## Implements low-level bit operations


import std/bitops
import std/strutils


import pieces
import moves


type
    Bitboard* = distinct uint64
        ## A bitboard

    Direction* = enum
        ## A move direction enumeration
        Forward,
        Backward,
        Left,
        Right
        ForwardLeft,
        ForwardRight,
        BackwardLeft,
        BackwardRight

# Overloaded operators and functions for our bitboard type
func `shl`*(a: Bitboard, x: Natural): Bitboard = Bitboard(a.uint64 shl x)
func `shr`*(a: Bitboard, x: Natural): Bitboard = Bitboard(a.uint64 shr x)
func `and`*(a, b: Bitboard): Bitboard = Bitboard(a.uint64 and b.uint64)
func `or`*(a, b: Bitboard): Bitboard = Bitboard(a.uint64 or b.uint64)
func `not`*(a: Bitboard): Bitboard = Bitboard(not a.uint64)
func `shr`*(a, b: Bitboard): Bitboard = Bitboard(a.uint64 and b.uint64)
func `xor`*(a, b: Bitboard): Bitboard = Bitboard(a.uint64 xor b.uint64)
func `+`*(a, b: Bitboard): Bitboard = Bitboard(a.uint64 + b.uint64)
func `-`*(a, b: Bitboard): Bitboard = Bitboard(a.uint64 - b.uint64)
func `div`*(a, b: Bitboard): Bitboard = Bitboard(a.uint64 div b.uint64)
func `*`*(a, b: Bitboard): Bitboard = Bitboard(a.uint64 * b.uint64)
func `+`*(a: Bitboard, b: SomeUnsignedInt): Bitboard = Bitboard(a.uint64 + b)
func `-`*(a: Bitboard, b: SomeUnsignedInt): Bitboard = Bitboard(a.uint64 - b.uint64)
func `div`*(a: Bitboard, b: SomeUnsignedInt): Bitboard = Bitboard(a.uint64 div b)
func `*`*(a: Bitboard, b: SomeUnsignedInt): Bitboard = Bitboard(a.uint64 * b)
func `*`*(a: SomeUnsignedInt, b: Bitboard): Bitboard = Bitboard(a.uint64 * b)
func `==`*(a, b: Bitboard): bool {.inline.} = a.uint64 == b.uint64
func `==`*(a: Bitboard, b: SomeInteger): bool {.inline.} = a.uint64 == b.uint64
func `!=`*(a, b: Bitboard): bool {.inline.} = a.uint64 != b.uint64
func `!=`*(a: Bitboard, b: SomeInteger): bool {.inline.} = a.uint64 != b.uint64


func getFileMask*(file: int): Bitboard = Bitboard(0x101010101010101'u64) shl file.uint64
func getRankMask*(rank: int): Bitboard = Bitboard(0xff) shl uint64(8 * rank)
func toBitboard*(square: SomeInteger): Bitboard = Bitboard(1'u64) shl square.uint64
func toBitboard*(square: Square): Bitboard = toBitboard(square.int8)

proc toSquare*(b: Bitboard): Square = Square(b.uint64.countTrailingZeroBits())
func createMove*(startSquare: Bitboard, targetSquare: Square, flags: varargs[MoveFlag]): Move =
    result = createMove(startSquare.toSquare(), targetSquare, flags)


func createMove*(startSquare: Square, targetSquare: Bitboard, flags: varargs[MoveFlag]): Move = 
    result = createMove(startSquare, targetSquare.toSquare(), flags)


func createMove*(startSquare, targetSquare: Bitboard, flags: varargs[MoveFlag]): Move =
    result = createMove(startSquare.toSquare(), targetSquare.toSquare(), flags)


func toBin*(x: Bitboard, b: Positive = 64): string = toBin(BiggestInt(x), b)
func toBin*(x: uint64, b: Positive = 64): string = toBin(Bitboard(x), b)

func contains*(self: Bitboard, square: Square): bool = (self and square.toBitboard()) != 0


iterator items*(self: Bitboard): Square =
    ## Iterates ove the given bitboard
    ## and returns all the squares that 
    ## are set
    var bits = self
    while bits != 0:
        yield bits.toSquare()
        bits = bits and bits - 1


iterator subsets*(self: Bitboard): Bitboard =
    ## Iterates over all the subsets of the given
    ## bitboard using the Carry-Rippler trick

