CPG/src/Chess/board.nim

# Copyright 2023 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.
import ../util/matrix

export matrix

import std/strutils
import std/strformat


type
    # Useful type aliases
    Location = tuple[row, col: int]

    Pieces = tuple[king: Location, queen: Location, rooks: seq[Location],
                   bishops: seq[Location], knights: seq[Location],
                   pawns: seq[Location]]

    PieceColor* = enum
        ## A piece color enumeration
        None = 0,
        White, 
        Black

    PieceKind* = enum
        ## A chess piece enumeration
        Empty = '\0',    # No piece
        Bishop = 'b', 
        King = 'k'
        Knight = 'n',
        Pawn = 'p', 
        Queen = 'q',
        Rook = 'r', 

    Piece* = object
        ## A chess piece
        color*: PieceColor
        kind*: PieceKind
            

    Move* = object
        ## A chess move
        piece*: Piece
        startSquare*: Location
        targetSquare*: Location

    ChessBoard* = ref object
        ## A chess board object
        grid: Matrix[Piece]
        # Currently active color
        turn: PieceColor
        # Number of half moves since
        # last piece capture or pawn movement.
        # Used for the 50-move rule
        halfMoveClock: int
        # Full move counter. Increments
        # every 2 ply
        fullMoveCount: int
        # Stores metadata for castling.
        castling: tuple[white, black: tuple[queen, king: bool]]
        # En passant target square (see https://en.wikipedia.org/wiki/En_passant)
        # If en passant is not possible, both the row and
        # column of the position will be set to -1
        enPassantSquare: Move
        # Locations of all pieces
        pieces: tuple[white: Pieces, black: Pieces]
        # Locations of all attacked squares and their 
        # respective attackers
        attacked: tuple[white: seq[tuple[attacker: Piece, loc: Location]], black: seq[tuple[attacker: Piece, loc: Location]]]


# Initialized only once, copied every time
var empty: seq[Piece] = @[]
for _ in countup(0, 63):
    empty.add(Piece(kind: Empty, color: None))


func emptyPiece*: Piece {.inline.} = Piece(kind: Empty, color: None)
func emptyLocation*: Location {.inline.} = (-1 , -1)
func emptyMove*: Move {.inline.} = Move(startSquare: emptyLocation(), targetSquare: emptyLocation(), piece: emptyPiece())
func opposite*(c: PieceColor): PieceColor {.inline.} = (if c == White: Black else: White)
proc algebraicToPosition*(s: string): Location {.inline.}
func `==`(a, b: Location): bool {.inline.} =  a.row == b.row and a.col == b.col


proc newChessboard: ChessBoard =
    ## Returns a new, empty chessboard
    new(result)
    # Turns our flat sequence into an 8x8 grid
    result.grid = newMatrixFromSeq[Piece](empty, (8, 8))
    result.attacked = (@[], @[])
    result.enPassantSquare = emptyMove()
    result.turn = White


