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 std/strutils
import std/strformat
import std/sequtils


type

    PieceColor* = enum
        ## A piece color enumeration
        None = 0'i8,
        White, 
        Black

    PieceKind* = enum
        ## A chess piece enumeration
        Empty = 0'i8,    # 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 = 0'u16,    # No flag
        EnPassant = 1,      # Move is a capture with en passant
        Capture = 2,        # Move is a capture
        DoublePush = 4,     # Move is a double pawn push
        # Castling metadata
        CastleLong = 8,
        CastleShort = 16,
        # Pawn promotion metadata
        PromoteToQueen = 32,
        PromoteToRook = 64,
        PromoteToBishop = 128,
        PromoteToKnight = 256

    # Useful type aliases
    Location* = tuple[row, col: int8]
    
    Attacked = seq[tuple[source, target, direction: Location]]

    Pieces = tuple[king: Location, queens: seq[Location], rooks: seq[Location],
                   bishops: seq[Location], knights: seq[Location],
                   pawns: seq[Location]]
    
    CountData = tuple[nodes: uint64, captures: uint64, castles: uint64, checks: uint64,  promotions: uint64, enPassant: uint64, checkmates: uint64]

    Move* = object
        ## A chess move
        startSquare*: Location
        targetSquare*: Location
        flags*: uint16


    Position* = ref object
        ## A chess position
        move: Move
        # Did the rooks on either side/the king move?
        castlingAvailable: tuple[white, black: tuple[queen, king: bool]]
        # Number of half-moves that were performed
        # to reach this position starting from the
        # root of the tree
        plyFromRoot: int16
        # Number of half moves since
        # last piece capture or pawn movement.
        # Used for the 50-move rule
        halfMoveClock: int8
        # Full move counter. Increments
        # every 2 ply
        fullMoveCount: int16
        # En passant target square (see https://en.wikipedia.org/wiki/En_passant)
        enPassantSquare*: Location
        # Locations of all pieces
        pieces: tuple[white: Pieces, black: Pieces]
        # Squares attacked by both sides
        attacked: tuple[white: Attacked, black: Attacked]
        # Pieces pinned by both sides
        pinned: tuple[white: Attacked, black: Attacked]
        # Active color
        turn: PieceColor

    ChessBoard* = ref object
        ## A chess board object
        
        # An 8x8 matrix we use for constant
        # time lookup of pieces by their location
        grid: seq[Piece]
        # The current position
        position: Position
        # List of all reached positions
        positions: seq[Position]


# A bunch of simple utility functions

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 makeMove*(self: ChessBoard, move: Move): Move {.discardable.}
func emptyMove*: Move {.inline.} = Move(startSquare: emptyLocation(), targetSquare: emptyLocation())
func `+`*(a, b: Location): Location = (a.row + b.row, a.col + b.col)
func `-`*(a: Location): Location = (-a.row, -a.col)
func `-`*(a, b: Location): Location = (a.row - b.row, a.col - b.col)
func isValid*(a: Location): bool {.inline.} = a.row in 0..7 and a.col in 0..7
proc generateMoves(self: ChessBoard, location: Location): seq[Move]
proc getAttackers*(self: ChessBoard, loc: Location, color: PieceColor): seq[Location]
proc getAttackFor*(self: ChessBoard, source, target: Location): tuple[source, target, direction: Location]
proc isAttacked*(self: ChessBoard, loc: Location, color: PieceColor = None): bool
proc isLegal(self: ChessBoard, move: Move): bool {.inline.}
proc doMove(self: ChessBoard, move: Move)
proc pretty*(self: ChessBoard): string
proc spawnPiece(self: ChessBoard, location: Location, piece: Piece)
proc updateAttackedSquares(self: ChessBoard)
proc getPinnedDirections(self: ChessBoard, loc: Location): seq[Location]
proc getAttacks*(self: ChessBoard, loc: Location): Attacked
proc getSlidingAttacks(self: ChessBoard, loc: Location): tuple[attacks: Attacked, pins: Attacked]
proc inCheck*(self: ChessBoard, color: PieceColor = None): bool


proc extend[T](self: var seq[T], other: openarray[T]) {.inline.} = 
    for x in other:
        self.add(x)


# Due to our board layout, directions of movement are reversed for white and black, so
# we need these helpers to avoid going mad with integer tuples and minus signs everywhere
func topLeftDiagonal(color: PieceColor): Location {.inline.} = (if color == White: (-1, -1) else: (1, 1))
func topRightDiagonal(color: PieceColor): Location {.inline.} = (if color == White: (-1, 1) else: (1, -1))
func bottomLeftDiagonal(color: PieceColor): Location {.inline.} = (if color == White: (1, -1) else: (-1, 1))
func bottomRightDiagonal(color: PieceColor): Location {.inline.} = (if color == White: (1, 1) else: (-1, -1))
func leftSide(color: PieceColor): Location {.inline.} = (if color == White: (0, -1) else: (0, 1))
func rightSide(color: PieceColor): Location {.inline.} = (if color == White: (0, 1) else: (0, -1))
func topSide(color: PieceColor): Location {.inline.} = (if color == White: (-1, 0) else: (1, 0))
func bottomSide(color: PieceColor): Location {.inline.} = (if color == White: (1, 0) else: (-1, 0))
func doublePush(color: PieceColor): Location {.inline.} = (if color == White: (-2, 0) else: (2, 0))
func longCastleKing: Location {.inline.} =  (0, -2)
func shortCastleKing: Location {.inline.} = (0, 2)
func longCastleRook: Location {.inline.} = (0, 3)
func shortCastleRook: Location {.inline.} = (0, -2)
func bottomLeftKnightMove(color: PieceColor, long: bool = true): Location {.inline.} = 
    if color == White: 
        if long:
            return (2, -1)
        else:
            return (1, -2)
    elif color == Black:
        if long:
            return (-2, 1)
        else:
            return (1, -2)


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


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


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

# These return absolute locations rather than relative direction offsets
func kingSideRook(color: PieceColor): Location {.inline.} = (if color == White: (7, 7) else: (0, 7))
func queenSideRook(color: PieceColor): Location {.inline.} = (if color == White: (7, 0) else: (0, 0))


# A bunch of getters
func getActiveColor*(self: ChessBoard): PieceColor {.inline.} =
    ## Returns the currently active color
    ## (turn of who has to move)
    return self.position.turn


func getEnPassantTarget*(self: ChessBoard): Location {.inline.} =
    ## Returns the current en passant target square
    return self.position.enPassantSquare


