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, queens: seq[Location], rooks: seq[Location],
                   bishops: seq[Location], knights: seq[Location],
                   pawns: seq[Location]]
    Castling* = tuple[white, black: tuple[queen, king: bool]]

    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

    MoveFlag* = enum
        ## An enumeration of move flags
        Default,   # Move is a regular move
        XRay,   # Move is an X-ray attack
        # Move is a pawn promotion
        PromoteToQueen,
        PromoteToRook,
        PromoteToBishop,
        PromoteToKnight       

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

    Position* = ref object
        ## A chess position
        move: Move
        # Stores castling metadata
        castling: Castling
        # Number of half-moves that were performed
        # to reach this position starting from the
        # root of the tree
        plyFromRoot: int
        # 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
        # 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]
        # Potential attacking moves for black and white
        attacked: tuple[white: seq[Move], black: seq[Move]]
        # Has any piece been captured to reach this position?
        captured: Piece
        # Active color
        turn: PieceColor


    ChessBoard* = ref object
        ## A chess board object
        grid: Matrix[Piece]
        position: Position
        # List of reached positions
        positions: seq[Position]


# 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 opposite*(c: PieceColor): PieceColor {.inline.} = (if c == White: Black else: White)
proc algebraicToLocation*(s: string): Location {.inline.}
proc getCapture*(self: ChessBoard, move: Move): Location
proc makeMove*(self: ChessBoard, startSquare, targetSquare: string): Move
proc makeMove*(self: ChessBoard, move: Move): Move
func emptyMove*: Move {.inline.} = Move(startSquare: emptyLocation(), targetSquare: emptyLocation(), piece: emptyPiece())
func `+`*(a, b: Location): Location = (a.row + b.row, a.col + b.col)
func isValid*(a: Location): bool = a.row in 0..7 and a.col in 0..7
proc generateMoves(self: ChessBoard, location: Location): seq[Move]

# Due to our board layout, directions of movement are reversed for white/black so
# we need these helpers to avoid going mad with integer tuples and minus signs
# everywhere
func topLeftDiagonal(piece: Piece): Location {.inline.} = (if piece.color == White: (-1, -1) else: (1, 1))
func topRightDiagonal(piece: Piece): Location {.inline.} = (if piece.color == White: (-1, 1) else: (1, -1))
func bottomLeftDiagonal(piece: Piece): Location {.inline.} = (if piece.color == White: (1, -1) else: (-1, 1))
func bottomRightDiagonal(piece: Piece): Location {.inline.} = (if piece.color == White: (1, 1) else: (-1, -1))
func leftSide(piece: Piece): Location {.inline.} = (if piece.color == White: (0, -1) else: (0, 1))
func rightSide(piece: Piece): Location {.inline.} = (if piece.color == White: (0, 1) else: (0, -1))
func topSide(piece: Piece): Location {.inline.} = (if piece.color == White: (-1, 0) else: (1, 0))
func bottomSide(piece: Piece): Location {.inline.} = (if piece.color == White: (1, 0) else: (-1, 0))
func forward(piece: Piece): Location {.inline.} = (if piece.color == White: (-1, 0) else: (1, 0))
func doublePush(piece: Piece): Location {.inline.} = (if piece.color == White: (-2, 0) else: (2, 0))


func bottomLeftKnightMove(piece: Piece, long: bool = true): Location {.inline.} = 
    if piece.color == White: 
        if long:
            return (-2, 1)
        else:
            return (1, -2)
    elif piece.color == Black:
        if long:
            return (2, -1)
        else:
            return (1, -2)


func bottomRightKnightMove(piece: Piece, long: bool = true): Location {.inline.} = 
    if piece.color == White: 
        if long:
            return (2, -1)
        else:
            return (1, 2)
    elif piece.color == Black:
        if long:
            return (2, 1)
        else:
            return (1, 2)


func topLeftKnightMove(piece: Piece, long: bool = true): Location {.inline.} = 
    if piece.color == White: 
        if long:
            return (-2, -1)
        else:
            return (-1, -2)
    elif piece.color == Black:
        if long:
            return (2, 1)
        else:
            return (1, 2)


func topRightKnightMove(piece: Piece, long: bool = true): Location {.inline.} = 
    if piece.color == White: 
        if long:
            return (-2, 1)
        else:
            return (-1, 2)
    elif piece.color == Black:
        if long:
            return (2, -1)
        else:
            return (-1, 2)


