Initial work on Chess
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# Copyright 2023 Mattia Giambirtone & All Contributors
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#
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# Licensed under the Apache License, Version 2.0 (the "License");
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# you may not use this file except in compliance with the License.
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# You may obtain a copy of the License at
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#
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# http://www.apache.org/licenses/LICENSE-2.0
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#
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# Unless required by applicable law or agreed to in writing, software
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# distributed under the License is distributed on an "AS IS" BASIS,
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# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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# See the License for the specific language governing permissions and
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# limitations under the License.
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import ../util/matrix
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import std/strutils
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type
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PieceColor* = enum
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None = 0,
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White,
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Black
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PieceKind* = enum
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Empty = 0, # No piece
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Bishop = 'b',
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King = 'k'
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Knight = 'n',
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Pawn = 'p',
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Queen = 'q',
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Rook = 'r',
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Piece* = object
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color*: PieceColor
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kind*: PieceKind
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Position* = object
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piece*: Piece
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location*: tuple[row, col: int]
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Board* = ref object
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grid: Matrix[Piece]
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ply: PieceColor
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# Number of half moves since
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# last piece capture. Used
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# for the 50-move rule
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halfMoveClock: int
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# Full move counter. Increments
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# every 2 ply
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fullMoveCount: int
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# Stores metadata for castling.
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castling: tuple[white, black: tuple[queen, king: bool]]
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# En passant target square (see https://en.wikipedia.org/wiki/En_passant)
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# If en passant is not possible, both the row and
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# column of the position will be set to -1
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enPassantSquare: Position
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# Initialized only once, copied every time
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var empty: seq[Piece] = @[]
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for _ in countup(0, 63):
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empty.add(Piece(kind: Empty, color: None))
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proc algebraicToPosition(s: string): tuple[row, col: int] {.inline.} =
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## Converts a square location from algebraic
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## notation to its corresponding row and column
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## in the chess grid (0 indexed)
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assert len(s) == 2
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result = (int(uint8(s[0]) - (uint8('a') - 1)), int(uint8(s[1]) - uint8('0')))
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proc newChessboardFromFEN*(state: string): Board =
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## Initializes a chessboard with the
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## state encoded by the given FEN string
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new(result)
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# Turns our flat sequence into an 8x8 grid
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result.grid = newMatrixFromSeq[Piece](empty, (8, 8))
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var
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# Current location in the grid
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row = 0
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column = 0
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# Current section in the FEN string
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section = 0
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# Current index into the FEN string
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index = 0
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# See https://en.wikipedia.org/wiki/Forsyth%E2%80%93Edwards_Notation
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while index <= state.high():
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var c = state[index]
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if c == ' ':
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# Next section
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inc(section)
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inc(index)
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continue
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case section:
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of 0:
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# Piece placement data
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case c.toLowerAscii():
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of 'r', 'n', 'b', 'q', 'k', 'p':
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result.grid[row, column] = Piece(kind: PieceKind(c.toLowerAscii()), color: if c.isUpperAscii(): White else: Black)
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inc(column)
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of '/':
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inc(row)
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column = 0
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of '0'..'9':
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let x = int(uint8(c) - uint8('0')) - 1
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if x > 7:
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raise newException(ValueError, "invalid skip value (> 8) in FEN string")
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column += x
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else:
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raise newException(ValueError, "invalid piece identifier in FEN string")
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of 1:
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# Active color
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case c:
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of 'w':
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result.ply = White
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of 'b':
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result.ply = Black
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else:
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raise newException(ValueError, "invalid next move identifier in FEN string")
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of 2:
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# Castling availability
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case c:
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of '-':
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# Neither side can castle anywhere: do nothing,
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# as the castling metadata is set to this state
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# by default
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discard
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of 'K':
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result.castling.white.king = true
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of 'Q':
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result.castling.white.queen = true
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of 'k':
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result.castling.black.king = true
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of 'q':
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result.castling.black.queen = true
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else:
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raise newException(ValueError, "invalid castling availability in FEN string")
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of 3:
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# En passant target square
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case c:
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of '-':
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result.enPassantSquare.location = (-1, -1)
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else:
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result.enPassantSquare.location = state[index..index+1].algebraicToPosition()
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# Just for cleanliness purposes, we fill in the other positional metadata as
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# well
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result.enPassantSquare.piece.color = if result.ply == Black: White else: Black
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result.enPassantSquare.piece.kind = Pawn
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# Square metadata is 2 bytes long
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inc(index)
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of 4:
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# Halfmove clock
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var s = ""
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while not state[index].isSpaceAscii():
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s.add(state[index])
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inc(index)
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# Backtrack so the space is seen by the
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# next iteration of the loop
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dec(index)
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result.halfMoveClock = parseInt(s)
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of 5:
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# Fullmove number
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var s = ""
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while index <= state.high():
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s.add(state[index])
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inc(index)
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result.fullMoveCount = parseInt(s)
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else:
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raise newException(ValueError, "too many fields in FEN string")
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inc(index)
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@ -0,0 +1,5 @@
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import board
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echo newChessboardFromFEN("rnbqkbnr/pppppppp/8/8/4P3/8/PPPP1PPP/RNBQKBNR b KQkq e3 0 1")[]
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