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


func getHalfMoveCount*(self: ChessBoard): int {.inline.} =
    ## 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 7
        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 = newSeqOfCap[Piece](64)
    result.position = Position(attacked: (@[], @[]),
                               enPassantSquare: emptyLocation(),
                               move: emptyMove(),
                               turn: White,
                               fullMoveCount: 1,
                               pieces: (white: (king: emptyLocation(), 
                                                queens: @[], 
                                                rooks: @[], 
                                                bishops: @[],
                                                knights: @[],
                                                pawns: @[]),
                                        black: (king: emptyLocation(),
                                                queens: @[],
                                                rooks: @[],
                                                bishops: @[],
                                                knights: @[],
                                                pawns: @[])))


func coordToIndex(row, col: int): int {.inline.} = (row * 8) + col
func `[]`(self: seq[Piece], row, column: Natural): Piece {.inline.} = self[coordToIndex(row, column)]
proc `[]=`(self: var seq[Piece], row, column: Natural, piece: Piece) {.inline.} = self[coordToIndex(row, column)] = piece


proc newChessboardFromFEN*(fen: string): ChessBoard =
    ## Initializes a chessboard with the
    ## position encoded by the given FEN string
    result = newChessboard()
    var
        # Current location in the grid
        row: int8 = 0
        column: int8 = 0
        # Current section in the FEN string
        section = 0
        # Current index into the FEN string
        index = 0
        # Temporary variable to store a piece
        piece: Piece
    # See https://en.wikipedia.org/wiki/Forsyth%E2%80%93Edwards_Notation
    while index <= fen.high():
        var c = fen[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:
                                        if result.position.pieces.black.king != emptyLocation():
                                            raise newException(ValueError, "invalid position: exactly one king of each color must be present")
                                        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:
                                        if result.position.pieces.white.king != emptyLocation():
                                            raise newException(ValueError, "invalid position: exactly one king of each color must be present")
                                        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'))
                        if x > 8:
                            raise newException(ValueError, &"invalid FEN: invalid column skip size ({x} > 8)")
                        column += int8(x)
                    else:
                        raise newException(ValueError, &"invalid FEN: unknown piece identifier '{c}'")
            of 1:
                # Active color
                case c:
                    of 'w':
                        result.position.turn = White
                    of 'b':
                        result.position.turn = Black
                    else:
                        raise newException(ValueError, &"invalid FEN: invalid active color identifier '{c}'")
            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.castlingAvailable.white.king = true
                    of 'Q':
                        result.position.castlingAvailable.white.queen = true
                    of 'k':
                        result.position.castlingAvailable.black.king = true
                    of 'q':
                        result.position.castlingAvailable.black.queen = true
                    else:
                        raise newException(ValueError, &"invalid FEN: unknown symbol '{c}' found in castling availability section")
            of 3:
                # En passant target square
                case c:
                    of '-':
                        # Field is already uninitialized to the correct state
                        discard
                    else:
                        result.position.enPassantSquare = fen[index..index+1].algebraicToLocation()
                        # Square metadata is 2 bytes long
                        inc(index)
            of 4:
                # Halfmove clock
                var s = ""
                while not fen[index].isSpaceAscii():
                    s.add(fen[index])
                    inc(index)
                # Backtrack so the space is seen by the
                # next iteration of the loop
                dec(index)
                result.position.halfMoveClock = parseInt(s).int8
            of 5:
                # Fullmove number
                var s = ""
                while index <= fen.high():
                    s.add(fen[index])
                    inc(index)
                result.position.fullMoveCount = parseInt(s).int8
            else:
                raise newException(ValueError, "invalid FEN: too many fields in FEN string")
        inc(index)
    result.updateAttackedSquares()
    if result.inCheck(result.getActiveColor().opposite):
        # Opponent king cannot be captured on the next move
        raise newException(ValueError, "invalid position: opponent king can be captured")
    if result.position.pieces.white.king == emptyLocation() or result.position.pieces.black.king == emptyLocation():
        # Both kings must be on the board
        raise newException(ValueError, "invalid position: exactly one king of each color must be present")


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


proc countPieces*(self: ChessBoard, kind: PieceKind, color: PieceColor): int =
    ## Returns the number of pieces with
    ## the given color and type in the given
    ## position
    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:
                    raise newException(ValueError, "invalid piece type")
        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:
                    raise newException(ValueError, "invalid piece type")        
        of None:
            raise newException(ValueError, "invalid piece color")


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


func rankToColumn(rank: int): int8 {.inline.} =
    ## 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: array[8, int8] = [7, 6, 5, 4, 3, 2, 1, 0]
    return indeces[rank - 1]


func rowToRank(row: int): int {.inline.} =
    ## Converts a row into our grid into
    ## a chess rank
    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 = int8(uint8(s[0]) - uint8('a'))
    # Convert the file character to a number
    let file = rankToColumn(int8(uint8(s[1]) - uint8('0')))
    return (file, rank)


func locationToAlgebraic*(loc: Location): string {.inline.} =
    ## 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)}"


func getPiece*(self: ChessBoard, loc: Location): Piece {.inline.} =
    ## Gets the piece at the given location
    return self.grid[loc.row, loc.col]


func getPiece*(self: ChessBoard, square: string): Piece {.inline.} =
    ## Gets the piece on the given square
    ## in algebraic notation
    return self.getPiece(square.algebraicToLocation())


func isPromotion*(move: Move): bool {.inline.} =
    ## Returns whether the given move is a 
    ## pawn promotion
    for promotion in [PromoteToBishop, PromoteToKnight, PromoteToRook, PromoteToQueen]:
        if (move.flags and promotion.uint16) != 0:
            return true


func getPromotionType*(move: Move): MoveFlag {.inline.} =
    ## Returns the promotion type of the given move.
    ## The return value of this function is only valid
    ## if isPromotion() returns true
    for promotion in [PromoteToBishop, PromoteToKnight, PromoteToRook, PromoteToQueen]:
        if (move.flags and promotion.uint16) != 0:
            return promotion


func isCapture*(move: Move): bool {.inline.} =
    ## Returns whether the given move is a
    ## cature
    result = (move.flags and Capture.uint16) != 0


func isCastling*(move: Move): bool {.inline.} =
    ## Returns whether the given move is a
    ## castle
    for flag in [CastleLong, CastleShort]:
        if (move.flags and flag.uint16) != 0:
            return true


func getCastlingType*(move: Move): MoveFlag {.inline.} =
    ## Returns the castling type of the given move.
    ## The return value of this function is only valid 
    ## if isCastling() returns true
    for flag in [CastleLong, CastleShort]:
        if (move.flags and flag.uint16) != 0:
            return flag


func isEnPassant*(move: Move): bool {.inline.} =
    ## Returns whether the given move is an
    ## en passant capture
    result = (move.flags and EnPassant.uint16) != 0


func isDoublePush*(move: Move): bool {.inline.} =
    ## Returns whether the given move is a
    ## double pawn push
    result = (move.flags and DoublePush.uint16) != 0


