but you may have to add it.

Out of the Box Support


Chess is a two-player game where both players start with a color and the same set of pieces on an eight by eight board, and must engage in a fruitful battle until one side is victorious. For the sake of brevity, we'll assume you're well-acquainted with the rules of chess (if not, feel free to read about them here), and simply focus on the parts relevant to monster-chess.

Ladder Checkmate

In chess, the end-goal of the game is checkmate, where the opponent's king is in check (in the line of sight of another piece), and has no moves that can escape said check (moving out of the way, blocking with another piece, etc.) The image above demonstrates this, with two rooks blocking all of the king's movement squares, and the opponent's king is simply unable to move out of the line of sight of the rooks. If the opponent is in check but can escape it, they must, and you cannot put yourself in check.

The way monster-chess handles this is using pseudolegal move generation. First, we generate all of the moves any piece would be able to make (aka. pseudolegal moves), and then we check to make sure your king isn't in check after you perform the move. To do this, we have to generate every possible move that the opponent can make after your move, and see if one of them allows for the king to be captured. This is expensive, but we have two ways to make this faster.

  • For pieces that move and capture differently, we only focus on the squares they capture.
  • For pieces that move in one direction until they hit another piece or the edge of the board (bishops, rooks, and queens), we check if the king is even in the line of sight of the piece before seeing if it's truly in the line of sight.

Despite all of that, this legality check is rather expensive. Fortunately, it won't matter much if you're building a chess engine, as in any given position, you'll only be searching a fraction of the moves. This means, instead of performing the legality check for every move in the position, if you search 3 moves and find an instant hit, you won't need to check the rest.

You can initialize the chess board as follows:

use monster_chess::games::chess::Chess;

let chess = Chess::create();
let mut board = chess.from_fen("rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1");

Then, we can generate the moves as follows:

let legal_moves = board.generate_legal_moves(NORMAL_MODE);
let pseudolegal_moves = board.generate_moves(NORMAL_MODE);

For testing and benchmarking purposes, monster-chess provides a method named perft, which will count the number of all possible moves possible that are depth half-moves ahead from the position (with a half-move being a move from one of the two players for reference.)

let perft = board.perft(5, true);
let perft_pseudolegal = board.perft(5, false);

From the benchmarks I've done, monster-chess can reach about 20,000,000 pseudo-legal moves per second, and 5,000,000 legal moves per second. This isn't ideal and if you're only interested in performance, I recommend using the cozy-chess crate which is at least 25x faster then the implementation of chess in monster-chess. However, monster-chess is a sound option for chess given you also want the ability to support chess variants or even other games.

It may be noted that monster-chess also aims to support Fischer Random Chess. As of now, Fischer Random Chess is theoretically supported in the implementation of monster-chess's Chess implementation, but as of now, it isn't tested, and FENs for the variant aren't supported. It would be trivial to add it in the framework of monster-chess as an extension of the existing Chess implementation, though.


Ataxx is a two-player game where both players start with a single stone on a seven by seven board, and must fight for who will end up controlling the most territory. The game is mainly known for how much the board can change in one move; positions are generally not tactically stable.

Ataxx Start Position

monster-chess can easily implement Ataxx simply by writing an implementation of StonePiece to represent an Ataxx piece. Moves can be found using a lookup table and then masked to avoid all of the pieces on the board. As for making the moves, all we have to do is check whether it's a single or double move, and then handle the move-making logic appropiately before converting stones around the moved piece.

You can still create an Ataxx board as follows:

use monster_chess::games::chess::Ataxx;

let ataxx = Ataxx::create();
let mut board = ataxx.from_fen("x5o/7/7/7/7/7/o5x x 0 1");

Then, we can generate the moves as follows:

let legal_moves = board.generate_legal_moves(NORMAL_MODE);
let pseudolegal_moves = board.generate_moves(NORMAL_MODE);

As the FEN Notation for Ataxx could be somewhat difficult to find, we follow this format: [piece placement] [moving team] [half moves] [full moves].

For piece placement, x is a piece from team one, black, and y is a piece from team two, white. In addition, - represents gaps, holes in the board. For the moving team, x is black, y is white.

For moves that the engine parses, single moves must be represented by only providing the destination square, and double moves are represented as [from][to].



monster-chess uses a general implementation of Bitboards to extend to larger board sizes, using a custom made BitBoard data type.

pub struct BitBoard<const T : usize> {
    pub data: [ u128; T ]

BitBoards are designed in such a way that if the BitBoard only has to store one u128, it would (ideally) be optimized to be as fast as it would natively. BitBoards support all bitwise operators, alongside addition, subtraction, and two custom methods for bitscans.

The library generates movement for pieces by taking in a bitboard with one toggled bit representing where the piece currently is, and applying bitwise operations to it. In native chess, there are three main types of pieces.

  • Delta Pieces, like the Knight and King, which move in the same way every time, no matter which square they are on (except out of bounds squares.)
  • Slider Pieces, like the Bishop, Rook, and Queen, which move almost in the same way every time looking in a certain direction, but can be blocked by pieces.
  • Pawns.

