## Low-level handling of squares, board indeces and pieces
import std/strutils
import std/strformat


type
    Square* = distinct int8
        ## A square

    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


func nullPiece*: Piece {.inline.} = Piece(kind: Empty, color: None)
func nullSquare*: Square {.inline.} = Square(-1'i8)
func opposite*(c: PieceColor): PieceColor {.inline.} = (if c == White: Black else: White)
func isValid*(a: Square): bool {.inline.} = a.int8 in 0..63
func isLightSquare*(a: Square): bool {.inline.} = (a.int8 and 2) == 0
func `==`*(a, b: Square): bool {.inline.} = a.int8 == b.int8
func `!=`*(a, b: Square): bool {.inline.} = a.int8 != b.int8
func `-`*(a, b: Square): Square {.inline.} = Square(a.int8 - b.int8)
func `-`*(a: Square, b: SomeInteger): Square {.inline.} = Square(a.int8 - b.int8)
func `-`*(a: SomeInteger, b: Square): Square {.inline.} = Square(a.int8 - b.int8)


func colFromSquare*(square: Square): int8 = square.int8 mod 8 + 1
func rowFromSquare*(square: Square): int8 = square.int8 div 8 + 1



func makeSquare*(rank, file: SomeInteger): Square = Square(rank * 8 + file)


proc toSquare*(s: string): Square {.discardable.} =
    ## Converts a square square 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]}')")

    return Square((s[0].uint8 - uint8('a')) + ((s[1].uint8 - uint8('1')) xor 7) * 8)


proc toAlgebraic*(square: Square): string {.inline.} =
    ## Converts a square from our internal rank/file
    ## notation to a square in algebraic notation
    let 
        file = char('a'.uint8 + (square.uint64 and 7))
        rank = char('1'.uint8 + ((square.uint64 div 8) xor 7))
    return &"{file}{rank}"


proc `$`*(square: Square): string = square.toAlgebraic()