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
    var color = color
    if color == None:
        color = self.getActiveColor()
    case color:
        of White:
            result = self.isAttacked(self.position.pieces.white.king, Black)
        of Black:
            result = self.isAttacked(self.position.pieces.black.king, White)
        else:
            # Unreachable
            discard


proc canCastle*(self: ChessBoard, color: PieceColor = None): tuple[queen, king: bool] {.inline.} =
    ## Returns the sides on which castling is allowed
    ## for the given color. If the color is None, the
    ## currently active color is used
    var color = color
    if color == None:
        color = self.getActiveColor()
    # Check if castling rights are still available for moving side
    case color:
        of White:
            result.king = self.position.castlingAvailable.white.king
            result.queen = self.position.castlingAvailable.white.queen
        of Black:
            result.king = self.position.castlingAvailable.black.king
            result.queen = self.position.castlingAvailable.black.queen
        of None:
            # Unreachable
            discard
    if self.inCheck(color):
        # King can not castle out of check
        return (false, false)
    if result.king or result.queen:
        var 
            loc: Location
            queenSide: Location
            kingSide: Location
        # If the path between the king and rook on a given side is blocked, or any of the
        # squares where the king would move to are attacked by the opponent, then castling
        # is temporarily prohibited on that side
        case color:
            of White:
                loc = self.position.pieces.white.king
                queenSide = color.leftSide()
                kingSide = color.rightSide()
            of Black:
                loc = self.position.pieces.black.king
                queenSide = color.rightSide()
                kingSide = color.leftSide()
            of None:
                # Unreachable
                discard

        if result.king:
            # Short castle
            var 
                location = loc
                otherPiece: Piece
                moveKing: bool = true
            while true:
                location = location + kingSide
                if location == color.kingSideRook():
                    # No need to do any extra checks: if the piece
                    # on this square were not a rook of the same color
                    # as the castling king, then we wouldn't have gotten
                    # here in the first place (it would've had to be either
                    # moved or captured, and both of those actions are detected 
                    # and accounted for way before this point)
                    break
                if location == loc + shortCastleKing():
                    moveKing = false
                otherPiece = self.grid[location.row, location.col]

                if otherPiece.color != None:
                    result.king = false
                    break
                if moveKing and self.isAttacked(location, color.opposite()):
                    result.king = false
                    break

        if result.queen:
            # Long castle
            var 
                location = loc
                otherPiece: Piece
                moveKing: bool = true
            while true:
                location = location + queenSide
                if location == color.queenSideRook():
                    break
                if location == loc + longCastleKing():
                    moveKing = false
                otherPiece = self.grid[location.row, location.col]

                if otherPiece.color != None:
                    result.queen = false
                    break
                if moveKing and self.isAttacked(location, color.opposite()):
                    result.queen = false
                    break


proc getCheckResolutions(self: ChessBoard, color: PieceColor): seq[Location] =
    ## Returns the squares that need to be covered to
    ## resolve the current check (including capturing
    ## the checking piece). In case of double check, an
    ## empty list is returned (as the king must move)
    var king: Location
    case color:
        of White:
            king = self.position.pieces.white.king
        of Black:
            king = self.position.pieces.black.king
        else:
            return

    let attackers: seq[Location] = self.getAttackers(king, color.opposite())
    if attackers.len() > 1:
        # Double checks require to move the king
        return @[]
    let 
        attacker = attackers[0]
        attackerPiece = self.grid[attacker.row, attacker.col]
        attack = self.getAttackFor(attacker, king)
    # Capturing the piece resolves the check
    result.add(attacker)
    # Blocking the attack is also a viable strategy
    # (unless the check is from a knight or a pawn, 
    # in which case either the king has to move or 
    # that piece has to be captured, but this is 
    # already implicitly handled by the loop below)
    var location = attacker
    while location != king:
        location = location + attack.direction
        if not location.isValid():
            break
        result.add(location)


func getKing(self: ChessBoard, color: PieceColor): Location {.inline.} =
    var color = color
    if color == None:
        color = self.getActiveColor()
    case color:
        of White:
            return self.position.pieces.white.king
        of Black:
            return self.position.pieces.black.king
        else:
            discard


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] = @[]
        flags: seq[MoveFlag] = @[]
    doAssert piece.kind == Pawn, &"generatePawnMoves called on a {piece.kind}"
    # Pawns can move forward one square
    let forward = piece.color.topSide() + location
    # Only if the square is empty though
    if forward.isValid() and self.grid[forward.row, forward.col].color == None:
        locations.add(forward)
        flags.add(Default)
    # If the pawn is on its first rank, it can push two squares
    if location.row == piece.getStartRow():
        let double = location + piece.color.doublePush()
        # Check that both squares are empty
        if double.isValid() and self.grid[forward.row, forward.col].color == None and self.grid[double.row, double.col].color == None:
            locations.add(double)
            flags.add(DoublePush)
    let enPassantPawn = self.getEnPassantTarget() + piece.color.opposite().topSide()
    # They can also move on either diagonal one
    # square, but only to capture or for en passant
    for diagonal in [location + piece.color.topRightDiagonal(), location + piece.color.topLeftDiagonal()]:
        if diagonal.isValid():
            let otherPiece = self.grid[diagonal.row, diagonal.col]
            if diagonal == self.position.enPassantSquare and self.grid[enPassantPawn.row, enPassantPawn.col].color == self.getActiveColor().opposite():
                # Ensure en passant doesn't create a check
                let king = self.getKing(piece.color)
                var ok = true
                if king.row == location.row:
                    var current = location + piece.color.rightSide()
                    while true:
                        current = current + piece.color.rightSide()
                        if not current.isValid():
                            break
                        let p = self.grid[current.row, current.col]
                        if p.color == piece.color:
                            break
                        if p.color == None:
                            continue
                        # Bishops can't create checks through en passant
                        if p.color == piece.color.opposite() and p.kind in [Queen, Rook]:
                            ok = false
                if ok:
                    locations.add(diagonal)
                    flags.add(EnPassant)
            elif otherPiece.color == piece.color.opposite() and otherPiece.kind != King:
                locations.add(diagonal)
                flags.add(Capture)
    var 
        newLocation: Location
        newFlags: seq[MoveFlag]
        newLocations: seq[Location]
    # Check for pins
    let pins = self.getPinnedDirections(location)
    for pin in pins:
        newLocation = location + pin
        let loc = locations.find(newLocation)
        if loc != -1:
            # Pin direction is legal for this piece
            newLocations.add(newLocation)
            newFlags.add(flags[loc])
    if pins.len() > 0:
        locations = newLocations
        flags = newFlags
    let checked = self.inCheck()
    let resolutions = if not checked: @[] else: self.getCheckResolutions(piece.color)
    var targetPiece: Piece
    for (target, flag) in zip(locations, flags):
        if checked and target notin resolutions:
            continue
        targetPiece = self.grid[target.row, target.col]
        if target.row == piece.color.getLastRow():
            # Pawn reached the other side of the board: generate all potential piece promotions
            for promotionType in [PromoteToKnight, PromoteToBishop, PromoteToRook, PromoteToQueen]:
                result.add(Move(startSquare: location, targetSquare: target, flags: promotionType.uint16 or flag.uint16))
            continue
        result.add(Move(startSquare: location, targetSquare: target, flags: flag.uint16))


proc generateSlidingMoves(self: ChessBoard, location: Location): seq[Move] =
    ## Generates moves for the sliding piece in the given location
    let 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.color.topLeftDiagonal())
        directions.add(piece.color.topRightDiagonal())
        directions.add(piece.color.bottomLeftDiagonal())
        directions.add(piece.color.bottomRightDiagonal())
    if piece.kind in [Queen, Rook]:
        directions.add(piece.color.topSide())
        directions.add(piece.color.bottomSide())
        directions.add(piece.color.rightSide())
        directions.add(piece.color.leftSide())
    let pinned = self.getPinnedDirections(location)
    if pinned.len() > 0:
        directions = pinned
    let checked = self.inCheck()
    let resolutions = if not checked: @[] else: self.getCheckResolutions(piece.color)
    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 checked and square notin resolutions:
                break
            if otherPiece.color == piece.color.opposite:
                # Target square contains an enemy piece: capture
                # it and stop going any further
                if otherPiece.kind != King:
                    # Can't capture the king
                    result.add(Move(startSquare: location, targetSquare: square, flags: Capture.uint16))
                break
            # Target square is empty
            result.add(Move(startSquare: location, targetSquare: square))