To optimize move generation for Delta Pieces and Slider Pieces, monster-chess provides an attack lookup table. Once a board is initialized, if a piece chooses to enable attack table lookups for speeding up move generation, its moves will be generated for every possible square it can go to. This means for kings and knights, move generation is effectively instant, only requiring an attack table lookup. For rooks, bishops, and queens, the attack table is used to retrieve the directions they can move in from any given square, but additional logic is needed to block out any pieces in the way.

Attack table lookups are stored as an AttackDirections once retrieved, which is an alias for Vec<BitBoard>. This means for delta pieces, they can just store their moves in the first slot, but for sliding pieces, they can store a bitboard for each direction they can go in.

Here's an example of what the King piece would look like with monster-chess's piece system.

impl Piece for KingPiece {
    // get_piece_symbol will show the symbol used for a given piece in FEN notation
    fn get_piece_symbol(&self) -> &str {

    // generate_lookup_moves will generate moves for attack tables
    fn generate_lookup_moves(&self, board: &Board, mut from: BitBoard) -> AttackDirections {
        let moves = ...;
        vec![ moves ]

    // can_lookup tells us if we should generate an attack table beforehand
    fn can_lookup(&self) -> bool {

    // get_moves will fetch the moves for move generation itself
    fn get_moves(&self, board: &Board, from: BitBoard, team: u32) -> BitBoard {
        let lookup = self.get_attack_lookup(board);
        match lookup {
            Some(lookup) => lookup[from.bitscan_reverse() as usize][0],
            None => self.generate_moves(board, from)[0]

FEN Representation

Board State

Because monster-chess is aiming to support all chess variants, a general modification of FEN is used for this (with a specific version of FEN for the base game of chess itself.) This version of FEN for the board state itself is the same as the typical chess FEN, with the following additions:

  • The FEN variant can support more or less than 8 rows and 8 columns.
  • The FEN variant can support custom pieces, by defining the custom piece symbol (eg. A for Archbishops) in the piece's get_piece_symbol method.
  • Additional information can be specified for individual pieces.
    • There are two ways to specify the piece type and the team, depending on the individual piece.
      • PieceSymbol::Char defines the piece as a single char (eg. p.) If there are two teams, P will represent the first team (team 0), and p will represent the second team (team 1.) If there are more than two teams, the teams will be represented with braces after the piece. (eg. p{2} for team three.)
      • PieceSymbol::Teams changes what char is used for the piece depending on the team. (eg. x for player one, o for player two.)
    • If a game supports first move notation, then if the ! marker follows a piece (eg. p!), that piece has moved at least once already. This is a general way to handle things like first pawn moves and castling rights.

For instance, p!{3} is a pawn that has moved once before on the fourth team. (We're using zero as the first index, much like arrays do in programming.)

There's only one option for FEN States, and that's first_move. In most games, first moves don't have any impact on the game state, or the FEN representation has other, more concise ways to represent first moves (eg. in chess, pawn first moves are detected based on if pawns are on the 2nd or 6th ranks.)

Fen Arguments

FEN Notation for games like chess also have additional information provided that isn't in the board state representation itself. For instance, the en passant square, or castling rights, or the side to move. monster-chess does not support these natively as part of the Board implementation. Instead, individual games have to manage the additional arguments for their respective FEN notations themselves, by implementing the FenArgument trait.

FenArgument provides two main methods, encode, and decode.

pub trait FenArgument {
    /// `encode` takes in a board, and outputs what this FEN argument's encoded result would be (eg. for a team argument, it could be `"b"`)
    fn encode(&self, board: &Board) -> String;

    /// `decode` takes in a board and an existing argument, and will modify the board to meet the argument (eg. changing the team to reflect the given arg team of `w`)
    fn decode(&self, board: &mut Board, arg: &str) -> Result<(), FenDecodeError>;

monster-chess provides an implementation of one argument for you, which is FenTeamArgument, defined like this:

pub enum FenTeamArgument {

If your game needs an argument to represent which side has to move (which it almost certainly does), using FenTeamArgument is necessary, unless you decide to define your own argument representing which side has to move.

monster-chess also provides implementations for Turns, SubMoves (half moves), and FullMoves out of the box.


monster-chess finally provides a struct called Game, which is used to describe the rules of your chess-adjacent game. It would be declared as follows:

pub struct Game<const T: usize> {
    pub pieces: Vec<&'static dyn Piece<T>>,
    pub controller: Box<dyn MoveController<T>>,
    pub resolution: Box<dyn Resolution<T>>,
    pub fen_options: FenOptions<T>

Note: For games like Go, where you're able to drop pieces onto squares, you'll need to handle that manually, by implementing an add_moves method on MoveController to generate drop moves (moves with from as None), and implementing a make_drop_move method on MoveController for making those moves.


monster-chess available under the MIT license. See LICENSE for the full license text.


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