proc getActiveColor*(self: ChessBoard): PieceColor =
    ## Returns the currently active color
    ## (turn of who has to move)
    return self.position.turn


proc getCastlingInformation*(self: ChessBoard): tuple[queen, king: bool] =
    ## Returns whether castling is possible
    ## for the given color
    case self.getActiveColor():
        of White:
            return self.position.castling.white
        of Black:
            return self.position.castling.black
        else:
            discard


proc getEnPassantTarget*(self: ChessBoard): Location =
    ## Returns the current en passant target square
    return self.position.enPassantSquare.targetSquare


proc getMoveCount*(self: ChessBoard): int =
    ## Returns the number of full moves that
    ## have been played
    return self.position.fullMoveCount


proc getHalfMoveCount*(self: ChessBoard): int =
    ## Returns the current number of half-moves
    ## since the last irreversible move
    return self.position.halfMoveClock


func getStartRow(piece: Piece): int {.inline.} =
    ## Retrieves the starting row of
    ## the given piece inside our 8x8
    ## grid
    case piece.color:
        of None:
            return -1
        of White:
            case piece.kind:
                of Pawn:
                    return 6
                else:
                    return 5
        of Black:
            case piece.kind:
                of Pawn:
                    return 1
                else:
                    return 0


func getLastRow(color: PieceColor): int {.inline.} =
    ## Retrieves the location of the last
    ## row relative to the given color
    case color:
        of White:
            return 0
        of Black:
            return 7
        else:
            return -1


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.position = Position(attacked: (@[], @[]),
                               enPassantSquare: emptyMove(),
                               move: emptyMove(),
                               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.position.pieces.black.pawns.add((row, column))
                                    of Bishop:
                                        result.position.pieces.black.bishops.add((row, column))
                                    of Knight:
                                        result.position.pieces.black.knights.add((row, column))
                                    of Rook:
                                        result.position.pieces.black.rooks.add((row, column))
                                    of Queen:
                                        result.position.pieces.black.queens.add((row, column))
                                    of King:
                                        result.position.pieces.black.king = (row, column)
                                    else:
                                        discard
                            of White:
                                case piece.kind:
                                    of Pawn:
                                        result.position.pieces.white.pawns.add((row, column))
                                    of Bishop:
                                        result.position.pieces.white.bishops.add((row, column))
                                    of Knight:
                                        result.position.pieces.white.knights.add((row, column))
                                    of Rook:
                                        result.position.pieces.white.rooks.add((row, column))
                                    of Queen:
                                        result.position.pieces.white.queens.add((row, column))
                                    of King:
                                        result.position.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.position.turn = White
                    of 'b':
                        result.position.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.position.castling.white.king = true
                    of 'Q':
                        result.position.castling.white.queen = true
                    of 'k':
                        result.position.castling.black.king = true
                    of 'q':
                        result.position.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.position.enPassantSquare.targetSquare = state[index..index+1].algebraicToLocation()
                        # Just for cleanliness purposes, we fill in the other metadata as
                        # well
                        result.position.enPassantSquare.piece.color = result.getActiveColor()
                        result.position.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.position.halfMoveClock = parseInt(s)
            of 5:
                # Fullmove number
                var s = ""
                while index <= state.high():
                    s.add(state[index])
                    inc(index)
                result.position.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.position.pieces.white.pawns.len()
                of Bishop:
                    return self.position.pieces.white.bishops.len()
                of Knight:
                    return self.position.pieces.white.knights.len()
                of Rook:
                    return self.position.pieces.white.rooks.len()
                of Queen:
                    return self.position.pieces.white.queens.len()
                of King:
                    # There shall be only one, forever
                    return 1
                else:
                    discard
        of Black:
            case kind:
                of Pawn:
                    return self.position.pieces.black.pawns.len()
                of Bishop:
                    return self.position.pieces.black.bishops.len()
                of Knight:
                    return self.position.pieces.black.knights.len()
                of Rook:
                    return self.position.pieces.black.rooks.len()
                of Queen:
                    return self.position.pieces.black.queens.len()
                of King:
                    # In perpetuity
                    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]


func rowToRank(row: int): int =
    const indeces = [8, 7, 6, 5, 4, 3, 2, 1]
    return indeces[row]


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


proc locationToAlgebraic*(loc: Location): string =
    ## Converts a location from our internal row, column
    ## notation to a square in algebraic notation
    return &"{char(uint8(loc.col) + uint8('a'))}{rowToRank(loc.row)}"