proc newChessboardFromFEN*(state: string): ChessBoard =
    ## Initializes a chessboard with the
    ## state encoded by the given FEN string
    result = newChessboard()
    var
        # Current location in the grid
        row = 0
        column = 0
        # Current section in the FEN string
        section = 0
        # Current index into the FEN string
        index = 0
        # Temporary variable to store the piece
        piece: Piece
    # See https://en.wikipedia.org/wiki/Forsyth%E2%80%93Edwards_Notation
    while index <= state.high():
        var c = state[index]
        if c == ' ':
            # Next section
            inc(section)
            inc(index)
            continue
        case section:
            of 0:
                # Piece placement data
                case c.toLowerAscii():
                    # Piece
                    of 'r', 'n', 'b', 'q', 'k', 'p':
                        # We know for a fact these values are in our 
                        # enumeration, so all is good
                        {.push.}
                        {.warning[HoleEnumConv]:off.}
                        piece = Piece(kind: PieceKind(c.toLowerAscii()), color: if c.isUpperAscii(): White else: Black)
                        {.pop.}
                        case piece.color:
                            of Black:
                                case piece.kind:
                                    of Pawn:
                                        result.pieces.black.pawns.add((row, column))
                                    of Bishop:
                                        result.pieces.black.bishops.add((row, column))
                                    of Knight:
                                        result.pieces.black.knights.add((row, column))
                                    of Rook:
                                        result.pieces.black.rooks.add((row, column))
                                    of Queen:
                                        result.pieces.black.queen = (row, column)
                                    of King:
                                        result.pieces.black.king = (row, column)
                                    else:
                                        discard
                            of White:
                                case piece.kind:
                                    of Pawn:
                                        result.pieces.white.pawns.add((row, column))
                                    of Bishop:
                                        result.pieces.white.bishops.add((row, column))
                                    of Knight:
                                        result.pieces.white.knights.add((row, column))
                                    of Rook:
                                        result.pieces.white.rooks.add((row, column))
                                    of Queen:
                                        result.pieces.white.queen = (row, column)
                                    of King:
                                        result.pieces.white.king = (row, column)
                                    else:
                                        discard
                            else:
                                discard
                        result.grid[row, column] = piece
                        inc(column)
                    of '/':
                        # Next row
                        inc(row)
                        column = 0
                    of '0'..'9':
                        # Skip x columns
                        let x = int(uint8(c) - uint8('0')) - 1
                        if x > 7:
                            raise newException(ValueError, "invalid skip value (> 8) in FEN string")
                        column += x
                    else:
                        raise newException(ValueError, "invalid piece identifier in FEN string")
            of 1:
                # Active color
                case c:
                    of 'w':
                        result.turn = White
                    of 'b':
                        result.turn = Black
                    else:
                        raise newException(ValueError, "invalid active color identifier in FEN string")
            of 2:
                # Castling availability
                case c:
                    of '-':
                        # Neither side can castle anywhere: do nothing,
                        # as the castling metadata is set to this state
                        # by default
                        discard
                    of 'K':
                        result.castling.white.king = true
                    of 'Q':
                        result.castling.white.queen = true
                    of 'k':
                        result.castling.black.king = true
                    of 'q':
                        result.castling.black.queen = true
                    else:
                        raise newException(ValueError, "invalid castling availability in FEN string")
            of 3:
                # En passant target square
                case c:
                    of '-':
                        # Field is already uninitialized to the correct state
                        discard
                    else:
                        result.enPassantSquare.targetSquare = state[index..index+1].algebraicToPosition()
                        # Just for cleanliness purposes, we fill in the other positional metadata as
                        # well
                        result.enPassantSquare.piece.color = if result.turn == Black: White else: Black
                        result.enPassantSquare.piece.kind = Pawn
                        # Square metadata is 2 bytes long
                        inc(index)
            of 4:
                # Halfmove clock
                var s = ""
                while not state[index].isSpaceAscii():
                    s.add(state[index])
                    inc(index)
                # Backtrack so the space is seen by the
                # next iteration of the loop
                dec(index)
                result.halfMoveClock = parseInt(s)
            of 5:
                # Fullmove number
                var s = ""
                while index <= state.high():
                    s.add(state[index])
                    inc(index)
                result.fullMoveCount = parseInt(s)
            else:
                raise newException(ValueError, "too many fields in FEN string")
        inc(index)


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


proc countPieces*(self: ChessBoard, kind: PieceKind, color: PieceColor): int =
    ## Counts the number of pieces with
    ## the given color and type
    case color:
        of White:
            case kind:
                of Pawn:
                    return self.pieces.white.pawns.len()
                of Bishop:
                    return self.pieces.white.bishops.len()
                of Knight:
                    return self.pieces.white.knights.len()
                of Rook:
                    return self.pieces.white.rooks.len()
                of Queen:
                    return int(self.pieces.white.queen != (-1, -1))
                of King:
                    # There shall be only one, forever
                    return 1
                else:
                    discard
        of Black:
            case kind:
                of Pawn:
                    return self.pieces.black.pawns.len()
                of Bishop:
                    return self.pieces.black.bishops.len()
                of Knight:
                    return self.pieces.black.knights.len()
                of Rook:
                    return self.pieces.black.rooks.len()
                of Queen:
                    return int(self.pieces.black.queen != emptyLocation())
                of King:
                    return 1
                else:
                    discard        
        of None:
            raise newException(ValueError, "invalid piece type")


proc countPieces*(self: ChessBoard, piece: Piece): int =
    ## Returns the number of pieces on the board that
    ## are of the same type and color of the given piece
    return self.countPieces(piece.kind, piece.color)


