Class: PGN::Board

Inherits:
Object
  • Object
show all
Defined in:
lib/pgn/board.rb

Overview

Board represents the squares of a chess board and the pieces on each square. It is responsible for translating between a human readable format (white queen's rook on the bottom left) and the obvious internal representation (white queen's rook is position [0,0]). It takes care of converting square names (e4) to actual locations, and can convert to unicode chess pieces for display purposes.

Constant Summary collapse

START =

The starting, internal representation of a chess board

[
  ['R', 'P', nil, nil, nil, nil, 'p', 'r'],
  ['N', 'P', nil, nil, nil, nil, 'p', 'n'],
  ['B', 'P', nil, nil, nil, nil, 'p', 'b'],
  ['Q', 'P', nil, nil, nil, nil, 'p', 'q'],
  ['K', 'P', nil, nil, nil, nil, 'p', 'k'],
  ['B', 'P', nil, nil, nil, nil, 'p', 'b'],
  ['N', 'P', nil, nil, nil, nil, 'p', 'n'],
  ['R', 'P', nil, nil, nil, nil, 'p', 'r']
].freeze
FILE_TO_INDEX = 
('a'..'h').each_with_index.to_h
INDEX_TO_FILE = 
FILE_TO_INDEX.invert
RANK_TO_INDEX = 
('1'..'8').each_with_index.to_h
INDEX_TO_RANK = 
RANK_TO_INDEX.invert
KNIGHT_OFFS =

0x88 knight offsets (a1 + 33 = b3, etc.).

[33, 31, -31, -33, 18, 14, -14, -18].freeze
KING_OFFS = 
[-1, 1, -16, 16, -15, 15, -17, 17].freeze
KNIGHT_ATTACKS =

Precomputed on-board attack masks: entry idx is the frozen Array of on-board 0x88 target indices reachable from idx by that piece. Built once at load time so the per-call offset + off-board test is replaced by a direct array iteration.

Array.new(128) do |idx|
  next nil if (idx & 0x88) != 0 # rubocop:disable Style/BitwisePredicate

  KNIGHT_OFFS.each_with_object([]) do |off, a|
    t = idx + off
    a << t if (t & 0x88).zero? # rubocop:disable Style/BitwisePredicate
  end.freeze
end.freeze
KING_ATTACKS = 
Array.new(128) do |idx|
  next nil if (idx & 0x88) != 0 # rubocop:disable Style/BitwisePredicate

  KING_OFFS.each_with_object([]) do |off, a|
    t = idx + off
    a << t if (t & 0x88).zero? # rubocop:disable Style/BitwisePredicate
  end.freeze
end.freeze
UNICODE_PIECES =

algebraic to unicode piece lookup

{
  'k' => "\u{265A}",
  'q' => "\u{265B}",
  'r' => "\u{265C}",
  'b' => "\u{265D}",
  'n' => "\u{265E}",
  'p' => "\u{265F}",
  'K' => "\u{2654}",
  'Q' => "\u{2655}",
  'R' => "\u{2656}",
  'B' => "\u{2657}",
  'N' => "\u{2658}",
  'P' => "\u{2659}",
  nil => '_'
}.freeze

Instance Attribute Summary collapse

Class Method Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(squares) ⇒ Board

Returns a new instance of Board.

Examples:

PGN::Board.new(
  [
    ["R", "P", nil, nil, nil, nil, "p", "r"],
    ["N", "P", nil, nil, nil, nil, "p", "n"],
    ["B", "P", nil, nil, nil, nil, "p", "b"],
    ["Q", "P", nil, nil, nil, nil, "p", "q"],
    ["K", "P", nil, nil, nil, nil, "p", "k"],
    ["B", "P", nil, nil, nil, nil, "p", "b"],
    ["N", "P", nil, nil, nil, nil, "p", "n"],
    ["R", "P", nil, nil, nil, nil, "p", "r"],
  ]
)

Parameters:

  • squares (<Array<Array<String>>>) —

    the squares of the board



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# File 'lib/pgn/board.rb', line 111

def initialize(squares)
  self.squares = squares
end

Instance Attribute Details

#squares ⇒ Array<Array<String>>

Returns the board as a file-major 8x8 array (squares[rank]). Built on demand from the 0x88 array; equality with the START constant and other boards is preserved.

Returns:

  • (Array<Array<String>>) —

    the board as a file-major 8x8 array (squares[rank]). Built on demand from the 0x88 array; equality with the START constant and other boards is preserved.