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


proc getCapture*(self: ChessBoard, move: Move): Location =
    ## Returns the location that would be captured if this
    ## move were played on the board, taking en passant and
    ## other things into account (the move is assumed to be
    ## already valid). An empty location is returned if no
    ## piece is captured by the given move
    result = emptyLocation()
    let target = self.grid[move.targetSquare.row, move.targetSquare.col]
    if target.color == None:
        if move.targetSquare != self.position.enPassantSquare.targetSquare:
            return
        else:
            return ((if move.piece.color == White: move.targetSquare.row + 1 else: move.targetSquare.row - 1), move.targetSquare.col)
    if target.color == move.piece.color.opposite():
        return move.targetSquare


proc isCapture*(self: ChessBoard, move: Move): bool {.inline.} =
    ## Returns whether the given move is a capture
    ## or not
    return self.getCapture(move) != emptyLocation()


proc generatePawnMoves(self: ChessBoard, location: Location): seq[Move] =
    ## Generates the possible moves for the pawn in the given
    ## location
    var 
        piece = self.grid[location.row, location.col]
        locations: seq[Location] = @[]
    doAssert piece.kind == Pawn, &"generatePawnMoves called on a {piece.kind}"
    # Pawns can move forward one square
    let forwardOffset = piece.forward()
    let forward = (forwardOffset + location)
    if forward.isValid() and self.grid[forward.row, forward.col].color == None:
        locations.add(forwardOffset)
    # If the pawn is on its first rank, it can push two squares
    if location.row == piece.getStartRow():
        locations.add(piece.doublePush())
    if self.position.enPassantSquare.piece.color == piece.color.opposite:
        if abs(self.position.enPassantSquare.targetSquare.col - location.col) == 1 and abs(self.position.enPassantSquare.targetSquare.row - location.row) == 1:
            # Only viable if the piece is on the diagonal of the target
            locations.add(self.position.enPassantSquare.targetSquare)
    # They can also move on either diagonal one
    # square, but only to capture
    if location.col in 1..6:
        # Top right diagonal
        locations.add(piece.topRightDiagonal())
    if location.row in 1..6:
        # Top left diagonal
        locations.add(piece.topLeftDiagonal())
    
    # Pawn is at the right side, can only capture
    # on the left one
    if location.col == 7 and location.row < 7:
        locations.add(piece.topLeftDiagonal())
    # Pawn is at the left side, can only capture
    # on the right one
    if location.col == 0 and location.row < 7:
        locations.add(piece.topRightDiagonal())
    var
        newLocation: Location
        targetPiece: Piece
    for target in locations:
        newLocation = location + target
        if not newLocation.isValid():
            continue
        targetPiece = self.grid[newLocation.row, newLocation.col]
        if targetPiece.color == piece.color:
            # Can't move over a friendly piece
            continue
        if location.col != newLocation.col and not self.isCapture(Move(piece: piece, startSquare: location, targetSquare: newLocation)):
            # Can only move diagonally when capturing
            continue
        if newLocation.row == piece.color.getLastRow():
            # Generate all promotion moves
            for promotionType in [PromoteToKnight, PromoteToBishop, PromoteToRook, PromoteToQueen]:
                result.add(Move(startSquare: location, targetSquare: newLocation, piece: piece, flag: promotionType))
            continue
        # Move is just a pawn push
        result.add(Move(startSquare: location, targetSquare: newLocation, piece: piece))


proc generateSlidingMoves(self: ChessBoard, location: Location): seq[Move] =
    ## Generates moves for the sliding piece in the given location
    var 
        piece = self.grid[location.row, location.col]
    doAssert piece.kind in [Bishop, Rook, Queen], &"generateSlidingMoves called on a {piece.kind}"
    var directions: seq[Location] = @[]
    # Only check in the right directions for the chosen piece
    if piece.kind in [Bishop, Queen]:
        directions.add(piece.topLeftDiagonal())
        directions.add(piece.topRightDiagonal())
        directions.add(piece.bottomLeftDiagonal())
        directions.add(piece.bottomRightDiagonal())
    if piece.kind in [Queen, Rook]:
        directions.add(piece.topSide())
        directions.add(piece.bottomSide())
        directions.add(piece.rightSide())
        directions.add(piece.leftSide())
    for direction in directions:
        # Slide in this direction as long as it's possible
        var
            square: Location = location
            otherPiece: Piece
        while true:
            square = square + direction
            # End of board reached
            if not square.isValid():
                break
            otherPiece = self.grid[square.row, square.col]
            # A friendly piece is in the way
            if otherPiece.color == piece.color:
                break
            if otherPiece.color == piece.color.opposite:
                # Target square contains an enemy piece: capture
                # it and stop going any further
                result.add(Move(startSquare: location, targetSquare: square, piece: piece))
                break
            # Target square is empty
            result.add(Move(startSquare: location, targetSquare: square, piece: piece))