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.color.topLeftDiagonal(),
                                      piece.color.topRightDiagonal(),
                                      piece.color.bottomRightDiagonal(),
                                      piece.color.bottomLeftDiagonal(),
                                      piece.color.topSide(),
                                      piece.color.bottomSide(),
                                      piece.color.leftSide(),
                                      piece.color.rightSide()]
    # Castling
    let canCastle = self.canCastle(piece.color)
    if canCastle.queen:
        directions.add(longCastleKing())
    if canCastle.king:
        directions.add(shortCastleKing())
    var flag = Default
    for direction in directions:
        # Step in this direction once
        let square: Location = location + direction
        # End of board reached
        if not square.isValid():
            continue
        if self.isAttacked(square, piece.color.opposite()):
            continue
        if direction == longCastleKing():
            flag = CastleLong
        elif direction == shortCastleKing():
            flag = CastleShort
        else:
            flag = Default
        let otherPiece = self.grid[square.row, square.col]
        if otherPiece.color == self.getActiveColor.opposite():
            flag = Capture
        # A friendly piece is in the way, move onto the next direction
        if otherPiece.color == piece.color:
            continue
        # Target square is empty or contains an enemy piece:
        # All good for us!
        result.add(Move(startSquare: location, targetSquare: square, flags: flag.uint16))


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.color.bottomLeftKnightMove(),
                                      piece.color.bottomRightKnightMove(),
                                      piece.color.topLeftKnightMove(),
                                      piece.color.topRightKnightMove(),
                                      piece.color.bottomLeftKnightMove(long=false),
                                      piece.color.bottomRightKnightMove(long=false),
                                      piece.color.topLeftKnightMove(long=false),
                                      piece.color.topRightKnightMove(long=false)]
    let pinned = self.getPinnedDirections(location)
    if pinned.len() > 0:
        # Knight is pinned: can't move!
        return @[]
    let checked = self.inCheck()
    let resolutions = if not checked: @[] else: self.getCheckResolutions(piece.color)
    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 or the opponent king is is in the way
        if otherPiece.color == piece.color or otherPiece.kind == King:
            continue

        if checked and square notin resolutions:
            continue
        if otherPiece.color != None:
            # Target square contains an enemy piece: capture
            # it
            result.add(Move(startSquare: location, targetSquare: square, flags: Capture.uint16))
        else:
            # Target square is empty
            result.add(Move(startSquare: location, targetSquare: square))


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 generateAllMoves*(self: ChessBoard): seq[Move] =
    ## Returns the list of all possible legal moves
    ## in the current position
    for i in 0..7:
        for j in 0..7:
            if self.grid[i, j].color == self.getActiveColor():
                for move in self.generateMoves((int8(i), int8(j))):
                    result.add(move)
      

proc isAttacked*(self: ChessBoard, loc: Location, color: PieceColor = None): bool =
    ## Returns whether the given location is attacked
    ## by the given color
    var color = color
    if color == None:
        color = self.getActiveColor().opposite()
    case color:
        of Black:
            for attack in self.position.attacked.black:
                if attack.target == loc:
                    return true
        of White:
            for attack in self.position.attacked.white:
                if attack.target == loc:
                    return true
        of None:
            discard


proc getAttackers*(self: ChessBoard, loc: Location, color: PieceColor): seq[Location] =
    ## Returns all the attackers of the given color
    ## for the given square
    case color:
        of Black:
            for attack in self.position.attacked.black:
                if attack.target == loc:
                    result.add(attack.source)
        of White:
            for attack in self.position.attacked.white:
                if attack.target == loc:
                    result.add(attack.source)
        of None:
            discard


proc getAttacks*(self: ChessBoard, loc: Location): Attacked =
    ## Returns all the squares attacked by the piece in the given
    ## location
    let piece = self.grid[loc.row, loc.col]
    case piece.color:
        of Black:
            for attack in self.position.attacked.black:
                if attack.source == loc:
                    result.add(attack)
        of White:
            for attack in self.position.attacked.white:
                if attack.source == loc:
                    result.add(attack)
        of None:
            discard


proc getAttackFor*(self: ChessBoard, source, target: Location): tuple[source, target, direction: Location] =
    ## Returns the first attacks of the piece in the given
    ## source location that also attacks the target location 
    let piece = self.grid[source.row, source.col]
    case piece.color:
        of Black:
            for attack in self.position.attacked.black:
                if attack.target == target and attack.source == source:
                    return attack
        of White:
            for attack in self.position.attacked.white:
                if attack.target == target and attack.source == source:
                    return attack
        of None:
            discard


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


func addAttack(self: ChessBoard, attack: tuple[source, target, direction: Location], color: PieceColor) {.inline.} =
    if attack.source.isValid() and attack.target.isValid():
        case color:
            of White:
                self.position.attacked.white.add(attack)
            of Black:
                self.position.attacked.black.add(attack)
            else:
                discard


proc getPinnedDirections(self: ChessBoard, loc: Location): seq[Location] =
    let piece = self.grid[loc.row, loc.col]
    case piece.color:
        of None:
            discard
        of White:
            for pin in self.position.pinned.black:
                if pin.target == loc:
                    result.add(pin.direction)
        of Black:
            for pin in self.position.pinned.white:
                if pin.target == loc:
                    result.add(pin.direction)


proc updatePawnAttacks(self: ChessBoard) =
    ## Internal helper of updateAttackedSquares
    for loc in self.position.pieces.white.pawns:
        # Pawns are special in how they capture (i.e. the
        # squares they can move to do not match the squares
        # they can capture on. Sneaky fucks)
        self.addAttack((loc, loc + White.topRightDiagonal(), White.topRightDiagonal()), White)
        self.addAttack((loc, loc + White.topLeftDiagonal(), White.topRightDiagonal()), White)
    # We do the same thing for black
    for loc in self.position.pieces.black.pawns:
        self.addAttack((loc, loc + Black.topRightDiagonal(), Black.topRightDiagonal()), Black)
        self.addAttack((loc, loc + Black.topLeftDiagonal(), Black.topRightDiagonal()), Black)