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# File 'lib/pgn/board.rb', line 15

class Board
  # The starting, internal representation of a chess board
  #
  START = [
    ['R', 'P', nil, nil, nil, nil, 'p', 'r'],
    ['N', 'P', nil, nil, nil, nil, 'p', 'n'],
    ['B', 'P', nil, nil, nil, nil, 'p', 'b'],
    ['Q', 'P', nil, nil, nil, nil, 'p', 'q'],
    ['K', 'P', nil, nil, nil, nil, 'p', 'k'],
    ['B', 'P', nil, nil, nil, nil, 'p', 'b'],
    ['N', 'P', nil, nil, nil, nil, 'p', 'n'],
    ['R', 'P', nil, nil, nil, nil, 'p', 'r']
  ].freeze

  FILE_TO_INDEX = ('a'..'h').each_with_index.to_h
  INDEX_TO_FILE = FILE_TO_INDEX.invert

  RANK_TO_INDEX = ('1'..'8').each_with_index.to_h
  INDEX_TO_RANK = RANK_TO_INDEX.invert

  # 0x88 knight offsets (a1 + 33 = b3, etc.).
  KNIGHT_OFFS = [33, 31, -31, -33, 18, 14, -14, -18].freeze
  KING_OFFS = [-1, 1, -16, 16, -15, 15, -17, 17].freeze

  # Precomputed on-board attack masks: entry `idx` is the frozen Array of
  # on-board 0x88 target indices reachable from `idx` by that piece. Built
  # once at load time so the per-call offset + off-board test is replaced by
  # a direct array iteration.
  KNIGHT_ATTACKS = Array.new(128) do |idx|
    next nil if (idx & 0x88) != 0 # rubocop:disable Style/BitwisePredicate

    KNIGHT_OFFS.each_with_object([]) do |off, a|
      t = idx + off
      a << t if (t & 0x88).zero? # rubocop:disable Style/BitwisePredicate
    end.freeze
  end.freeze
  KING_ATTACKS = Array.new(128) do |idx|
    next nil if (idx & 0x88) != 0 # rubocop:disable Style/BitwisePredicate

    KING_OFFS.each_with_object([]) do |off, a|
      t = idx + off
      a << t if (t & 0x88).zero? # rubocop:disable Style/BitwisePredicate
    end.freeze
  end.freeze

  # algebraic to unicode piece lookup
  #
  UNICODE_PIECES = {
    'k' => "\u{265A}",
    'q' => "\u{265B}",
    'r' => "\u{265C}",
    'b' => "\u{265D}",
    'n' => "\u{265E}",
    'p' => "\u{265F}",
    'K' => "\u{2654}",
    'Q' => "\u{2655}",
    'R' => "\u{2656}",
    'B' => "\u{2657}",
    'N' => "\u{2658}",
    'P' => "\u{2659}",
    nil => '_'
  }.freeze

  # 0x88 board representation (see chess.js / the classic 0x88 move-generation
  # algorithm). A square is addressed by a single integer index
  # `rank * 16 + file`; the extra files/ranks make off-board detection a
  # single bitmask test -- `(idx & 0x88) != 0` -- which is faster than the
  # four-integer comparison a 0..7 bounds check needs, and lets ray
  # stepping be a single integer add. The public `squares` 8x8 API is built
  # from this array on demand (it is off the replay hot path), and the
  # MoveCalculator hot path works entirely in integer indices.
  #
  #   file = idx & 0x0F   (0..7)
  #   rank = idx >> 4      (0..7)

  # @return [PGN::Board] a board in the starting position
  #
  def self.start
    PGN::Board.new(START)
  end

  # @param squares [<Array<Array<String>>>] the squares of the board
  # @example
  #   PGN::Board.new(
  #     [
  #       ["R", "P", nil, nil, nil, nil, "p", "r"],
  #       ["N", "P", nil, nil, nil, nil, "p", "n"],
  #       ["B", "P", nil, nil, nil, nil, "p", "b"],
  #       ["Q", "P", nil, nil, nil, nil, "p", "q"],
  #       ["K", "P", nil, nil, nil, nil, "p", "k"],
  #       ["B", "P", nil, nil, nil, nil, "p", "b"],
  #       ["N", "P", nil, nil, nil, nil, "p", "n"],
  #       ["R", "P", nil, nil, nil, nil, "p", "r"],
  #     ]
  #   )
  #
  def initialize(squares)
    self.squares = squares
  end

  # @return [Array<Array<String>>] the board as a file-major 8x8 array
  #   (squares[file][rank]). Built on demand from the 0x88 array; equality
  #   with the START constant and other boards is preserved.
  #
  def squares
    (0..7).map { |f| (0..7).map { |r| @cells[(r * 16) + f] } }
  end

  def squares=(squares)
    @cells = Array.new(128)
    8.times do |f|
      8.times do |r|
        @cells[(r * 16) + f] = squares[f][r]
      end
    end
  end