    # Thanks analog-hors :D
    var subset = Bitboard(0)
    while true:
        subset = (subset - self) and self
        if subset == 0:
            break
        yield subset
    

iterator pairs*(self: Bitboard): tuple[i: int, sq: Square] =
    var i = 0
    for item in self:
        yield (i, item)
        inc(i)


func pretty*(self: Bitboard): string =

    iterator items(self: Bitboard): uint8 =
        ## Iterates over all the bits in the
        ## given bitboard
        for i in 0..63:
            yield self.uint64.bitsliced(i..i).uint8


    iterator pairs(self: Bitboard): (int, uint8) =
        var i = 0
        for bit in self:
            yield (i, bit)
            inc(i)

    ## Returns a prettyfied version of
    ## the given bitboard
    result &= "- - - - - - - -\n"
    for i, bit in self:
        if i > 0 and i mod 8 == 0:
            result &= "\n"
        result &= $bit & " "
    result &= "\n- - - - - - - -"


func `$`*(self: Bitboard): string = self.pretty()


func getDirectionMask*(bitboard: Bitboard, color: PieceColor, direction: Direction): Bitboard =
    ## Get a bitmask relative to the given bitboard 
    ## for the given direction for a piece of the 
    ## given color 
    case color:
        of White:
            case direction:
                of Forward:
                    return bitboard shr 8
                of Backward:
                    return bitboard shl 8
                of ForwardRight:
                    return bitboard shr 7
                of ForwardLeft:
                    return bitboard shr 9
                of BackwardRight:
                    return bitboard shl 9
                of BackwardLeft:
                    return bitboard shl 7
                of Left:
                    return bitboard shr 1
                of Right:
                    return bitboard shl 1
        of Black:
            # The directions for black are just the opposite of those for white,
            # so we avoid duplicating any code
            case direction:
                of Forward:
                    return bitboard shl 8
                of Backward:
                    return bitboard shr 8
                of ForwardLeft:
                    return bitboard shl 9
                of ForwardRight:
                    return bitboard shl 7
                of BackwardRight:
                    return bitboard shr 9
                of BackwardLeft:
                    return bitboard shr 7
                of Left:
                    return bitboard shl 1
                of Right:
                    return bitboard shr 1
        else:
            discard


func getLastRank*(color: PieceColor): Bitboard = (if color == White: getRankMask(0) else: getRankMask(7))
func getFirstRank*(color: PieceColor): Bitboard = (if color == White: getRankMask(7) else: getRankMask(0))
func getLeftmostFile*(color: PieceColor): Bitboard = (if color == White: getFileMask(0) else: getFileMask(7))
func getRightmostFile*(color: PieceColor): Bitboard = (if color == White: getFileMask(7) else: getFileMask(0))


func getDirectionMask*(square: Square, color: PieceColor, direction: Direction): Bitboard =
    ## Get a bitmask for the given direction for a piece
    ## of the given color located at the given square
    result = getDirectionMask(toBitboard(square), color, direction)


func forwardRelativeTo*(self: Bitboard, side: PieceColor): Bitboard = getDirectionMask(self, side, Forward)
func doubleForwardRelativeTo*(self: Bitboard, side: PieceColor): Bitboard = self.forwardRelativeTo(side).forwardRelativeTo(side)

func backwardRelativeTo*(self: Bitboard, side: PieceColor): Bitboard = getDirectionMask(self, side, Backward)
func doubleBackwardRelativeTo*(self: Bitboard, side: PieceColor): Bitboard = self.backwardRelativeTo(side).backwardRelativeTo(side)

func leftRelativeTo*(self: Bitboard, side: PieceColor): Bitboard = getDirectionMask(self, side, Left) and not getRightmostFile(side)
func rightRelativeTo*(self: Bitboard, side: PieceColor): Bitboard = getDirectionMask(self, side, Right) and not getLeftmostFile(side)


