Class: PGN::MoveCalculator

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

Overview

MoveCalculator is responsible for computing all of the ways that a specific move changes the current position. This includes which squares on the board need to be updated, new castling restrictions, the en passant square and whether to update fullmove and halfmove counters.

Constant Summary collapse

DIRECTIONS =

Specifies the movement of pieces who are allowed to move in a given direction until they reach an obstacle or the end of the board.

{
  'b' => [[1, 1], [-1,  1], [-1, -1], [1, -1]],
  'r' => [[-1, 0], [1,  0], [0, -1], [0,  1]],
  'q' => [[1,  1], [-1,  1], [-1, -1], [1, -1],
          [-1, 0], [1,  0], [0, -1], [0, 1]]
}.freeze
MOVES =

Specifies the movement of pieces that have a limited set of moves they are allowed to make.

{
  'k' => [[-1, -1], [0, -1], [1, -1], [1,  0],
          [1,  1], [0,  1], [-1,  1], [-1, 0]],
  'n' => [[-1, -2], [-1, 2], [1, -2], [1,  2],
          [-2, -1], [2, -1], [-2, 1], [2,  1]]
}.freeze
PAWN_MOVES =

Specifies possible pawn movements. It may seem backwards since it is used to compute the origin square and not the destination.

{
  'P' => {
    capture: [[-1, -1], [1, -1]],
    normal: [[0, -1]],
    double: [[0, -2]]
  },
  'p' => {
    capture: [[-1, 1], [1, 1]],
    normal: [[0,  1]],
    double: [[0,  2]]
  }
}.freeze
CASTLING =

The squares to update for each possible castling move.

{
  'Q' => {
    'a1' => nil,
    'c1' => 'K',
    'd1' => 'R',
    'e1' => nil
  },
  'K' => {
    'e1' => nil,
    'f1' => 'R',
    'g1' => 'K',
    'h1' => nil
  },
  'q' => {
    'a8' => nil,
    'c8' => 'k',
    'd8' => 'r',
    'e8' => nil
  },
  'k' => {
    'e8' => nil,
    'f8' => 'r',
    'g8' => 'k',
    'h8' => nil
  }
}.freeze
ROOK_RESTRICTIONS =

Frozen rook-origin -> castling-restriction lookup, shared by both white ('R') and black ('r') since their rook origins (a1/h1, a8/h8) are distinct keys. Replaces a per-call hash literal.

{ 'a1' => 'Q', 'h1' => 'K', 'a8' => 'q', 'h8' => 'k' }.freeze

Instance Attribute Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(board, move) ⇒ MoveCalculator

Returns a new instance of MoveCalculator.

Parameters:



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# File 'lib/pgn/move_calculator.rb', line 95

def initialize(board, move)
  self.board = board
  self.move  = move
  self.origin = compute_origin
end

Instance Attribute Details

#board ⇒ PGN::Board

Returns the current board.

Returns:



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# File 'lib/pgn/move_calculator.rb', line 18

class MoveCalculator
  # Specifies the movement of pieces who are allowed to move in a
  # given direction until they reach an obstacle or the end of the
  # board.
  #
  DIRECTIONS = {
    'b' => [[1, 1], [-1,  1], [-1, -1], [1, -1]],
    'r' => [[-1, 0], [1,  0], [0, -1], [0,  1]],
    'q' => [[1,  1], [-1,  1], [-1, -1], [1, -1],
            [-1, 0], [1,  0], [0, -1], [0, 1]]
  }.freeze

  # Specifies the movement of pieces that have a limited set of moves
  # they are allowed to make.
  #
  MOVES = {
    'k' => [[-1, -1], [0, -1], [1, -1], [1,  0],
            [1,  1], [0,  1], [-1,  1], [-1, 0]],
    'n' => [[-1, -2], [-1, 2], [1, -2], [1,  2],
            [-2, -1], [2, -1], [-2, 1], [2,  1]]
  }.freeze

  # Specifies possible pawn movements. It may seem backwards since it is
  # used to compute the origin square and not the destination.
  #
  PAWN_MOVES = {
    'P' => {
      capture: [[-1, -1], [1, -1]],
      normal: [[0, -1]],
      double: [[0, -2]]
    },
    'p' => {
      capture: [[-1, 1], [1, 1]],
      normal: [[0,  1]],
      double: [[0,  2]]
    }
  }.freeze

