chesskit-swift/Sources/ChessKit/Bitboards/Bitboard.swift

//
//  Bitboard.swift
//  ChessKit
//

/// Contains `UInt64`-based utilities for manipulating
/// chess bitboards.
typealias Bitboard = UInt64

extension Bitboard {

  /// Bitboard representing all the squares on the A file.
  static let aFile: Bitboard = 0x0101010101010101
  /// Bitboard representing all the squares on the H file.
  static let hFile: Bitboard = aFile << 7

  /// Bitboard representing all the squares on the 1st rank.
  static let rank1: Bitboard = 0xFF
  /// Bitboard representing all the squares on the 8th rank.
  static let rank8: Bitboard = rank1 << (8 * 7)

  /// Bitboard representing all the dark squares on the board.
  static let dark: Bitboard = 0xAA55AA55AA55AA55
  /// Bitboard representing all the light squares on the board.
  static let light: Bitboard = ~dark

  /// Translates the receiver `n` column "east" on an 8x8 grid.
  ///
  /// `n` should be in the range `[1, 7]`.
  func east(_ n: Int = 1) -> Bitboard {
    (self & ~Self.hFile) << n
  }

  /// Translates the receiver `n` columns "west" on an 8x8 grid.
  ///
  /// `n` should be in the range `[1, 7]`.
  func west(_ n: Int = 1) -> Bitboard {
    (self & ~Self.aFile) >> n
  }

  /// Translates the receiver `n` rows "north" on an 8x8 grid.
  ///
  /// `n` should be in the range `[1, 7]`.
  func north(_ n: Int = 1) -> Bitboard {
    self << (8 * n)
  }

  /// Translates the receiver `n` rows "south" on an 8x8 grid.
  ///
  /// `n` should be in the range `[1, 7]`.
  func south(_ n: Int = 1) -> Bitboard {
    self >> (8 * n)
  }

  /// Translates the receiver `n` rows "north" and `n` columns "east" on an 8x8 grid.
  ///
  /// `n` should be in the range `[1, 7]`.
  func northEast(_ n: Int = 1) -> Bitboard {
    (self & ~Self.hFile) << (9 * n)
  }

  /// Translates the receiver `n` rows "north" and `n` columns "west" on an 8x8 grid.
  ///
  /// `n` should be in the range `[1, 7]`.
  func northWest(_ n: Int = 1) -> Bitboard {
    (self & ~Self.aFile) << (7 * n)
  }

  /// Translates the receiver `n` rows "south" and `n` columns "east" on an 8x8 grid.
  ///
  /// `n` should be in the range `[1, 7]`.
  func southEast(_ n: Int = 1) -> Bitboard {
    (self & ~Self.hFile) >> (7 * n)
  }

  /// Translates the receiver `n` rows "south" and `n` columns "west" on an 8x8 grid.
  ///
  /// `n` should be in the range `[1, 7]`.
  func southWest(_ n: Int = 1) -> Bitboard {
    (self & ~Self.aFile) >> (9 * n)
  }

}

extension Bitboard {

  /// Converts the `Bitboard` to an 8x8 board representation string.
  ///
  /// - parameter occupied: The character with which to represent occupied squares.
  /// - parameter empty: The character with which to represent unoccupied squares.
  /// - parameter labelRanks: Whether or not to label ranks (i.e. 1, 2, 3, ...).
  /// - parameter labelFiles: Whether or not to label ranks (i.e. a, b, c, ...).
  /// - returns: A string representing an 8x8 chess board.
  ///
  // periphery:ignore
  func chessString(
    _ occupied: Character = "⨯",
    _ empty: Character = "·",
    labelRanks: Bool = true,
    labelFiles: Bool = true
  ) -> String {
    var s = ""

    for rank in Square.Rank.range.reversed() {
      s += labelRanks ? "\(rank) " : ""
      var newLine = ""

      for file in Square.File.allCases {
        let sq = Square(file, .init(rank)).bb
        newLine += (self & sq != 0) ? "\(occupied) " : "\(empty) "
      }

      s += newLine.trimmingCharacters(in: .whitespaces) + "\n"
    }

    let fileLabels = (labelFiles ? "\n  a b c d e f g h" : "")
    return s.trimmingCharacters(in: .whitespacesAndNewlines) + fileLabels
  }

}

extension Bitboard {
  var squares: [Square] {
    var indices: [Int] = []
    var bb = self

    while bb != 0 {
      let index = bb.trailingZeroBitCount
      indices.append(index)
      bb &= bb &- 1
    }

    return indices.compactMap(Square.init)
  }
}