CPG/Chess/nimfish/nimfishpkg/board.nim

# Copyright 2024 Mattia Giambirtone & All Contributors
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#    http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

## Implementation of a simple chessboard


import pieces
import magics
import moves
import rays
import bitboards
import position
import zobrist



export pieces, position, bitboards, moves, magics, rays, zobrist



type 
    Chessboard* = object
        ## A chessboard
        
        # The current position
        position*: Position
        # List of all previously reached positions
        positions*: seq[Position]


func toFEN*(self: Chessboard): string


proc newChessboard*: Chessboard =
    ## Returns a new, empty chessboard
    result.position = Position(enPassantSquare: nullSquare(), sideToMove: White)
    for i in Square(0)..Square(63):
        result.position.mailbox[i] = nullPiece()


proc newChessboardFromFEN*(fen: string): Chessboard =
    ## Initializes a chessboard with the
    ## position encoded by the given FEN string
    result = newChessboard()
    result.position = loadFEN(fen)


proc newDefaultChessboard*: Chessboard {.inline.} =
    ## Initializes a chessboard with the
    ## starting position
    return newChessboardFromFEN("rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1")


func inCheck*(self: Chessboard): bool {.inline.} =
    ## Returns if the current side to move is in check
    return self.position.inCheck()


proc canCastle*(self: Chessboard): tuple[queen, king: bool] {.inline.} =
    ## Returns if the current side to move can castle
    return self.position.canCastle()


func `$`*(self: Chessboard): string = $self.position


func pretty*(self: Chessboard): string =
    ## Returns a colored version of the
    ## current position for easier visualization
    return self.position.pretty()


func toFEN*(self: Chessboard): string =
    ## Returns a FEN string of the current
    ## position in the chessboard
    return self.position.toFEN()


func drawnByRepetition*(self: Chessboard): bool =
    ## Returns whether the current position is a draw
    ## by repetition
    # TODO: Improve this
    var i = self.positions.high()
    var count = 0
    while i >= 0:
        if self.position.zobristKey == self.positions[i].zobristKey:
            inc(count)
            if count == 2:
                return true
        if self.positions[i].halfMoveClock == 0:
            # Position was reached via a pawn move or
            # capture: cannot repeat beyond this point!
            return false
        dec(i)


proc isInsufficientMaterial*(self: Chessboard): bool =
    ## Returns whether the current position is drawn
    ## due to insufficient mating material. Note that
    ## this is not a strict implementation of the FIDE
    ## rule about material draws due to the fact that
    ## it would be basically impossible to implement those
    ## efficiently
    
    # Break out early if there's more than 4 pieces on the
    # board
    let occupancy = self.position.getOccupancy()
    if occupancy.countSquares() > 4:
        return false

    # KvK is a draw
    if occupancy.countSquares() == 2:
        # Only the two kings are left
        return true

    let
        sideToMove = self.position.sideToMove
        nonSideToMove = sideToMove.opposite()

    # Break out early if there's any pawns left on the board
    if self.position.getBitboard(Pawn, sideToMove) != 0:
        return false
    if self.position.getBitboard(Pawn, nonSideToMove) != 0:
        return false

    # If there's any queens or rooks on the board, break out early too
    let 
        friendlyQueens = self.position.getBitboard(Queen, sideToMove)
        enemyQueens = self.position.getBitboard(Queen, nonSideToMove)
        friendlyRooks = self.position.getBitboard(Rook, sideToMove)
        enemyRooks = self.position.getBitboard(Rook, nonSideToMove)
    
    if (friendlyQueens or enemyQueens or friendlyRooks or enemyRooks).countSquares() != 0:
        return false

    # KNvK is a draw

    let knightCount = (self.position.getBitboard(Knight, sideToMove) or self.position.getBitboard(Knight, nonSideToMove)).countSquares()

    # More than one knight (doesn't matter which side), not a draw
    if knightCount > 1:
        return false

    # KBvK is a draw
    let bishopCount = (self.position.getBitboard(Bishop, sideToMove) or self.position.getBitboard(Bishop, nonSideToMove)).countSquares()

    if bishopCount + knightCount > 1:
        return false

    # Maybe TODO: KBBvK and KBvKB (these should be handled by search anyway)

    return true



func isDrawn*(self: Chessboard): bool =
    ## Returns whether the given position is
    ## drawn
    if self.position.halfMoveClock >= 100:
        # Draw by 50 move rule
        return true

    if self.drawnByRepetition():
        return true

    if self.isInsufficientMaterial():
        return true