  # The squares to update for each possible castling move.
  #
  CASTLING = {
    'Q' => {
      'a1' => nil,
      'c1' => 'K',
      'd1' => 'R',
      'e1' => nil
    },
    'K' => {
      'e1' => nil,
      'f1' => 'R',
      'g1' => 'K',
      'h1' => nil
    },
    'q' => {
      'a8' => nil,
      'c8' => 'k',
      'd8' => 'r',
      'e8' => nil
    },
    'k' => {
      'e8' => nil,
      'f8' => 'r',
      'g8' => 'k',
      'h8' => nil
    }
  }.freeze

  # Frozen rook-origin -> castling-restriction lookup, shared by both
  # white ('R') and black ('r') since their rook origins (a1/h1, a8/h8)
  # are distinct keys. Replaces a per-call hash literal.
  ROOK_RESTRICTIONS = { 'a1' => 'Q', 'h1' => 'K', 'a8' => 'q', 'h8' => 'k' }.freeze

  attr_accessor :board, :move, :origin

  # @param board [PGN::Board] the current board
  # @param move [PGN::Move] the current move
  #
  def initialize(board, move)
    self.board = board
    self.move  = move
    self.origin = compute_origin
  end

  # @return [PGN::Board] the board after the move is made
  #
  def result_board
    new_board = board.dup
    new_board.change!(changes)

    new_board
  end

  # @return [Array<String>] which castling moves are no longer available
  #
  def castling_restrictions
    restrict = []

    # when a king or rook is moved
    case move.piece
    when 'K'
      restrict += %w[K Q]
    when 'k'
      restrict += %w[k q]
    when 'R', 'r'
      restrict << ROOK_RESTRICTIONS[origin]
    end

    # when castling occurs
    restrict += %w[K Q] if %w[K Q].include?(move.castle)
    restrict += %w[k q] if %w[k q].include?(move.castle)

    # when a rook is taken
    restrict << 'Q' if move.destination == 'a1'
    restrict << 'q' if move.destination == 'a8'
    restrict << 'K' if move.destination == 'h1'
    restrict << 'k' if move.destination == 'h8'

    restrict.empty? ? restrict : restrict.compact.uniq
  end

  # @return [Boolean] whether to increment the halfmove clock
  #
  def increment_halfmove?
    !(move.capture || move.pawn?)
  end

  # @return [Boolean] whether to increment the fullmove counter
  #
  def increment_fullmove?
    move.black?
  end

  # @return [String, nil] the en passant square if applicable
  #
  def en_passant_square
    return nil if move.castle

    return unless move.pawn? && (origin[1].to_i - move.destination[1].to_i).abs == 2

    if move.white?
      "#{origin[0]}3"
    else
      "#{origin[0]}6"
    end
  end

  private

  def changes
    changes = {}
    changes.merge!(CASTLING[move.castle]) if move.castle
    changes.merge!(
      origin => nil,
      move.destination => move.piece,
      en_passant_capture => nil
    )
    changes[move.destination] = move.promotion if move.promotion

    changes.reject! { |key, _| key.nil? or key.empty? }

    changes
  end

  # Using the current position and move, figure out where the piece
  # came from.
  #
  def compute_origin
    return nil if move.castle

    possibilities = case move.piece
                    when 'B', 'R', 'Q', 'b', 'r', 'q' then direction_origins
                    when 'K', 'N', 'k', 'n' then move_origins
                    when 'P', 'p' then pawn_origins
                    else # don't care move, used in variations
                      return nil
                    end

    possibilities = disambiguate(possibilities) if possibilities.length > 1

    board.position_for(possibilities.first)
  end

  # From the destination square, move in each direction stopping if we
  # reach the end of the board. If we encounter a piece, add it to the
  # list of origin possibilities if it is the moving piece, or else
  # check the next direction.
  #
  def direction_origins
    directions    = DIRECTIONS[move.piece.downcase]
    possibilities = []

    directions.each do |dir|
      square = first_piece(destination_coords, dir)
      possibilities << square if piece_at(square) == move.piece
    end