proc generateKingMoves(self: ChessBoard, location: Location): seq[Move] =
    ## Generates moves for the king in the given location
    var 
        piece = self.grid[location.row, location.col]
    doAssert piece.kind == King, &"generateKingMoves called on a {piece.kind}"
    var directions: seq[Location] = @[piece.topLeftDiagonal(),
                                      piece.topRightDiagonal(),
                                      piece.bottomRightDiagonal(),
                                      piece.bottomLeftDiagonal(),
                                      piece.topSide(),
                                      piece.bottomSide(),
                                      piece.leftSide(),
                                      piece.rightSide()]
    for direction in directions:
        # Step in this direction once
        let square: Location = location + direction
        # End of board reached
        if not square.isValid():
            continue
        let otherPiece = self.grid[square.row, square.col]
        # A friendly piece is in the way
        if otherPiece.color == piece.color:
            continue
        if otherPiece.color == piece.color.opposite:
            # Target square contains an enemy piece: capture
            # it
            result.add(Move(startSquare: location, targetSquare: square, piece: piece))
            continue
        # Target square is empty
        result.add(Move(startSquare: location, targetSquare: square, piece: piece))


proc generateKnightMoves(self: ChessBoard, location: Location): seq[Move] =
    ## Generates moves for the knight in the given location
    var 
        piece = self.grid[location.row, location.col]
    doAssert piece.kind == Knight, &"generateKnightMoves called on a {piece.kind}"
    var directions: seq[Location] = @[piece.bottomLeftKnightMove(),
                                      piece.bottomRightKnightMove(),
                                      piece.topLeftKnightMove(),
                                      piece.topRightKnightMove(),
                                      piece.bottomLeftKnightMove(long=false),
                                      piece.bottomRightKnightMove(long=false),
                                      piece.topLeftKnightMove(long=false),
                                      piece.topRightKnightMove(long=false)]
    for direction in directions:
        # Jump to this square
        let square: Location = location + direction
        # End of board reached
        if not square.isValid():
            continue
        let otherPiece = self.grid[square.row, square.col]
        # A friendly piece is in the way
        if otherPiece.color == piece.color:
            continue
        if otherPiece.color == piece.color.opposite:
            # Target square contains an enemy piece: capture
            # it
            result.add(Move(startSquare: location, targetSquare: square, piece: piece))
            continue
        # Target square is empty
        result.add(Move(startSquare: location, targetSquare: square, piece: piece))


proc generateMoves(self: ChessBoard, location: Location): seq[Move] =
    ## Returns the list of possible legal chess moves for the
    ## piece in the given location
    let piece = self.grid[location.row, location.col]
    case piece.kind:
        of Queen, Bishop, Rook:
            return self.generateSlidingMoves(location)
        of Pawn:
            return self.generatePawnMoves(location)
        of King:
            return self.generateKingMoves(location)
        of Knight:
            return self.generateKnightMoves(location)
        else:
            return @[]


proc getAttackers*(self: ChessBoard, square: string): seq[Piece] =
    ## Returns all the attackers of the given square
    let loc = square.algebraicToLocation()
    for move in self.position.attacked.black:
        if move.targetSquare == loc:
            result.add(move.piece)
    for move in self.position.attacked.white:
        if move.targetSquare == loc:
            result.add(move.piece)


proc getAttackersFor*(self: ChessBoard, square: string, color: PieceColor): seq[Piece] =
    ## Returns all the attackers of the given square
    ## for the given color
    let loc = square.algebraicToLocation()
    case color:
        of White:
            for move in self.position.attacked.black:
                if move.targetSquare == loc:
                    result.add(move.piece)
        of Black:
            for move in self.position.attacked.white:
                if move.targetSquare == loc:
                    result.add(move.piece)
        else:
            discard