proc updateKingAttacks(self: ChessBoard) =
    ## Internal helper of updateAttackedSquares
    var king = self.position.pieces.white.king
    self.addAttack((king, king + White.topRightDiagonal(), White.topRightDiagonal()), White)
    self.addAttack((king, king + White.topLeftDiagonal(), White.topLeftDiagonal()), White)
    self.addAttack((king, king + White.bottomLeftDiagonal(), White.bottomLeftDiagonal()), White)
    self.addAttack((king, king + White.bottomRightDiagonal(), White.bottomRightDiagonal()), White)
    king = self.position.pieces.black.king
    self.addAttack((king, king + Black.topRightDiagonal(), Black.topRightDiagonal()), Black)
    self.addAttack((king, king + Black.topLeftDiagonal(), Black.topLeftDiagonal()), Black)
    self.addAttack((king, king + Black.bottomLeftDiagonal(), Black.bottomLeftDiagonal()), Black)
    self.addAttack((king, king + Black.bottomRightDiagonal(), Black.bottomRightDiagonal()), Black)


proc updateKnightAttacks(self: ChessBoard) =
    ## Internal helper of updateAttackedSquares
    for loc in self.position.pieces.white.knights:
        self.addAttack((loc, loc + White.topLeftKnightMove(), White.topLeftKnightMove()), White)
        self.addAttack((loc, loc + White.topRightKnightMove(), White.topRightKnightMove()), White)
        self.addAttack((loc, loc + White.bottomLeftKnightMove(), White.bottomLeftKnightMove()), White)
        self.addAttack((loc, loc + White.bottomRightKnightMove(), White.bottomRightKnightMove()), White)
        self.addAttack((loc, loc + White.topLeftKnightMove(long=false), White.topLeftKnightMove(long=false)), White)
        self.addAttack((loc, loc + White.topRightKnightMove(long=false), White.topRightKnightMove(long=false)), White)
        self.addAttack((loc, loc + White.bottomLeftKnightMove(long=false), White.bottomLeftKnightMove(long=false)), White)
        self.addAttack((loc, loc + White.bottomRightKnightMove(long=false), White.bottomRightKnightMove(long=false)), White)

    for loc in self.position.pieces.black.knights:
        self.addAttack((loc, loc + Black.topLeftKnightMove(), Black.topLeftKnightMove()), Black)
        self.addAttack((loc, loc + Black.topRightKnightMove(), Black.topRightKnightMove()), Black)
        self.addAttack((loc, loc + Black.bottomLeftKnightMove(), Black.bottomLeftKnightMove()), Black)
        self.addAttack((loc, loc + Black.bottomRightKnightMove(), Black.bottomRightKnightMove()), Black)
        self.addAttack((loc, loc + Black.topLeftKnightMove(long=false), Black.topLeftKnightMove(long=false)), Black)
        self.addAttack((loc, loc + Black.topRightKnightMove(long=false), Black.topRightKnightMove(long=false)), Black)
        self.addAttack((loc, loc + Black.bottomLeftKnightMove(long=false), Black.bottomLeftKnightMove(long=false)), Black)
        self.addAttack((loc, loc + Black.bottomRightKnightMove(long=false), Black.bottomRightKnightMove(long=false)), Black)


proc getSlidingAttacks(self: ChessBoard, loc: Location): tuple[attacks: Attacked, pins: Attacked] =
    ## Internal helper of updateSlidingAttacks
    var 
        directions: seq[Location] = @[]
    let piece = self.grid[loc.row, loc.col]
    if piece.kind in [Bishop, Queen]:
        directions.add(piece.color.topLeftDiagonal())
        directions.add(piece.color.topRightDiagonal())
        directions.add(piece.color.bottomLeftDiagonal())
        directions.add(piece.color.bottomRightDiagonal())

    if piece.kind in [Queen, Rook]:
        directions.add(piece.color.topSide())
        directions.add(piece.color.bottomSide())
        directions.add(piece.color.rightSide())
        directions.add(piece.color.leftSide())

    for direction in directions:
        var 
            square = loc
            otherPiece: Piece
        # Slide in this direction as long as it's possible
        while true:
            square = square + direction
            # End of board reached
            if not square.isValid():
                break
            otherPiece = self.grid[square.row, square.col]
            # Target square is attacked (even if a friendly piece
            # is present, because in this case we're defending
            # it)
            result.attacks.add((loc, square, direction))
            # Empty square, keep going
            if otherPiece.color == None:
                continue
            if otherPiece.color == piece.color.opposite():
                if otherPiece.kind != King:
                    # We found an enemy piece that is not
                    # the enemy king. We don't break out
                    # immediately because we first want 
                    # to check if we've pinned a piece
                    var 
                        otherSquare: Location = square
                        behindPiece: Piece
                    while true:
                        otherSquare = otherSquare + direction
                        if not otherSquare.isValid():
                            break
                        behindPiece = self.grid[otherSquare.row, otherSquare.col]
                        if behindPiece.color == None:
                            continue
                        if behindPiece.color == piece.color.opposite and behindPiece.kind == King:
                            # The enemy king is behind this enemy piece: pin it along
                            # this axis in both directions
                            result.pins.add((loc, square, direction))
                            result.pins.add((loc, square, -direction))
                        else:
                            break
                else:
                    # Enemy king is here: ensure it cannot move backwards by
                    # attacking the square behind it (if one exists and is
                    # valid)
                    let target = square + direction
                    if target.isValid():
                        result.attacks.add((loc, target, direction))
            break


proc updateSlidingAttacks(self: ChessBoard) =
    ## Internal helper of updateAttackedSquares
    
    var data: tuple[attacks: Attacked, pins: Attacked]
    for loc in self.position.pieces.white.bishops:
        data = self.getSlidingAttacks(loc)
        self.position.attacked.white.extend(data.attacks)
        self.position.pinned.white.extend(data.pins)
    for loc in self.position.pieces.white.rooks:
        data = self.getSlidingAttacks(loc)
        self.position.attacked.white.extend(data.attacks)
        self.position.pinned.white.extend(data.pins)
    for loc in self.position.pieces.white.queens:
        data = self.getSlidingAttacks(loc)
        self.position.attacked.white.extend(data.attacks)
        self.position.pinned.white.extend(data.pins)
    for loc in self.position.pieces.black.bishops:
        data = self.getSlidingAttacks(loc)
        self.position.attacked.black.extend(data.attacks)
        self.position.pinned.black.extend(data.pins)
    for loc in self.position.pieces.black.rooks:
        data = self.getSlidingAttacks(loc)
        self.position.attacked.black.extend(data.attacks)
        self.position.pinned.black.extend(data.pins)
    for loc in self.position.pieces.black.queens:
        data = self.getSlidingAttacks(loc)
        self.position.attacked.black.extend(data.attacks)
        self.position.pinned.black.extend(data.pins)


proc updateAttackedSquares(self: ChessBoard) =
    ## Updates internal metadata about which squares
    ## are attacked
    
    self.position.attacked.white.setLen(0)
    self.position.attacked.black.setLen(0)
    # Pawns
    self.updatePawnAttacks()
    # Sliding pieces
    self.updateSlidingAttacks()
    # Knights
    self.updateKnightAttacks()
    # Kings
    self.updateKingAttacks()