    possibilities
  end

  # From the destination square, make each move. If it is a valid
  # square and matches the moving piece, add it to the list of origin
  # possibilities.
  #
  def move_origins(moves = nil)
    moves         ||= MOVES[move.piece.downcase]
    possibilities   = []
    file, rank      = destination_coords

    moves.each do |i, j|
      f = file + i
      r = rank + j

      possibilities << [f, r] if valid_square?(f, r) && board.at(f, r) == move.piece
    end

    possibilities
  end

  # Computes the possbile pawn origins based on the destination square
  # and whether or not the move is a capture.
  #
  def pawn_origins
    _, rank     = destination_coords
    double_rank = (rank == 3 && move.white?) || (rank == 4 && move.black?)

    pawn_moves = PAWN_MOVES[move.piece]

    moves = move.capture ? pawn_moves[:capture] : pawn_moves[:normal]
    moves += pawn_moves[:double] if double_rank

    move_origins(moves)
  end

  def disambiguate(possibilities)
    possibilities = disambiguate_san(possibilities)
    possibilities = disambiguate_pawns(possibilities)            if possibilities.length > 1
    possibilities = disambiguate_discovered_check(possibilities) if possibilities.length > 1

    possibilities
  end

  # Try to disambiguate based on the standard algebraic notation.
  #
  def disambiguate_san(possibilities)
    if move.disambiguation
      possibilities.select { |p| board.position_for(p).match(move.disambiguation) }
    else
      possibilities
    end
  end

  # A pawn can't move two spaces if there is a pawn in front of it.
  #
  def disambiguate_pawns(possibilities)
    if move.piece.match(/p/i) && !move.capture
      possibilities.reject { |p| board.position_for(p).match(/2|7/) }
    else
      possibilities
    end
  end

  # A piece can't move if it would result in a discovered check.
  #
  def disambiguate_discovered_check(possibilities)
    king_pos = king_position

    DIRECTIONS.each do |attacking_piece, directions|
      attacking_piece = attacking_piece.upcase if move.black?

      directions.each do |dir|
        square = first_piece(king_pos, dir)
        next unless piece_at(square) == move.piece && possibilities.include?(square)

        next_square = first_piece(square, dir)
        possibilities.reject! { |p| p == square } if piece_at(next_square) == attacking_piece
      end
    end

    possibilities
  end

  # Walks from `from` in `direction` until it reaches the edge of the board
  # or the first occupied square. Returns that square's `[file, rank]`
  # coordinates, or `nil` if no piece was encountered before the edge. The
  # caller reads the piece off the board itself, so this avoids allocating
  # the `[piece, square]` wrapper tuple per direction scan.
  #
  def first_piece(from, direction)
    file, rank = from
    i,    j    = direction

    loop do
      file += i
      rank += j
      return nil if file.negative? || file > 7 || rank.negative? || rank > 7

      square = [file, rank]
      return square if board.at(file, rank)
    end
  end

  # Reads the piece on a square, tolerating a nil square (returned by
  # {#first_piece} when the scan ran off the edge). Kept as a helper so the
  # callers read the piece once instead of unpacking a `[piece, square]`
  # tuple per direction scan.
  #
  def piece_at(square)
    square && board.at(square[0], square[1])
  end

  # If the move is a capture and there is no piece on the
  # destination square, it must be an en passant capture.
  #
  def en_passant_capture
    return nil if move.castle

    move.destination[0] + origin[1] if !board.at(move.destination) && move.capture
  end

  def king_position
    king = move.white? ? 'K' : 'k'

    0.upto(7) do |file|
      0.upto(7) do |rank|
        return [file, rank] if board.at(file, rank) == king
      end
    end

    nil
  end

  def valid_square?(file, rank)
    file >= 0 && file < 8 && rank >= 0 && rank < 8
  end

  def destination_coords
    @destination_coords ||= board.coordinates_for(move.destination)
  end
end

#move ⇒ PGN::Move

Returns the current move.