  # @overload at(str)
  #   Looks up a piece based on the string representation of a square (e4)
  #   @param str [String] the square in algebraic notation
  # @overload at(file, rank)
  #   Looks up a piece based on zero-indexed coordinates (4, 3)
  #   @param file [Integer] the file the piece is on
  #   @param rank [Integer] the rank the piece is on
  # @return [String, nil] the piece on the square, or nil if it is
  #   empty
  # @example
  #   board.at(4,3)  #=> "P"
  #   board.at("e4") #=> "P"
  #
  def at(arg0, arg1 = nil)
    return at_index(index_for(arg0, arg1)) unless arg1.nil?

    at_index(index_of(arg0))
  end

  # @param changes [Hash<String, <String, nil>>] changes to make to the board
  # @return [self]
  # @example
  #   board.change!({"e2" => nil, "e4" => "P"})
  #
  def change!(changes)
    changes.each { |square, piece| update(square, piece) }
    self
  end

  # @param square [String] the square in algebraic notation
  # @param piece [String, nil] the piece to put on the square
  # @return [self]
  # @example
  #   board.update("e4", "P")
  #
  def update(square, piece)
    update_index(index_of(square), piece)
  end

  # @param position [String] the square in algebraic notation
  # @return [Array<Integer>] the coordinates of the square
  # @example
  #   board.coordinates_for("e4") #=> [4, 3]
  #
  def coordinates_for(position)
    [file_of(position), rank_of(position)]
  end

  # @param coordinates [Array<Integer>] the coordinates of the square
  # @return [String] the square in algebraic notation
  # @example
  #   board.position_for([4, 3]) #=> "e4"
  #
  def position_for(coordinates)
    file, rank = coordinates
    INDEX_TO_FILE[file] + INDEX_TO_RANK[rank]
  end

  # @return [String] the board in human readable format with unicode
  #   pieces
  #
  def inspect
    squares.transpose.reverse.map do |row|
      row.map { |chr| UNICODE_PIECES[chr] }.join(' ')
    end.join("\n")
  end

  # Build a {Board} directly from a 0x88 cell array, bypassing the 8x8
  # -> 0x88 conversion in {#initialize}. Used by {#dup} (which runs every
  # move) to skip the per-square rebuild; the cell array is already in the
  # canonical 128-cell layout.
  #
  # @param cells [Array<String, nil>] a 128-cell 0x88 array
  # @return [PGN::Board]
  #
  def self.from_cells(cells)
    board = allocate
    board.instance_variable_set(:@cells, cells)
    board
  end

  # @return [PGN::Board] a copy of self. Copies the 128-cell 0x88 array;
  #   mutations to the copy do not affect the original.
  #
  def dup
    self.class.from_cells(@cells.dup)
  end

  # -- 0x88 hot-path API (integer indices) ---------------------------------

  # The 0x88 index of an algebraic square name.
  #
  # @param square [String] e.g. "e4"
  # @return [Integer] idx = rank * 16 + file
  #
  def index_of(square)
    (rank_of(square) * 16) + file_of(square)
  end

  # The 0x88 index of zero-indexed file/rank coordinates.
  #
  # @return [Integer] idx = rank * 16 + file
  #
  def index_for(file, rank)
    (rank * 16) + file
  end

  # Looks up a piece by 0x88 index. The caller is responsible for having
  # already verified the index is on-board (`(idx & 0x88).zero?`); reading
  # an off-board index simply returns nil.
  #
  # @param idx [Integer] a 0x88 square index
  # @return [String, nil] the piece on that square
  #
  def at_index(idx)
    @cells[idx]
  end

  # Places a piece on a 0x88 index. Returns self.
  #
  # @param idx [Integer] a 0x88 square index
  # @param piece [String, nil]
  # @return [self]
  #
  def update_index(idx, piece)
    @cells[idx] = piece
    self
  end

  # Applies a batch of integer-indexed changes. The replay hot path uses
  # this so it never allocates square-name strings or `[file, rank]`
  # coordinate arrays.
  #
  # @param changes [Hash<Integer, <String, nil>>]
  # @return [self]
  #
  def apply!(changes)
    changes.each { |idx, piece| @cells[idx] = piece }
    self
  end

  # Whether a 0x88 index is on the board (see the class doc for the
  # bitmask this tests).
  #
  # @param idx [Integer] a 0x88 square index
  # @return [Boolean]
  #
  def on_board?(idx)
    (idx & 0x88).zero? # rubocop:disable Style/BitwisePredicate
  end