proc removePiece(self: ChessBoard, location: Location, attack: bool = true) =
    ## 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.black.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
    if attack:
        self.updateAttackedSquares()


proc movePiece(self: ChessBoard, move: Move, attack: bool = true) =
    ## Internal helper to move a piece. If attack
    ## is set to false, then this function does
    ## not update attacked squares metadata, just 
    ## positional info and the grid itself
    let piece = self.grid[move.startSquare.row, move.startSquare.col]
    case piece.color:
        of White:
            case 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:
                    self.position.pieces.white.queens.delete(self.position.pieces.white.queens.find(move.startSquare))
                    self.position.pieces.white.queens.add(move.targetSquare)
                of King:
                    self.position.pieces.white.king = move.targetSquare
                else:
                    discard
        of Black:
            case 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] = piece
    if attack:
        self.updateAttackedSquares()



proc movePiece(self: ChessBoard, startSquare, targetSquare: Location, attack: bool = true) =
    ## Like the other movePiece(), but with two locations
    self.movePiece(Move(startSquare: startSquare, targetSquare: targetSquare), attack)

proc doMove(self: ChessBoard, move: Move) =
    ## Internal function called by makeMove after
    ## performing legality checks. Can be used in 
    ## performance-critical paths where a move is
    ## already known to be legal
    
    # Record final position for future reference
    self.positions.add(self.position)

    # Final checks
    let piece = self.grid[move.startSquare.row, move.startSquare.col]

    var 
        halfMoveClock = self.position.halfMoveClock
        fullMoveCount = self.position.fullMoveCount
        castlingAvailable = self.position.castlingAvailable
        enPassantTarget = self.getEnPassantTarget()
    # Needed to detect draw by the 50 move rule
    if piece.kind == Pawn or move.isCapture():
        halfMoveClock = 0
    else:
        inc(halfMoveClock)
    if piece.color == Black:
        inc(fullMoveCount)
    
    # En passant check
    if enPassantTarget != emptyLocation():
        let enPassantPawn = enPassantTarget + piece.color.topSide()
        if self.grid[enPassantPawn.row, enPassantPawn.col].color == piece.color.opposite():
            enPassantTarget = emptyLocation()

    if move.isDoublePush():
        enPassantTarget = move.targetSquare + piece.color.bottomSide()

    # Castling check: have the rooks moved?
    if piece.kind == Rook:
        case piece.color:
            of White:
                if move.startSquare.row == piece.getStartRow():
                    if move.startSquare.col == 0:
                        # Queen side
                        castlingAvailable.white.queen = false
                    elif move.startSquare.col == 7:
                        # King side
                        castlingAvailable.white.king = false
            of Black:
                if move.startSquare.row == piece.getStartRow():
                    if move.startSquare.col == 0:
                        # Queen side
                        castlingAvailable.black.queen = false
                    elif move.startSquare.col == 7:
                        # King side
                        castlingAvailable.black.king = false
            else:
                discard
    # Has a rook been captured?
    if move.isCapture():
        let captured = self.grid[move.targetSquare.row, move.targetSquare.col]
        if captured.kind == Rook:
            case piece.color:
                of White:
                    if move.targetSquare == captured.color.queenSideRook():
                        # Queen side
                        castlingAvailable.white.queen = false
                    elif move.targetSquare == captured.color.kingSideRook():
                        # King side
                        castlingAvailable.white.king = false
                of Black:
                    if move.targetSquare == captured.color.queenSideRook():
                        # Queen side
                        castlingAvailable.black.queen = false
                    elif move.targetSquare == captured.color.kingSideRook():
                        # King side
                        castlingAvailable.black.king = false
                else:
                    # Unreachable
                    discard
    # Has the king moved?
    if piece.kind == King or move.isCastling():
        # Revoke all castling rights for the moving king
        case piece.color:
            of White:
                castlingAvailable.white.king = false
                castlingAvailable.white.queen = false
            of Black:
                castlingAvailable.black.king = false
                castlingAvailable.black.queen = false
            else:
                discard
    # Create new position
    self.position = Position(plyFromRoot: self.position.plyFromRoot + 1,
                             halfMoveClock: halfMoveClock,
                             fullMoveCount: fullMoveCount,
                             turn: self.getActiveColor().opposite,
                             castlingAvailable: castlingAvailable,
                             move: move,
                             pieces: self.position.pieces,
                             enPassantSquare: enPassantTarget
                            )
    # Update position metadata

    if move.isCastling():
        # Move the rook onto the
        # correct file when castling
        var 
            location: Location
            target: Location
        if move.getCastlingType() == CastleShort:
            location = piece.color.kingSideRook()
            target = shortCastleRook()
        else:
            location = piece.color.queenSideRook()
            target = longCastleRook()
        let rook = self.grid[location.row, location.col]
        let move = Move(startSquare: location, targetSquare: location + target, flags: move.flags)
        self.movePiece(move, attack=false)

    if move.isCapture():
        # Get rid of captured pieces
        self.removePiece(move.targetSquare, attack=false)

    if move.isEnPassant():
        # Make the en passant pawn disappear
        self.removePiece(move.targetSquare + piece.color.bottomSide(), attack=false)

    self.movePiece(move, attack=false)

    if move.isPromotion():
        # Move is a pawn promotion: get rid of the pawn
        # and spawn a new piece
        self.removePiece(move.targetSquare, attack=false)
        case move.getPromotionType():
            of PromoteToBishop:
                self.spawnPiece(move.targetSquare, Piece(kind: Bishop, color: piece.color))
            of PromoteToKnight:
                self.spawnPiece(move.targetSquare, Piece(kind: Knight, color: piece.color))
            of PromoteToRook:
                self.spawnPiece(move.targetSquare, Piece(kind: Rook, color: piece.color))
            of PromoteToQueen:
                self.spawnPiece(move.targetSquare, Piece(kind: Queen, color: piece.color))
            else:
                discard
    # Update attack metadata
    self.updateAttackedSquares()