Returns:



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# File 'lib/pgn/move_calculator.rb', line 18

class MoveCalculator
  # Specifies the movement of pieces who are allowed to move in a
  # given direction until they reach an obstacle or the end of the
  # board.
  #
  DIRECTIONS = {
    'b' => [[1, 1], [-1,  1], [-1, -1], [1, -1]],
    'r' => [[-1, 0], [1,  0], [0, -1], [0,  1]],
    'q' => [[1,  1], [-1,  1], [-1, -1], [1, -1],
            [-1, 0], [1,  0], [0, -1], [0, 1]]
  }.freeze

  # Specifies the movement of pieces that have a limited set of moves
  # they are allowed to make.
  #
  MOVES = {
    'k' => [[-1, -1], [0, -1], [1, -1], [1,  0],
            [1,  1], [0,  1], [-1,  1], [-1, 0]],
    'n' => [[-1, -2], [-1, 2], [1, -2], [1,  2],
            [-2, -1], [2, -1], [-2, 1], [2,  1]]
  }.freeze

  # Specifies possible pawn movements. It may seem backwards since it is
  # used to compute the origin square and not the destination.
  #
  PAWN_MOVES = {
    'P' => {
      capture: [[-1, -1], [1, -1]],
      normal: [[0, -1]],
      double: [[0, -2]]
    },
    'p' => {
      capture: [[-1, 1], [1, 1]],
      normal: [[0,  1]],
      double: [[0,  2]]
    }
  }.freeze

  # The squares to update for each possible castling move.
  #
  CASTLING = {
    'Q' => {
      'a1' => nil,
      'c1' => 'K',
      'd1' => 'R',
      'e1' => nil
    },
    'K' => {
      'e1' => nil,
      'f1' => 'R',
      'g1' => 'K',
      'h1' => nil
    },
    'q' => {
      'a8' => nil,
      'c8' => 'k',
      'd8' => 'r',
      'e8' => nil
    },
    'k' => {
      'e8' => nil,
      'f8' => 'r',
      'g8' => 'k',
      'h8' => nil
    }
  }.freeze

  # Frozen rook-origin -> castling-restriction lookup, shared by both
  # white ('R') and black ('r') since their rook origins (a1/h1, a8/h8)
  # are distinct keys. Replaces a per-call hash literal.
  ROOK_RESTRICTIONS = { 'a1' => 'Q', 'h1' => 'K', 'a8' => 'q', 'h8' => 'k' }.freeze

  attr_accessor :board, :move, :origin

  # @param board [PGN::Board] the current board
  # @param move [PGN::Move] the current move
  #
  def initialize(board, move)
    self.board = board
    self.move  = move
    self.origin = compute_origin
  end

  # @return [PGN::Board] the board after the move is made
  #
  def result_board
    new_board = board.dup
    new_board.change!(changes)

    new_board
  end

  # @return [Array<String>] which castling moves are no longer available
  #
  def castling_restrictions
    restrict = []

    # when a king or rook is moved
    case move.piece
    when 'K'
      restrict += %w[K Q]
    when 'k'
      restrict += %w[k q]
    when 'R', 'r'
      restrict << ROOK_RESTRICTIONS[origin]
    end

    # when castling occurs
    restrict += %w[K Q] if %w[K Q].include?(move.castle)
    restrict += %w[k q] if %w[k q].include?(move.castle)

    # when a rook is taken
    restrict << 'Q' if move.destination == 'a1'
    restrict << 'q' if move.destination == 'a8'
    restrict << 'K' if move.destination == 'h1'
    restrict << 'k' if move.destination == 'h8'

    restrict.empty? ? restrict : restrict.compact.uniq
  end

  # @return [Boolean] whether to increment the halfmove clock
  #
  def increment_halfmove?
    !(move.capture || move.pawn?)
  end

  # @return [Boolean] whether to increment the fullmove counter
  #
  def increment_fullmove?
    move.black?
  end

  # @return [String, nil] the en passant square if applicable
  #
  def en_passant_square
    return nil if move.castle

    return unless move.pawn? && (origin[1].to_i - move.destination[1].to_i).abs == 2

    if move.white?
      "#{origin[0]}3"
    else
      "#{origin[0]}6"
    end
  end

  private

  def changes
    changes = {}
    changes.merge!(CASTLING[move.castle]) if move.castle
    changes.merge!(
      origin => nil,
      move.destination => move.piece,
      en_passant_capture => nil
    )
    changes[move.destination] = move.promotion if move.promotion

