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]]

    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

    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 opposite*(c: PieceColor): PieceColor {.inline.} = (if c == White: Black else: White)
proc algebraicToPosition*(s: string): Location {.inline.}
proc getCapture*(self: ChessBoard, move: Move): Location
func emptyMove*: Move {.inline.} = Move(startSquare: emptyLocation(), targetSquare: emptyLocation(), piece: emptyPiece())


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


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.queens.add((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.queens.add((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 self.pieces.white.queens.len()
                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 self.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]


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 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}"
    case piece.color:
        of White:
            # Pawns can move forward one square. In our flipped
            # board configuration, that means moving up one row
            # while keeping the column the same
            if location.row in 1..6 and self.grid[location.row - 1, location.col].color == None:
                locations.add((location.row - 1, location.col))
            if self.enPassantSquare.piece.color == piece.color.opposite:
                if abs(self.enPassantSquare.targetSquare.col - location.col) == 1 and abs(self.enPassantSquare.targetSquare.row - location.row) == 1:
                    # Only viable if the piece is on the diagonal of the target
                    locations.add(self.enPassantSquare.targetSquare)
            # They can also move on either diagonal one
            # square, but only to capture
            if location.col in 1..6 and location.row in 1..6:
                if self.grid[location.row + 1, location.col + 1].color == Black:
                    # Top right diagonal (white side)
                    locations.add((location.row + 1, location.col + 1))
                if self.grid[location.row - 1, location.col - 1].color == Black:
                    # Top left diagonal
                    locations.add((location.row + 1, location.col + 1))
            # Pawn is at the right side, can only capture
            # on the left one
            elif location.col == 0 and location.row < 7 and self.grid[location.row + 1, location.col + 1].color == Black:
                locations.add((location.row + 1, location.col + 1))
            # Pawn is at the left side, can only capture
            # on the right one
            elif location.col == 7 and location.row < 7 and self.grid[location.row + 1, location.col - 1].color == Black:
                locations.add((location.row - 1, location.col - 1))
        of Black:
            # Pawns can move forward one square. In our flipped
            # board configuration, that means moving down one row
            # while keeping the column the same
            if location.row in 1..6 and self.grid[location.row - 1, location.col].color == None:
                locations.add((1, 0))
            if self.enPassantSquare.piece.color == piece.color.opposite:
                if abs(self.enPassantSquare.targetSquare.col - location.col) == 1 and abs(self.enPassantSquare.targetSquare.row - location.row) == 1:
                    # Only viable if the piece is on the diagonal of the target
                    locations.add(self.enPassantSquare.targetSquare)
            # They can also move on either diagonal one
            # square, but only to capture
            if location.col in 1..6 and location.row in 1..6:
                if self.grid[location.row - 1, location.col - 1].color == White:
                    # Top right diagonal (black side)
                    locations.add((1, 1))
                if self.grid[location.row + 1, location.col + 1].color == White:
                    # Top left diagonal
                    locations.add((-1, -1))
            # Pawn is at the right side, can only capture
            # on the left one
            elif location.col > 0 and location.row > 0 and self.grid[location.row - 1, location.col + 1].color == White:
                locations.add((-1, -1))
            # Pawn is at the left side, can only capture
            # on the right one
            elif location.col == 7 and location.row > 0 and self.grid[location.row + 1, location.col + 1].color == White:
                locations.add((1, 1))
        else:
            discard
    for target in locations:
        if target.row == piece.color.getLastRow():
            # Generate all promotion moves
            for promotionType in [PromoteToKnight, PromoteToBishop, PromoteToRook, PromoteToQueen]:
                result.add(Move(startSquare: location, targetSquare: target, piece: self.grid[location.row, location.col], flag: promotionType))
        else:
            result.add(Move(startSquare: location, targetSquare: target, piece: self.grid[location.row, location.col]))
        


proc generateSlidingMoves(self: ChessBoard, location: Location): seq[Move] =
    ## Generates sliding 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}"


proc generateMoves(self: ChessBoard, location: Location): seq[Move] =
    ## Returns the list of possible 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)
        else:
            return @[]