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
    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)
                of King:
                    self.position.pieces.white.king = 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)
                of King:
                    self.position.pieces.black.king = location
                else:
                    discard
        else:
            # Unreachable
            discard
    self.grid[location.row, location.col] = piece


proc resetBoard*(self: ChessBoard) =
    ## Resets the internal grid representation
    ## according to the positional data stored
    ## in the chessboard. Warning: this can be
    ## expensive, especially in critical paths
    ## or tight loops
    for i in 0..63:
        self.grid[i] = emptyPiece()
    for loc in self.position.pieces.white.pawns:
        self.grid[loc.row, loc.col] = Piece(color: White, kind: Pawn)
    for loc in self.position.pieces.black.pawns:
        self.grid[loc.row, loc.col] = Piece(color: Black, kind: Pawn)
    for loc in self.position.pieces.white.bishops:
        self.grid[loc.row, loc.col] = Piece(color: White, kind: Bishop)
    for loc in self.position.pieces.black.bishops:
        self.grid[loc.row, loc.col] = Piece(color: Black, kind: Bishop)
    for loc in self.position.pieces.white.knights:
        self.grid[loc.row, loc.col] = Piece(color: White, kind: Knight)
    for loc in self.position.pieces.black.knights:
        self.grid[loc.row, loc.col] = Piece(color: Black, kind: Knight)
    for loc in self.position.pieces.white.rooks:
        self.grid[loc.row, loc.col] = Piece(color: White, kind: Rook)
    for loc in self.position.pieces.black.rooks:
        self.grid[loc.row, loc.col] = Piece(color: Black, kind: Rook)
    for loc in self.position.pieces.white.queens:
        self.grid[loc.row, loc.col] = Piece(color: White, kind: Queen)
    for loc in self.position.pieces.black.queens:
        self.grid[loc.row, loc.col] = Piece(color: Black, kind: Queen)
    self.grid[self.position.pieces.white.king.row, self.position.pieces.white.king.col] = Piece(color: White, kind: King)
    self.grid[self.position.pieces.black.king.row, self.position.pieces.black.king.col] = Piece(color: Black, kind: King)


proc undoLastMove*(self: ChessBoard) =
    if self.positions.len() == 0:
        return
    self.position = self.positions.pop()
    self.resetBoard()


proc isLegal(self: ChessBoard, move: Move): bool {.inline.} =
    ## Returns whether the given move is legal
    if self.grid[move.startSquare.row, move.startSquare.col].color != self.getActiveColor():
        return false
    return move in self.generateMoves(move.startSquare)


proc makeMove*(self: ChessBoard, move: Move): Move {.discardable.} =
    ## Makes a move on the board
    result = move
    if not self.isLegal(move):
        return emptyMove()
    self.doMove(move)


proc toChar*(piece: Piece): char =
    if piece.color == White:
        return char(piece.kind).toUpperAscii()
    return char(piece.kind)


proc `$`*(self: ChessBoard): string =
    result &= "- - - - - - - -"
    for i in 0..7:
        result &= "\n"
        for j in 0..7:
            let piece = self.grid[i, j]
            if piece.kind == Empty:
                result &= "x "
                continue
            result &= &"{piece.toChar()} "
        result &= &"{rankToColumn(i + 1) + 1}"
    result &= "\n- - - - - - - -"
    result &= "\na b c d e f g h"


proc toPretty*(piece: Piece): string =
    case piece.color:
        of Black:
            case piece.kind:
                of King:
                    return "\U2654"
                of Queen:
                    return "\U2655"
                of Rook:
                    return "\U2656"
                of Bishop:
                    return "\U2657"
                of Knight:
                    return "\U2658"
                of Pawn:
                    return "\U2659"
                else:
                    discard
        of White:
            case piece.kind:
                of King:
                    return "\U265A"
                of Queen:
                    return "\U265B"
                of Rook:
                    return "\U265C"
                of Bishop:
                    return "\U265D"
                of Knight:
                    return "\U265E"
                of Pawn:
                    return "\U265F"
                else:
                    discard
        else:
            discard


proc pretty*(self: ChessBoard): string =
    ## Returns a colorized version of the
    ## board for easier visualization
    for i in 0..7:
        if i > 0:
            result &= "\n"
        for j in 0..7:
            if ((i + j) mod 2) == 0:
                result &= "\x1b[39;44;1m"
            else:
                result &= "\x1b[39;40;1m"
            let piece = self.grid[i, j]
            if piece.kind == Empty:
                result &= "  \x1b[0m"
            else:
                result &= &"{piece.toPretty()} \x1b[0m"
        result &= &" \x1b[33;1m{rankToColumn(i + 1) + 1}\x1b[0m"

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



proc toFEN*(self: ChessBoard): string =
    ## Returns a FEN string of the current
    ## position in the chessboard
    var skip: int
    # Piece placement data
    for i in 0..7:
        skip = 0
        for j in 0..7:
            let piece = self.grid[i, j]
            if piece.kind == Empty:
                inc(skip)
            elif skip > 0:
                result &= &"{skip}{piece.toChar()}"
                skip = 0
            else:
                result &= piece.toChar()
        if skip > 0:
            result &= $skip
        if i < 7:
            result &= "/"
    result &= " "
    # Active color
    result &= (if self.getActiveColor() == White: "w" else: "b")
    result &= " "
    # Castling availability
    let castleWhite = self.position.castlingAvailable.white
    let castleBlack = self.position.castlingAvailable.black
    if not (castleBlack.king or castleBlack.queen or castleWhite.king or castleWhite.queen):
        result &= "-"
    else:
        if castleWhite.king:
            result &= "K"
        if castleWhite.queen:
            result &= "Q"
        if castleBlack.king:
            result &= "k"
        if castleBlack.queen:
            result &= "q"
    result &= " "
    # En passant target
    if self.getEnPassantTarget() == emptyLocation():
        result &= "-"
    else:
        result &= self.getEnPassantTarget().locationToAlgebraic()
    result &= " "
    # Halfmove clock
    result &= $self.getHalfMoveCount()
    result &= " "
    # Fullmove number
    result &= $self.getMoveCount()


proc perft*(self: ChessBoard, ply: int, verbose: bool = false, divide: bool = false, bulk: bool = false): CountData =
    ## Counts (and debugs) the number of legal positions reached after
    ## the given number of ply

    let moves = self.generateAllMoves()
    if len(moves) == 0:
        result.checkmates = 1
    if ply == 0:
        result.nodes = 1
        return
    if ply == 1 and bulk:
        if divide:
            var postfix = ""
            for move in moves:
                case move.getPromotionType():
                    of PromoteToBishop:
                        postfix = "b"
                    of PromoteToKnight:
                        postfix = "n"
                    of PromoteToRook:
                        postfix = "r"
                    of PromoteToQueen:
                        postfix = "q"
                    else:
                        postfix = ""
                echo &"{move.startSquare.locationToAlgebraic()}{move.targetSquare.locationToAlgebraic()}{postfix}: 1"
                if verbose:
                    echo ""
        return (uint64(len(moves)), 0, 0, 0, 0, 0, 0)