# We don't use getAttackers because this one only cares about whether
# the square is attacked or not (and can therefore exit earlier than
# getAttackers)
proc isAttacked*(self: ChessBoard, loc: Location): bool =
    ## Returns whether the given location is attacked
    ## by the opponent. If the location is empty, this
    ## function returns true regardless of which color
    ## the attackers are
    let piece = self.grid[loc.row, loc.col]
    case piece.color:
        of White:
            for move in self.position.attacked.black:
                if move.targetSquare == loc:
                    return true
        of Black:
            for move in self.position.attacked.white:
                if move.targetSquare == loc:
                    return true
        of None:
            case self.getActiveColor():
                of White:
                    for move in self.position.attacked.black:
                        if move.targetSquare == loc:
                            return true
                of Black:
                    for move in self.position.attacked.white:
                        if move.targetSquare == loc:
                            return true
                else:
                    discard


proc isAttacked*(self: ChessBoard, square: string): bool = 
    ## Returns whether the given square is attacked
    ## by its opponent
    return self.isAttacked(square.algebraicToLocation())


proc updateAttackedSquares(self: ChessBoard) =
    ## Updates internal metadata about which squares
    ## are attacked. Called internally by doMove
    
    # We refresh the attack metadata at every move. This is an
    # O(1) operation, because we're only updating the length 
    # field without deallocating the memory, which will promptly
    # be reused by us again. Neat!
    self.position.attacked.white.setLen(0)
    self.position.attacked.black.setLen(0)
    # Go over each piece one by one and see which squares
    # it currently attacks

    # Pawns
    for loc in self.position.pieces.white.pawns:
        for move in self.generateMoves(loc):
            self.position.attacked.white.add(move)
    # Bishops
    for loc in self.position.pieces.white.bishops:
        for move in self.generateMoves(loc):
            self.position.attacked.white.add(move)
    # Rooks
    for loc in self.position.pieces.white.rooks:
        for move in self.generateMoves(loc):
            self.position.attacked.white.add(move)
    # Queens
    for loc in self.position.pieces.white.queens:
        for move in self.generateMoves(loc):
            self.position.attacked.white.add(move)
    # King
    for move in self.generateMoves(self.position.pieces.white.king):
        self.position.attacked.white.add(move)
    
    # Same for black
    for loc in self.position.pieces.black.pawns:
        for move in self.generateMoves(loc):
            self.position.attacked.black.add(move)
    for loc in self.position.pieces.black.bishops:
        for move in self.generateMoves(loc):
            self.position.attacked.black.add(move)
    for loc in self.position.pieces.black.rooks:
        for move in self.generateMoves(loc):
            self.position.attacked.black.add(move)
    for loc in self.position.pieces.black.queens:
        for move in self.generateMoves(loc):
            self.position.attacked.black.add(move)
    for move in self.generateMoves(self.position.pieces.black.king):
        self.position.attacked.black.add(move)


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


proc movePiece(self: ChessBoard, move: Move) =
    ## Internal helper to move a piece. Does
    ## not update attacked squares, just position
    ## metadata and the grid itself
    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.position.pieces.white.pawns.delete(self.position.pieces.white.pawns.find(move.startSquare))
                    self.position.pieces.white.pawns.add(move.targetSquare)
                of Bishop:
                    self.position.pieces.white.bishops.delete(self.position.pieces.white.bishops.find(move.startSquare))
                    self.position.pieces.white.bishops.add(move.targetSquare)
                of Knight:
                    self.position.pieces.white.knights.delete(self.position.pieces.white.knights.find(move.startSquare))
                    self.position.pieces.white.knights.add(move.targetSquare)
                of Rook:
                    self.position.pieces.white.rooks.delete(self.position.pieces.white.rooks.find(move.startSquare))
                    self.position.pieces.white.rooks.add(move.targetSquare)
                of Queen:
                    try:
                        self.position.pieces.white.queens.delete(self.position.pieces.white.queens.find(move.startSquare))
                    except:
                        echo self.position.pieces.white.queens
                        echo move.startSquare
                        raise getCurrentException()
                    self.position.pieces.white.queens.add(move.targetSquare)
                of King:
                    self.position.pieces.white.king = move.targetSquare
                else:
                    discard
        of Black:
            case move.piece.kind:
                of Pawn:
                    self.position.pieces.black.pawns.delete(self.position.pieces.black.pawns.find(move.startSquare))
                    self.position.pieces.black.pawns.add(move.targetSquare)
                of Bishop:
                    self.position.pieces.black.bishops.delete(self.position.pieces.black.bishops.find(move.startSquare))
                    self.position.pieces.black.bishops.add(move.targetSquare)
                of Knight:
                    self.position.pieces.black.knights.delete(self.position.pieces.black.knights.find(move.startSquare))
                    self.position.pieces.black.knights.add(move.targetSquare)
                of Rook:
                    self.position.pieces.black.rooks.delete(self.position.pieces.black.rooks.find(move.startSquare))
                    self.position.pieces.black.rooks.add(move.targetSquare)
                of Queen:
                    self.position.pieces.black.queens.delete(self.position.pieces.black.queens.find(move.startSquare))
                    self.position.pieces.black.queens.add(move.targetSquare)
                of King:
                    self.position.pieces.black.king = move.targetSquare
                else:
                    discard
        else:
            discard
        # Empty out the starting square
    self.grid[move.startSquare.row, move.startSquare.col] = emptyPiece()
    # Actually move the piece
    self.grid[move.targetSquare.row, move.targetSquare.col] = move.piece 