func rankToColumn(rank: int): int =
    ## Converts a chess rank (1-indexed) 
    ## into a 0-indexed column value for our 
    ## board. This converter is necessary because
    ## chess positions are indexed differently with
    ## respect to our internal representation
    const indeces = [7, 6, 5, 4, 3, 2, 1, 0]
    return indeces[rank - 1]


proc algebraicToPosition*(s: string): Location {.inline.} =
    ## Converts a square location from algebraic
    ## notation to its corresponding row and column
    ## in the chess grid (0 indexed)
    if len(s) != 2:
        raise newException(ValueError, "algebraic position must be of length 2")

    var s = s.toLowerAscii()
    if s[0] notin 'a'..'h':
        raise newException(ValueError, &"algebraic position has invalid first character ('{s[0]}')")
    if s[1] notin '1'..'8':
        raise newException(ValueError, &"algebraic position has invalid second character ('{s[1]}')")

    let file = int(uint8(s[0]) - uint8('a'))
    # Convert the rank character to a number
    let rank = rankToColumn(int(uint8(s[1]) - uint8('0')))
    return (rank, file)


proc getPiece*(self: ChessBoard, square: string): Piece =
    ## Gets the piece on the given square
    ## in algebraic notation
    let loc = square.algebraicToPosition()
    return self.grid[loc.row, loc.col]


proc isCapture*(self: ChessBoard, move: Move): bool {.inline.} =
    ## Returns whether the given move is a capture
    ## or not
    let target = self.grid[move.targetSquare.row, move.targetSquare.col]
    return target.color == move.piece.color.opposite()


proc testMoveOffsets(self: ChessBoard, move: Move): bool =
    ## Returns true if the piece in the given
    ## move is allowed to move in the direction
    ## specified. This does not take pins nor checks
    ## into account, but other rules like double pawn
    ## pushes and en passant are validated here. Note
    ## that this is an internal method called by checkMove
    ## and it does not validate whether the target square
    ## is occupied or not (it is assumed the check has been
    ## performed beforehand, like checkMove does)
    case move.piece.kind:
        of Pawn:
            if move.targetSquare.col != move.startSquare.col:
                # Pawn can only change column in case of capture or en passant
                if self.enPassantSquare == emptyMove():
                    # No en passant possible, only possibility
                    # is a capture
                    return self.isCapture(move)
                # En passant is possible, check if the destination is
                # its target square
                elif self.enPassantSquare.targetSquare != move.targetSquare:
                    # We still need to check for captures even if en passant
                    # is possible
                    return self.isCapture(move)
            # Number of rows traveled
            var rows: int
            # Due to our unique board layout, we need to do this nonsense
            if move.piece.color == White:
                rows = move.startSquare.row - move.targetSquare.row
            else:
                rows = move.targetSquare.row - move.startSquare.row
            if rows < 0 or rows > 2:
                # Pawns don't go backwards, I'm afraid. They also can't
                # go any further than 2 squares
                return false
            if rows == 2:
                # Check if double pawn pushing is possible (only the first
                # move)
                let startRow = if move.piece.color == White: 6 else: 1
                if move.startSquare.row != startRow:
                    # Pawn has already moved more than once, double push
                    # is not allowed
                    return false
            return true
        else:
            return false


proc updateAttackedSquares(self: ChessBoard) =
    ## Updates internal metadata about which squares
    ## are attacked. Called internally by doMove
    
    # Go over each piece one by one and see which squares
    # it currently attacks
    for piece in self.pieces.white.pawns:
        discard