  # The algebraic square name of a 0x88 index.
  #
  # @param idx [Integer] a 0x88 square index
  # @return [String] e.g. "e4"
  #
  def square_name(idx)
    INDEX_TO_FILE[idx & 0x0F] + INDEX_TO_RANK[idx >> 4]
  end

  # Serializes the board to the FEN board-string portion (ranks 8→1,
  # files a→h, runs of empty squares collapsed to a digit) by walking
  # the 0x88 `@cells` array directly. This avoids rebuilding the 8x8
  # `squares` array on every FEN generation.
  #
  # @return [String] e.g. "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR"
  def fen_board_string
    rows = []
    7.downto(0) do |rank|
      s = String.new
      run = 0
      0.upto(7) do |file|
        piece = @cells[(rank * 16) + file]
        if piece.nil?
          run += 1
        else
          s << run.to_s if run.positive?
          run = 0
          s << piece
        end
      end
      s << run.to_s if run.positive?
      rows << s
    end
    rows.join('/')
  end

  # Boards are equal when every cell holds the same piece (or is
  # empty), including off-board padding, which is always nil on both.
  def eql?(other)
    other.is_a?(Board) && cells == other.cells
  end

  alias == eql?

  protected

  attr_reader :cells

  private

  def file_of(square)
    square.getbyte(0) - 97
  end

  def rank_of(square)
    square.getbyte(1) - 49
  end
end

Class Method Details

.from_cells(cells) ⇒ PGN::Board

Build a PGN::Board directly from a 0x88 cell array, bypassing the 8x8 -> 0x88 conversion in #initialize. Used by #dup (which runs every move) to skip the per-square rebuild; the cell array is already in the canonical 128-cell layout.

Parameters:

  • cells (Array<String, nil>) —

    a 128-cell 0x88 array

Returns:



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# File 'lib/pgn/board.rb', line 207

def self.from_cells(cells)
  board = allocate
  board.instance_variable_set(:@cells, cells)
  board
end

.start ⇒ PGN::Board

Returns a board in the starting position.

Returns:

  • (PGN::Board) —

    a board in the starting position



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# File 'lib/pgn/board.rb', line 92

def self.start
  PGN::Board.new(START)
end

Instance Method Details

#apply!(changes) ⇒ self

Applies a batch of integer-indexed changes. The replay hot path uses this so it never allocates square-name strings or [file, rank] coordinate arrays.

Parameters:

  • changes (Hash<Integer, <String, nil>>)

Returns:

  • (self) 


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# File 'lib/pgn/board.rb', line 268

def apply!(changes)
  changes.each { |idx, piece| @cells[idx] = piece }
  self
end

#at(str) ⇒ String? #at(file, rank) ⇒ String?

Returns the piece on the square, or nil if it is empty.

Examples:

board.at(4,3)  #=> "P"
board.at("e4") #=> "P"

Overloads:

  • #at(str) ⇒ String?

    Looks up a piece based on the string representation of a square (e4)

    Parameters:

    • str (String) —

      the square in algebraic notation

  • #at(file, rank) ⇒ String?

    Looks up a piece based on zero-indexed coordinates (4, 3)

    Parameters:

    • file (Integer) —

      the file the piece is on

    • rank (Integer) —

      the rank the piece is on

Returns:

  • (String, nil) —

    the piece on the square, or nil if it is empty



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# File 'lib/pgn/board.rb', line 145

def at(arg0, arg1 = nil)
  return at_index(index_for(arg0, arg1)) unless arg1.nil?

  at_index(index_of(arg0))
end

#at_index(idx) ⇒ String?

Looks up a piece by 0x88 index. The caller is responsible for having already verified the index is on-board ((idx & 0x88).zero?); reading an off-board index simply returns nil.

Parameters:

  • idx (Integer) —

    a 0x88 square index

Returns:

  • (String, nil) —

    the piece on that square



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# File 'lib/pgn/board.rb', line 246

def at_index(idx)
  @cells[idx]
end

#change!(changes) ⇒ self

Examples:

board.change!({"e2" => nil, "e4" => "P"})

Parameters:

  • changes (Hash<String, <String, nil>>) —

    changes to make to the board

Returns:

  • (self) 


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# File 'lib/pgn/board.rb', line 156

def change!(changes)
  changes.each { |square, piece| update(square, piece) }
  self
end

#coordinates_for(position) ⇒ Array<Integer>

Returns the coordinates of the square.