proc updatePositions(self: ChessBoard, move: Move) =
    ## Internal helper to update the position of
    ## the pieces on the board after a move
    let capture = self.getCapture(move)
    if capture != emptyLocation():
        self.position.captured = self.grid[capture.row, capture.col]
    # Update the positional metadata of the moving piece
    self.movePiece(move)


proc inCheck*(self: ChessBoard, color: PieceColor = None): bool =
    ## Returns whether the given color's
    ## king is in check. If the color is
    ## set to None, checks are checked
    ## for the active color's king
    case color:
        of White:
            return self.isAttacked(self.position.pieces.white.king)
        of Black:
            return self.isAttacked(self.position.pieces.black.king)
        of None:
            case self.getActiveColor():
                of White:
                    return self.isAttacked(self.position.pieces.white.king)
                of Black:
                    return self.isAttacked(self.position.pieces.black.king)
                else:
                    # Unreachable
                    discard


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

    # Final checks
    
    # Needed to detect draw by the 50 move rule
    var 
        halfMoveClock = self.position.halfMoveClock
        fullMoveCount = self.position.fullMoveCount
    if move.piece.kind == Pawn or self.isCapture(move):
        halfMoveClock = 0
    else:
        inc(halfMoveClock)
    if move.piece.color == Black:
        inc(fullMoveCount)
    # TODO: Castling

    # Record the move in the position
    self.position.move = move

    # Update position and attack metadata
    self.updatePositions(move)    
    self.updateAttackedSquares()
    
    # Record final position for future reference
    self.positions.add(self.position)
    # Create new position with
    var newPos = Position(plyFromRoot: self.position.plyFromRoot + 1,
                          halfMoveClock: halfMoveClock,
                          fullMoveCount: fullMoveCount,
                          captured: emptyPiece(),
                          turn: self.getActiveColor().opposite,
                          # Inherit values from current position
                          # (they are already up to date by this point)
                          castling: self.position.castling,
                          attacked: self.position.attacked,
                          # Updated at the next call to doMove()
                          move: emptyMove(),
                          pieces: self.position.pieces,
                          )
    # Check for double pawn push
    if move.piece.kind == Pawn and abs(move.startSquare.row - move.targetSquare.row) == 2:
        newPos.enPassantSquare = Move(piece: move.piece,
                                      startSquare: (move.startSquare.row, move.startSquare.col),
                                      targetSquare: move.targetSquare + move.piece.bottomSide())
    else:
        newPos.enPassantSquare = emptyMove()

    self.position = newPos


proc spawnPiece(self: ChessBoard, location: Location, piece: Piece) =
    ## Internal helper to "spawn" a given piece at the given
    ## location. Note that this will overwrite whatever piece
    ## was previously located there: use with caution. Does
    ## not automatically update the attacked square metadata
    ## or other positional information
    doAssert piece.kind != King, "spawnPiece: cannot spawn a king"
    case piece.color:
        of White:
            case piece.kind:
                of Pawn:
                    self.position.pieces.white.pawns.add(location)
                of Knight:
                    self.position.pieces.white.knights.add(location)
                of Bishop:
                    self.position.pieces.white.bishops.add(location)
                of Rook:
                    self.position.pieces.white.rooks.add(location)
                of Queen:
                    self.position.pieces.white.queens.add(location)
                else:
                    discard
        of Black:
            case piece.kind:
                of Pawn:
                    self.position.pieces.black.pawns.add(location)
                of Knight:
                    self.position.pieces.black.knights.add(location)
                of Bishop:
                    self.position.pieces.black.bishops.add(location)
                of Rook:
                    self.position.pieces.black.rooks.add(location)
                of Queen:
                    self.position.pieces.black.queens.add(location)
                else:
                    discard
        else:
            # Unreachable
            discard
    self.grid[location.row, location.col] = piece