    for move in moves:
        if verbose:
            let canCastle = self.canCastle(self.getActiveColor())
            echo &"Ply (from root): {self.position.plyFromRoot}"
            echo &"Move: {move.startSquare.locationToAlgebraic()}{move.targetSquare.locationToAlgebraic()}, from ({move.startSquare.row}, {move.startSquare.col}) to ({move.targetSquare.row}, {move.targetSquare.col})"
            echo &"Turn: {self.getActiveColor()}"
            echo &"Piece: {self.grid[move.startSquare.row, move.startSquare.col].kind}"
            echo &"Flag: {move.flags}"
            echo &"In check: {(if self.inCheck(): \"yes\" else: \"no\")}"
            echo &"Can castle:\n  - King side: {(if canCastle.king: \"yes\" else: \"no\")}\n  - Queen side: {(if canCastle.queen: \"yes\" else: \"no\")}"
            echo &"Position before move: {self.toFEN()}"
            stdout.write("En Passant target: ")
            if self.getEnPassantTarget() != emptyLocation():
                echo self.getEnPassantTarget().locationToAlgebraic()
            else:
                echo "None"
            echo "\n", self.pretty()
        self.doMove(move)
        if move.isCapture():
            inc(result.captures)
        if move.isCastling():
            inc(result.castles)
        if move.isPromotion():
            inc(result.promotions)
        if move.isEnPassant():
            inc(result.enPassant)
        if self.inCheck():
            # Opponent king is in check
            inc(result.checks)
        if verbose:
            let canCastle = self.canCastle(self.getActiveColor())
            echo "\n"
            echo &"Opponent in check: {(if self.inCheck(): \"yes\" else: \"no\")}"
            echo &"Opponent can castle:\n  - King side: {(if canCastle.king: \"yes\" else: \"no\")}\n  - Queen side: {(if canCastle.queen: \"yes\" else: \"no\")}"
            echo &"Position after move: {self.toFEN()}"
            echo "\n", self.pretty()
            stdout.write("nextpos>> ")
            try:
                discard readLine(stdin)
            except IOError:
                discard
            except EOFError:
                discard
        let next = self.perft(ply - 1, verbose, bulk=bulk)
        self.undoLastMove()
        if divide and (not bulk or ply > 1):
            var postfix = ""
            if move.isPromotion():
                case move.getPromotionType():
                    of PromoteToBishop:
                        postfix = "b"
                    of PromoteToKnight:
                        postfix = "n"
                    of PromoteToRook:
                        postfix = "r"
                    of PromoteToQueen:
                        postfix = "q"
                    else:
                        discard
            echo &"{move.startSquare.locationToAlgebraic()}{move.targetSquare.locationToAlgebraic()}{postfix}: {next.nodes}"
            if verbose:
                echo ""
        result.nodes += next.nodes
        result.captures += next.captures
        result.checks += next.checks
        result.promotions += next.promotions
        result.castles += next.castles
        result.enPassant += next.enPassant
        result.checkmates += next.checkmates


proc handleGoCommand(board: ChessBoard, command: seq[string]) =
    if len(command) < 2:
        echo &"Error: go: invalid number of arguments"
        return
    case command[1]:
        of "perft":
            if len(command) == 2:
                echo &"Error: go: perft: invalid number of arguments"
                return
            var 
                args = command[2].splitWhitespace()
                bulk = false
                verbose = false
            if args.len() > 1:
                var ok = true
                for arg in args[1..^1]:
                    case arg:
                        of "bulk-count", "bulk":
                            bulk = true
                        of "verbose":
                            verbose = true
                        else:
                            echo &"Error: go: perft: invalid argument '{args[1]}'"
                            ok = false
                            break
                if not ok:
                    return
            try:
                let ply = parseInt(args[0])
                if bulk:
                    echo &"\nNodes searched (bulk-counting: on): {board.perft(ply, divide=true, bulk=true, verbose=verbose).nodes}\n"
                else:
                    let data = board.perft(ply, divide=true, verbose=verbose)
                    echo &"\nNodes searched (bulk-counting: off): {data.nodes}"
                    echo &"  - Captures: {data.captures}"
                    echo &"  - Checks: {data.checks}"
                    echo &"  - E.P: {data.enPassant}"
                    echo &"  - Checkmates: {data.checkmates}"
                    echo &"  - Castles: {data.castles}"
                    echo &"  - Promotions: {data.promotions}"
                    echo ""
            except ValueError:
                echo "Error: go: perft: invalid depth"
        else:
            echo &"Error: go: unknown subcommand '{command[1]}'"


proc handlePositionCommand(board: var ChessBoard, command: seq[string]) =
    case len(command):
        of 2:
            case command[1]:
                of "startpos":
                    board = newDefaultChessboard()
                of "current", "cur":
                    echo &"Current position: {board.toFEN()}"
                of "pretty":
                    echo board.pretty()
                of "print", "show":
                    echo board
                else:
                    echo &"Error: position: invalid argument '{command[1]}'"
        of 3:
            case command[1]:
                of "fen":
                    try:
                        board = newChessboardFromFEN(command[2])
                    except ValueError:
                        echo &"Error: position: invalid FEN string '{command[2]}': {getCurrentExceptionMsg()}"
                else:
                    echo &"Error: position: unknown subcommand '{command[1]}'"
        else:
            echo &"Error: position: invalid number of arguments"


proc handleMoveCommand(board: ChessBoard, command: seq[string]) =
    if len(command) != 2:
        echo &"Error: move: invalid number of arguments"
        return
    let moveString = command[1]
    if len(moveString) notin 4..5:
        echo &"Error: move: invalid move syntax"
        return
    var
        startSquare: Location
        targetSquare: Location
        flags: uint16
    
    try:
        startSquare = moveString[0..1].algebraicToLocation()
    except ValueError:
        echo &"Error: move: invalid start square"
        return
    try:
        targetSquare = moveString[2..3].algebraicToLocation()
    except ValueError:
        echo &"Error: move: invalid target square"
        return

    if board.grid[targetSquare.row, targetSquare.col].kind != Empty:
        flags = flags or Capture.uint16

    if board.grid[startSquare.row, startSquare.col].kind == Pawn and abs(startSquare.row - targetSquare.row) == 2:
        flags = flags or DoublePush.uint16

    if len(moveString) == 5:
        # Promotion
        case moveString[4]:
            of 'b':
                flags = flags or PromoteToBishop.uint16
            of 'n':
                flags = flags or PromoteToKnight.uint16
            of 'q':
                flags = flags or PromoteToQueen.uint16
            of 'r':
                flags = flags or PromoteToRook.uint16
            else:
                echo &"Error: move: invalid promotion type"
                return
    
    let move = Move(startSquare: startSquare, targetSquare: targetSquare, flags: flags)
    if board.makeMove(move) == emptyMove():
        echo "Error: move: illegal move"
    

proc main: int =
    ## Nimfish's control interface
    echo "Nimfish by nocturn9x (see LICENSE)"
    var board = newDefaultChessboard()
    while true:
        var
            cmd: seq[string]
            cmdStr: string
        try:
            stdout.write(">>> ")
            stdout.flushFile()
            cmdStr = readLine(stdin).strip(leading=true, trailing=true, chars={'\t', ' '})
            if cmdStr.len() == 0:
                continue
            cmd = cmdStr.splitWhitespace(maxsplit=2)

            case cmd[0]:
                of "clear":
                    echo "\x1Bc"
                of "help":
                    echo "TODO"
                of "go":
                    handleGoCommand(board, cmd)
                of "position":
                    handlePositionCommand(board, cmd)
                of "move":
                    handleMoveCommand(board, cmd)
                of "pretty":
                    echo board.pretty()
                of "undo":
                    board.undoLastMove()
                else:
                    echo &"Unknown command '{cmd[0]}'. Type 'help' for more information."
        except IOError:
            echo ""
            return -1
        except EOFError:
            echo ""
            return 0


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"

    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)
    setControlCHook(proc () {.noconv.} = quit(0))

    quit(main())