# We mask off the opposide files to make sure there are
# no weird wraparounds when moving diagonally
func forwardRightRelativeTo*(self: Bitboard, side: PieceColor): Bitboard = 
    getDirectionMask(self, side, ForwardRight) and not getLeftmostFile(side)


func forwardLeftRelativeTo*(self: Bitboard, side: PieceColor): Bitboard = 
    getDirectionMask(self, side, ForwardLeft) and not getRightmostFile(side)


func backwardRightRelativeTo*(self: Bitboard, side: PieceColor): Bitboard =
    getDirectionMask(self, side, BackwardRight) and not getLeftmostFile(side)


func backwardLeftRelativeTo*(self: Bitboard, side: PieceColor): Bitboard =
    getDirectionMask(self, side, BackwardLeft) and not getRightmostFile(side)


func longKnightUpLeftRelativeTo*(self: Bitboard, side: PieceColor): Bitboard = self.doubleForwardRelativeTo(side).leftRelativeTo(side)
func longKnightUpRightRelativeTo*(self: Bitboard, side: PieceColor): Bitboard = self.doubleForwardRelativeTo(side).rightRelativeTo(side)
func longKnightDownLeftRelativeTo*(self: Bitboard, side: PieceColor): Bitboard = self.doubleBackwardRelativeTo(side).leftRelativeTo(side)
func longKnightDownRightRelativeTo*(self: Bitboard, side: PieceColor): Bitboard = self.doubleBackwardRelativeTo(side).rightRelativeTo(side)

func shortKnightUpLeftRelativeTo*(self: Bitboard, side: PieceColor): Bitboard = self.forwardRelativeTo(side).leftRelativeTo(side).leftRelativeTo(side)
func shortKnightUpRightRelativeTo*(self: Bitboard, side: PieceColor): Bitboard = self.forwardRelativeTo(side).rightRelativeTo(side).rightRelativeTo(side)
func shortKnightDownLeftRelativeTo*(self: Bitboard, side: PieceColor): Bitboard = self.backwardRelativeTo(side).leftRelativeTo(side).leftRelativeTo(side)
func shortKnightDownRightRelativeTo*(self: Bitboard, side: PieceColor): Bitboard = self.backwardRelativeTo(side).rightRelativeTo(side).rightRelativeTo(side)

# We precompute as much stuff as possible: lookup tables are fast!


func computeKingBitboards: array[64, Bitboard] =
    ## Precomputes all the movement bitboards for the king
    for i in 0'u64..63:
        let king = i.toBitboard()
        # It doesn't really matter which side we generate
        # the move for, they're identical for both
        var movements = king.forwardRelativeTo(White)
        movements = movements or king.forwardLeftRelativeTo(White)
        movements = movements or king.leftRelativeTo(White)
        movements = movements or king.rightRelativeTo(White)
        movements = movements or king.backwardRelativeTo(White)
        movements = movements or king.forwardLeftRelativeTo(White)
        movements = movements or king.backwardRightRelativeTo(White)
        movements = movements or king.backwardLeftRelativeTo(White)
        # We don't *need* to mask the king off: the engine already masks off
        # the board's occupancy when generating moves, but it may be useful for
        # other parts of the movegen for this stuff not to say "the king can just
        # stay still", so we do it anyway
        movements = movements and not king
        result[i] = movements


func computeKnightBitboards: array[64, Bitboard] =
    ## Precomputes all the movement bitboards for knights
    for i in 0'u64..63:
        let knight = i.toBitboard()
        # It doesn't really matter which side we generate
        # the move for, they're identical for both
        var movements = knight.longKnightDownLeftRelativeTo(White)
        movements = movements or knight.longKnightDownRightRelativeTo(White)
        movements = movements or knight.longKnightUpLeftRelativeTo(White)
        movements = movements or knight.longKnightUpRightRelativeTo(White)
        movements = movements or knight.shortKnightDownLeftRelativeTo(White)
        movements = movements or knight.shortKnightDownRightRelativeTo(White)
        movements = movements or knight.shortKnightUpLeftRelativeTo(White)
        movements = movements or knight.shortKnightUpRightRelativeTo(White)
        result[i] = movements


# For some reason nim freaks out if I try to call computeKingBitboards()
# at compile-time. ¯\_(ツ)_/¯
let 
    KING_BITBOARDS* = computeKingBitboards()
    KNIGHT_BITBOARDS* = computeKnightBitboards()