proc removePiece(self: ChessBoard, location: Location) =
    ## Removes a piece from the board, updating necessary
    ## metadata
    var piece = self.grid[location.row, location.col]
    case piece.color:
        of White:
            case piece.kind:
                of Pawn:
                    self.pieces.white.pawns.delete(self.pieces.white.pawns.find(location))
                of Bishop:
                    self.pieces.white.pawns.delete(self.pieces.white.bishops.find(location))
                of Knight:
                    self.pieces.white.pawns.delete(self.pieces.white.knights.find(location))
                of Rook:
                    self.pieces.white.rooks.delete(self.pieces.white.rooks.find(location))
                of Queen:
                    self.pieces.white.queen = emptyLocation()
                of King:
                    doAssert false, "removePiece: attempted to remove the white king"
                else:
                    discard 
        of Black:
            case piece.kind:
                of Pawn:
                    self.pieces.black.pawns.delete(self.pieces.black.pawns.find(location))
                of Bishop:
                    self.pieces.black.pawns.delete(self.pieces.black.bishops.find(location))
                of Knight:
                    self.pieces.black.pawns.delete(self.pieces.black.knights.find(location))
                of Rook:
                    self.pieces.black.rooks.delete(self.pieces.black.rooks.find(location))
                of Queen:
                    self.pieces.black.queen = emptyLocation()
                of King:
                    doAssert false, "removePiece: attempted to remove the black king"
                else:
                    discard
        else:
            discard


proc updatePositions(self: ChessBoard, move: Move) =
    ## Internal helper to update the position of
    ## the pieces on the board after a move
    
    # Empty out the starting square
    self.grid[move.startSquare.row, move.startSquare.col] = emptyPiece()
    if self.isCapture(move):
        # Move has captured a piece: remove the destination square' piece as well.
        # We call a helper instead of doing it ourselves because there's a bunch
        # of metadata that needs to be updated to do this properly and I thought
        # it'd fit into its neat little function
        self.removePiece(move.targetSquare)
    # Update the positional metadata of the moving piece
    case move.piece.color:
        of White:
            case move.piece.kind:
                of Pawn:
                    # The way things are structured, we don't care about the order
                    # of this list, so we can add and remove entries as we please
                    self.pieces.white.pawns.delete(self.pieces.white.pawns.find(move.startSquare))
                    self.pieces.white.pawns.add(move.targetSquare)
                of Bishop:
                    self.pieces.white.bishops.delete(self.pieces.white.bishops.find(move.startSquare))
                    self.pieces.white.bishops.add(move.targetSquare)
                of Knight:
                    self.pieces.white.knights.delete(self.pieces.white.knights.find(move.startSquare))
                    self.pieces.white.knights.add(move.targetSquare)
                of Rook:
                    self.pieces.white.rooks.delete(self.pieces.white.rooks.find(move.startSquare))
                    self.pieces.white.rooks.add(move.targetSquare)
                of Queen:
                    self.pieces.white.queen = move.targetSquare
                of King:
                    self.pieces.white.king = move.targetSquare
                else:
                    discard 
        of Black:
            case move.piece.kind:
                of Pawn:
                    self.pieces.black.pawns.delete(self.pieces.black.pawns.find(move.startSquare))
                    self.pieces.black.pawns.add(move.targetSquare)
                of Bishop:
                    self.pieces.black.bishops.delete(self.pieces.black.bishops.find(move.startSquare))
                    self.pieces.black.bishops.add(move.targetSquare)
                of Knight:
                    self.pieces.black.knights.delete(self.pieces.black.knights.find(move.startSquare))
                    self.pieces.black.knights.add(move.targetSquare)
                of Rook:
                    self.pieces.black.rooks.delete(self.pieces.black.rooks.find(move.startSquare))
                    self.pieces.black.rooks.add(move.targetSquare)
                of Queen:
                    self.pieces.black.queen = move.targetSquare
                of King:
                    self.pieces.black.king = move.targetSquare
                else:
                    discard
        else:
            discard
    # Actually move the piece
    self.grid[move.targetSquare.row, move.targetSquare.col] = move.piece


proc doMove(self: ChessBoard, move: Move) =
    ## Internal function called by makeMove after
    ## performing legality checks on the given move. Can
    ## be used in performance-critical paths where
    ## a move is already known to be legal
    self.updatePositions(move)    
    self.updateAttackedSquares()
    self.turn = self.turn.opposite()


proc checkMove(self: ChessBoard, startSquare, targetSquare: string): Move =
    ## Internal function called by makeMove to check a move for legality
    var pieceToMove = self.getPiece(startSquare)
    # Start square doesn't contain a piece or it is of the wrong color for which
    # turn it is to move
    if pieceToMove.kind == Empty or pieceToMove.color != self.turn:
        return emptyMove()
    var destination = self.getPiece(targetSquare)
    # Destination square is occupied by a piece of the same color as the piece
    # being moved: illegal!
    if destination.kind != Empty and destination.color == self.turn:
        return emptyMove()
    var 
        startLocation = startSquare.algebraicToPosition()
        targetLocation = targetSquare.algebraicToPosition()    
    result = Move(startSquare: startLocation, targetSquare: targetLocation, piece: pieceToMove)
    if not self.testMoveOffsets(result):
        # Piece cannot move in this direction
        return emptyMove()
    # TODO: Check for checks and pins (moves are currently pseudo-legal)