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. 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.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() and move in self.generateMoves(move.startSquare):
        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 validatePawnMove(self: ChessBoard, move: Move): bool =
    ## Returns true if the given pawn move is allowed
    ## (internal helper to testMoveOffsets)
    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
        if 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 for each pawn)
        if move.startSquare.row != move.piece.getStartRow():
            # Pawn has already moved more than once, double push
            # is not allowed
            return false
        # En passant is now possible
        let targetSquare: Location = ((if move.piece.color == White: move.targetSquare.row + 1 else: move.targetSquare.row - 1), move.targetSquare.col)
        self.enPassantSquare = Move(piece: move.piece, startSquare: move.startSquare, targetSquare: targetSquare)
    # Captures are checked earlier, so we only need to make sure we aren't blocked by
    # a piece
    return self.grid[move.targetSquare.row, move.targetSquare.col].kind == Empty


proc validateSlidingMove(self: ChessBoard, move: Move): bool =
    ## Returns true if the given pawn move is allowed
    ## (internal helper to testMoveOffsets)
    
    var directions: seq[Location]
    

proc testMoveOffsets(self: ChessBoard, move: Move): bool =
    ## Returns true if the piece in the given
    ## move is pseudo-legal: 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:
            return self.validatePawnMove(move)
        of Bishop:
            return self.validateSlidingMove(move)
        else:
            return false


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.attacked.white.setLen(0)
    self.attacked.black.setLen(0)
    # Go over each piece one by one and see which squares
    # it currently attacks

    # White pawns
    for loc in self.pieces.white.pawns:
        for move in self.generateMoves(loc):
            self.attacked.white.add((move.piece, move.targetSquare))
    # Black pawns
    for loc in self.pieces.black.pawns:
        for move in self.generateMoves(loc):
            self.attacked.black.add((move.piece, move.targetSquare))
    # White bishops
    for loc in self.pieces.white.bishops:
        for move in self.generateMoves(loc):
            self.attacked.white.add((move.piece, move.targetSquare))
    # Black bishops
    for loc in self.pieces.black.bishops:
        for move in self.generateMoves(loc):
            self.attacked.black.add((move.piece, move.targetSquare))


proc getAttackers*(self: ChessBoard, square: string): seq[Piece] =
    ## Returns the attackers of the given square.
    ## If the square has no attackers, an empty
    ## seq is returned
    let loc = square.algebraicToPosition()
    case self.turn:
        of White:
            for (attacker, location) in self.attacked.black:
                if location == loc:
                    result.add(attacker)
        of Black:
            for (attacker, location) in self.attacked.white:
                if location == loc:
                    result.add(attacker)
        else:
            return @[]

# 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, square: string): bool = 
    ## Returns whether the given square is attacked
    ## by one of the enemy pieces
    let loc = square.algebraicToPosition()
    case self.turn:
        of White:
            for (attacker, location) in self.attacked.black:
                if location == loc:
                    return true
            return false
        of Black:
            for (attacker, location) in self.attacked.white:
                if location == loc:
                    return true
            return false
        else:
            discard


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.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.queens.delete(self.pieces.white.rooks.find(location))
                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.queens.delete(self.pieces.black.rooks.find(location))
                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
    
    let capture = self.getCapture(move)
    if capture != emptyLocation():
        # Move has captured a piece: remove the destination square's 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(capture)
    # 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.queens.delete(self.pieces.white.queens.find(move.startSquare))
                    self.pieces.white.queens.add(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.queens.delete(self.pieces.black.queens.find(move.startSquare))
                    self.pieces.white.queens.add(move.targetSquare)
                of King:
                    self.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 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()
    # En passant is possible only immediately after the
    # pawn has moved
    if self.enPassantSquare != emptyMove() and self.enPassantSquare.piece.color == self.turn.opposite():
        self.enPassantSquare = emptyMove()
    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 (and it isn't the en passant square) 
    # 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 arrive to destination (either
        # because it is blocked or because the moving
        # pattern is incorrect)
        return emptyMove()
    # TODO: Check for checks and pins (moves are currently pseudo-legal)



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)
    
    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 &= "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"