Examples:

board.coordinates_for("e4") #=> [4, 3]

Parameters:

  • position (String) —

    the square in algebraic notation

Returns:

  • (Array<Integer>) —

    the coordinates of the square



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# File 'lib/pgn/board.rb', line 176

def coordinates_for(position)
  [file_of(position), rank_of(position)]
end

#dup ⇒ PGN::Board

Returns a copy of self. Copies the 128-cell 0x88 array; mutations to the copy do not affect the original.

Returns:

  • (PGN::Board) —

    a copy of self. Copies the 128-cell 0x88 array; mutations to the copy do not affect the original.



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# File 'lib/pgn/board.rb', line 216

def dup
  self.class.from_cells(@cells.dup)
end

#eql?(other) ⇒ Boolean Also known as: ==

Boards are equal when every cell holds the same piece (or is empty), including off-board padding, which is always nil on both.

Returns:

  • (Boolean) 


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# File 'lib/pgn/board.rb', line 321

def eql?(other)
  other.is_a?(Board) && cells == other.cells
end

#fen_board_string ⇒ String

Serializes the board to the FEN board-string portion (ranks 8→1, files a→h, runs of empty squares collapsed to a digit) by walking the 0x88 @cells array directly. This avoids rebuilding the 8x8 squares array on every FEN generation.

Returns:

  • (String) —

    e.g. "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR"



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# File 'lib/pgn/board.rb', line 298

def fen_board_string
  rows = []
  7.downto(0) do |rank|
    s = String.new
    run = 0
    0.upto(7) do |file|
      piece = @cells[(rank * 16) + file]
      if piece.nil?
        run += 1
      else
        s << run.to_s if run.positive?
        run = 0
        s << piece
      end
    end
    s << run.to_s if run.positive?
    rows << s
  end
  rows.join('/')
end

#index_for(file, rank) ⇒ Integer

The 0x88 index of zero-indexed file/rank coordinates.

Returns:

  • (Integer) —

    idx = rank * 16 + file



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# File 'lib/pgn/board.rb', line 235

def index_for(file, rank)
  (rank * 16) + file
end

#index_of(square) ⇒ Integer

The 0x88 index of an algebraic square name.

Parameters:

  • square (String) —

    e.g. "e4"

Returns:

  • (Integer) —

    idx = rank * 16 + file



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# File 'lib/pgn/board.rb', line 227

def index_of(square)
  (rank_of(square) * 16) + file_of(square)
end

#inspect ⇒ String

Returns the board in human readable format with unicode pieces.

Returns:

  • (String) —

    the board in human readable format with unicode pieces



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# File 'lib/pgn/board.rb', line 193

def inspect
  squares.transpose.reverse.map do |row|
    row.map { |chr| UNICODE_PIECES[chr] }.join(' ')
  end.join("\n")
end

#on_board?(idx) ⇒ Boolean

Whether a 0x88 index is on the board (see the class doc for the bitmask this tests).

Parameters:

  • idx (Integer) —

    a 0x88 square index

Returns:

  • (Boolean) 


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# File 'lib/pgn/board.rb', line 279

def on_board?(idx)
  (idx & 0x88).zero? # rubocop:disable Style/BitwisePredicate
end

#position_for(coordinates) ⇒ String

Returns the square in algebraic notation.

Examples:

board.position_for([4, 3]) #=> "e4"

Parameters:

  • coordinates (Array<Integer>) —

    the coordinates of the square

Returns:

  • (String) —

    the square in algebraic notation



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# File 'lib/pgn/board.rb', line 185

def position_for(coordinates)
  file, rank = coordinates
  INDEX_TO_FILE[file] + INDEX_TO_RANK[rank]
end

#square_name(idx) ⇒ String

The algebraic square name of a 0x88 index.

Parameters:

  • idx (Integer) —

    a 0x88 square index

Returns:

  • (String) —

    e.g. "e4"



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# File 'lib/pgn/board.rb', line 288

def square_name(idx)
  INDEX_TO_FILE[idx & 0x0F] + INDEX_TO_RANK[idx >> 4]
end

#update(square, piece) ⇒ self

Examples:

board.update("e4", "P")

Parameters:

  • square (String) —

    the square in algebraic notation

  • piece (String, nil) —

    the piece to put on the square

Returns:

  • (self) 


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# File 'lib/pgn/board.rb', line 167

def update(square, piece)
  update_index(index_of(square), piece)
end

#update_index(idx, piece) ⇒ self

Places a piece on a 0x88 index. Returns self.

Parameters:

  • idx (Integer) —

    a 0x88 square index

  • piece (String, nil)

Returns:

  • (self) 


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# File 'lib/pgn/board.rb', line 256

def update_index(idx, piece)
  @cells[idx] = piece
  self
end