proc makeMove*(self: ChessBoard, startSquare, targetSquare: string): Move =
    ## Makes a move on the board from the chosen start square to
    ## the chosen target square, ensuring it is legal (turns are
    ## taken into account). This function returns a Move object: if the move
    ## is legal and has been performed, the fields will be populated properly.
    ## For efficiency purposes, no exceptions are raised if the move is 
    ## illegal, but the move's piece kind will be Empty (its color will be None
    ## too) and the locations will both be set to the tuple (-1, -1)
    result = self.checkMove(startSquare, targetSquare)
    if result == emptyMove():
        return
    self.doMove(result)



proc `$`*(self: ChessBoard): string =
    result &= "- - - - - - - -"
    for i, row in self.grid:
        result &= "\n"
        for piece in row:
            if piece.kind == Empty:
                result &= "  "
                continue
            if piece.color == White:
                result &= &"{char(piece.kind).toUpperAscii()} "
            else:
                result &= &"{char(piece.kind)} "
        result &= &"{rankToColumn(i + 1) + 1}"
    result &= "\n- - - - - - - -"
    result &= "\na b c d e f g h"



when isMainModule:
    proc testPiece(piece: Piece, kind: PieceKind, color: PieceColor) =
        doAssert piece.kind == kind and piece.color == color, &"expected piece of kind {kind} and color {color}, got {piece.kind} / {piece.color} instead"

    proc testPieceCount(board: ChessBoard, kind: PieceKind, color: PieceColor, count: int) =
        let pieces = board.countPieces(kind, color)
        doAssert pieces == count, &"expected {count} pieces of kind {kind} and color {color}, got {pieces} instead"

    echo "Running tests"
    var b = newDefaultChessboard()
    # Ensure correct number of pieces
    testPieceCount(b, Pawn, White, 8)
    testPieceCount(b, Pawn, Black, 8)
    testPieceCount(b, Knight, White, 2)
    testPieceCount(b, Knight, Black, 2)
    testPieceCount(b, Bishop, White, 2)
    testPieceCount(b, Bishop, Black, 2)
    testPieceCount(b, Rook, White, 2)
    testPieceCount(b, Rook, Black, 2)
    testPieceCount(b, Queen, White, 1)
    testPieceCount(b, Queen, Black, 1)
    testPieceCount(b, King, White, 1)
    testPieceCount(b, King, Black, 1)
    

    # Ensure pieces are in the correct location

    # Pawns
    for loc in ["a2", "b2", "c2", "d2", "e2", "f2", "g2", "h2"]:
        testPiece(b.getPiece(loc), Pawn, White)
    for loc in ["a7", "b7", "c7", "d7", "e7", "f7", "g7", "h7"]:
        testPiece(b.getPiece(loc), Pawn, Black)
    # Rooks
    testPiece(b.getPiece("a1"), Rook, White)
    testPiece(b.getPiece("h1"), Rook, White)
    testPiece(b.getPiece("a8"), Rook, Black)
    testPiece(b.getPiece("h8"), Rook, Black)
    # Knights
    testPiece(b.getPiece("b1"), Knight, White)
    testPiece(b.getPiece("g1"), Knight, White)
    testPiece(b.getPiece("b8"), Knight, Black)
    testPiece(b.getPiece("g8"), Knight, Black)
    # Bishops
    testPiece(b.getPiece("c1"), Bishop, White)
    testPiece(b.getPiece("f1"), Bishop, White)
    testPiece(b.getPiece("c8"), Bishop, Black)
    testPiece(b.getPiece("f8"), Bishop, Black)
    # Kings
    testPiece(b.getPiece("e1"), King, White)
    testPiece(b.getPiece("e8"), King, Black)
    # Queens
    testPiece(b.getPiece("d1"), Queen, White)
    testPiece(b.getPiece("d8"), Queen, Black)
    echo "All tests were successful"