proc undoLastMove*(self: ChessBoard): Move {.discardable.} =
    ## Undoes the last move, restoring the previous position.
    ## If there are no positions to roll back to to, this is a 
    ## no-op. Returns the move that was performed (may be empty)
    result = emptyMove()
    if self.positions.len() == 0:
        return
    var 
        previous = self.positions[^1]
        oppositeMove = Move(piece: previous.move.piece, targetSquare: previous.move.startSquare, startSquare: previous.move.targetSquare)
    self.removePiece(previous.move.startSquare)
    self.position = previous
    if previous.move != emptyMove():
        self.spawnPiece(previous.move.startSquare, previous.move.piece)
        self.updateAttackedSquares()
        self.updatePositions(oppositeMove)
    if previous.captured != emptyPiece():
        self.spawnPiece(previous.move.targetSquare, previous.captured)
    discard self.positions.pop()
    return self.position.move
    

proc isLegal(self: ChessBoard, move: Move): bool =
    ## Returns whether the given move is legal

    # Start square doesn't contain a piece (and it isn't the en passant square) 
    # or it's not this player's turn to move
    if (move.piece.kind == Empty and move.targetSquare != self.getEnPassantTarget()) or move.piece.color != self.getActiveColor():
        return false
    var destination = self.grid[move.targetSquare.row, move.targetSquare.col]
    # Destination square is occupied by a friendly piece
    if destination.kind != Empty and destination.color == self.getActiveColor():
        return false
    if move notin self.generateMoves(move.startSquare):
        # Piece cannot arrive to destination (blocked,
        # pinned or otherwise invalid move)
        return false
    self.doMove(move)
    defer: self.undoLastMove()
    # Move would reveal an attack
    # on our king: not allowed
    if self.inCheck(move.piece.color):
        return false
    # All checks have passed: move is legal
    result = true


proc makeMove*(self: ChessBoard, move: Move): Move {.discardable.} =
    ## Like the other makeMove(), but with a Move object
    result = move
    if not self.isLegal(move):
        return emptyMove()
    self.doMove(result)


proc makeMove*(self: ChessBoard, startSquare, targetSquare: string): Move {.discardable.} =
    ## 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)
    var 
        startLocation = startSquare.algebraicToLocation()
        targetLocation = targetSquare.algebraicToLocation()
    result = Move(startSquare: startLocation, targetSquare: targetLocation, piece: self.grid[startLocation.row, startLocation.col])
    return self.makeMove(result)


proc makeMove*(self: ChessBoard, startSquare, targetSquare: Location): Move {.discardable.} =
    ## Like the other makeMove(), but with two locations
    result = Move(startSquare: startSquare, targetSquare: targetSquare, piece: self.grid[startSquare.row, startSquare.col])
    return self.makeMove(result)


proc `$`*(self: ChessBoard): string =
    result &= "- - - - - - - -"
    for i, row in self.grid:
        result &= "\n"
        for piece in row:
            if piece.kind == Empty:
                result &= "x "
                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"


proc pretty*(self: ChessBoard): string =
    ## Returns a colorized version of the
    ## board for easier visualization
    result &= "- - - - - - - -"

    for i, row in self.grid:
        result &= "\n"
        for j, piece in row:
            if piece.kind == Empty:
                result &= "\x1b[36;1mx"
                # Avoids the color overflowing
                # onto the numbers
                if j < 7:
                    result &= " \x1b[0m"
                else:
                    result &= "\x1b[0m "
                continue
            if piece.color == White:
                result &= &"\x1b[37;1m{char(piece.kind).toUpperAscii()}\x1b[0m "
            else:
                result &= &"\x1b[30;1m{char(piece.kind)} "
        result &= &"\x1b[33;1m{rankToColumn(i + 1) + 1}\x1b[0m"

    result &= "\n- - - - - - - -"
    result &= "\n\x1b[31;1ma b c d e f g h"
    result &= "\x1b[0m"


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"