Version 1 release: major improvements, bug fixes and more

[genyrosk]

Version 1 of gym-chess is an almost complete rewrite of the original code with a bunch of improvements, bug-fixes and tests. The original implementation has been renamed V0, but further development or maintenance of it is not planned.

Environment settings

As chess is a 2-player game, you can choose to play against a random bot or against yourself (self-play).

• initial_state: initial board positions, the default value is the default chess starting board
• opponent: can be "random", "none" or a function. Tells the environment whether to use a random bot, play against self or use a specific bot policy (default: "random")
• log: True or False, specifies whether to log every move and render every new state (default: True)
• player_color: "WHITE" or "BLACK", only useful if playing against a bot

New move specification:

Moves are now specified as either:

• a tuple of coordinates ((from_x, from_y), (to_x, to_y))
• or a string e.g. "CASTLE_KING_SIDE_WHITE", "CASTLE_QUEEN_SIDE_BLACK", "RESIGN"

Moves are pre-calculated for every new state and stored in possible_moves.

A basic script would look like this:

env = ChessEnvV1(log=False)
moves = env.possible_moves
action = env.move_to_action(moves[0])
state, reward, done, info = env.step(action)
env.render()

You can also calculate the possible steps separately for every player:

white_moves = env.get_possible_moves(player="WHITE")
black_moves = env.get_possible_moves(player="BLACK")

State

The state is the board with pieces.

>>> print(env.state)

[[-3, -5, -4, -2, -1, -4, -5, -3],
 [-6, -6, -6, -6, -6, -6, -6, -6],
 [0, 0, 0, 0, 0, 0, 0, 0],
 [0, 0, 0, 0, 0, 0, 0, 0],
 [0, 0, 0, 0, 0, 0, 0, 0],
 [0, 0, 0, 0, 0, 0, 0, 0],
 [6, 6, 6, 6, 6, 6, 6, 6],
 [3, 5, 4, 2, 1, 4, 5, 3]]

It can be rendered in a prettier way with the render() method:

>>> env.render()

    -------------------------
 8 |  ♖  ♘  ♗  ♕  ♔  ♗  ♘  ♖ |
 7 |  ♙  ♙  ♙  ♙  ♙  ♙  ♙  ♙ |
 6 |  .  .  .  .  .  .  .  . |
 5 |  .  .  .  .  .  .  .  . |
 4 |  .  .  .  .  .  .  .  . |
 3 |  .  .  .  .  .  .  .  . |
 2 |  ♟  ♟  ♟  ♟  ♟  ♟  ♟  ♟ |
 1 |  ♜  ♞  ♝  ♛  ♚  ♝  ♞  ♜ |
    -------------------------
      a  b  c  d  e  f  g  h

Every integer represents a piece. Positive pieces are white and negative ones are black.

Piece IDs are stored in constants that can be imported.

from gym_chess.envs.chess_v1 import (
    KING_ID,
    QUEEN_ID,
    ROOK_ID,
    BISHOP_ID,
    KNIGHT_ID,
    PAWN_ID,
)

The schema is:

EMPTY_SQUARE_ID = 0
KING_ID = 1
QUEEN_ID = 2
ROOK_ID = 3
BISHOP_ID = 4
KNIGHT_ID = 5
PAWN_ID = 6

Additional information can be found in other attributes of the environment:

env.current_player
env.white_king_castle_possible
env.white_queen_castle_possible
env.black_king_castle_possible
env.black_queen_castle_possible
env.white_king_on_the_board
env.black_king_on_the_board

Other features

• The environment keeps track of all past states and can detect 3-fold repetitions, at which point the game is a draw.
• Render method uses background colors to better visualize the move

Code linting and fixing

Code fixing is done with black with max line width of 100 characters with the command black -l 100 . No config needed.

Notes:

En-passant moves are not currently supported in the V1 environment.