    changes.reject! { |key, _| key.nil? or key.empty? }

    changes
  end

  # Using the current position and move, figure out where the piece
  # came from.
  #
  def compute_origin
    return nil if move.castle

    possibilities = case move.piece
                    when 'B', 'R', 'Q', 'b', 'r', 'q' then direction_origins
                    when 'K', 'N', 'k', 'n' then move_origins
                    when 'P', 'p' then pawn_origins
                    else # don't care move, used in variations
                      return nil
                    end

    possibilities = disambiguate(possibilities) if possibilities.length > 1

    board.position_for(possibilities.first)
  end

  # From the destination square, move in each direction stopping if we
  # reach the end of the board. If we encounter a piece, add it to the
  # list of origin possibilities if it is the moving piece, or else
  # check the next direction.
  #
  def direction_origins
    directions    = DIRECTIONS[move.piece.downcase]
    possibilities = []

    directions.each do |dir|
      square = first_piece(destination_coords, dir)
      possibilities << square if piece_at(square) == move.piece
    end

    possibilities
  end

  # From the destination square, make each move. If it is a valid
  # square and matches the moving piece, add it to the list of origin
  # possibilities.
  #
  def move_origins(moves = nil)
    moves         ||= MOVES[move.piece.downcase]
    possibilities   = []
    file, rank      = destination_coords

    moves.each do |i, j|
      f = file + i
      r = rank + j

      possibilities << [f, r] if valid_square?(f, r) && board.at(f, r) == move.piece
    end

    possibilities
  end

  # Computes the possbile pawn origins based on the destination square
  # and whether or not the move is a capture.
  #
  def pawn_origins
    _, rank     = destination_coords
    double_rank = (rank == 3 && move.white?) || (rank == 4 && move.black?)

    pawn_moves = PAWN_MOVES[move.piece]

    moves = move.capture ? pawn_moves[:capture] : pawn_moves[:normal]
    moves += pawn_moves[:double] if double_rank

    move_origins(moves)
  end

  def disambiguate(possibilities)
    possibilities = disambiguate_san(possibilities)
    possibilities = disambiguate_pawns(possibilities)            if possibilities.length > 1
    possibilities = disambiguate_discovered_check(possibilities) if possibilities.length > 1

    possibilities
  end

  # Try to disambiguate based on the standard algebraic notation.
  #
  def disambiguate_san(possibilities)
    if move.disambiguation
      possibilities.select { |p| board.position_for(p).match(move.disambiguation) }
    else
      possibilities
    end
  end

  # A pawn can't move two spaces if there is a pawn in front of it.
  #
  def disambiguate_pawns(possibilities)
    if move.piece.match(/p/i) && !move.capture
      possibilities.reject { |p| board.position_for(p).match(/2|7/) }
    else
      possibilities
    end
  end

  # A piece can't move if it would result in a discovered check.
  #
  def disambiguate_discovered_check(possibilities)
    king_pos = king_position

    DIRECTIONS.each do |attacking_piece, directions|
      attacking_piece = attacking_piece.upcase if move.black?

      directions.each do |dir|
        square = first_piece(king_pos, dir)
        next unless piece_at(square) == move.piece && possibilities.include?(square)

        next_square = first_piece(square, dir)
        possibilities.reject! { |p| p == square } if piece_at(next_square) == attacking_piece
      end
    end

    possibilities
  end

  # Walks from `from` in `direction` until it reaches the edge of the board
  # or the first occupied square. Returns that square's `[file, rank]`
  # coordinates, or `nil` if no piece was encountered before the edge. The
  # caller reads the piece off the board itself, so this avoids allocating
  # the `[piece, square]` wrapper tuple per direction scan.
  #
  def first_piece(from, direction)
    file, rank = from
    i,    j    = direction

    loop do
      file += i
      rank += j
      return nil if file.negative? || file > 7 || rank.negative? || rank > 7

      square = [file, rank]
      return square if board.at(file, rank)
    end
  end

  # Reads the piece on a square, tolerating a nil square (returned by
  # {#first_piece} when the scan ran off the edge). Kept as a helper so the
  # callers read the piece once instead of unpacking a `[piece, square]`
  # tuple per direction scan.
  #
  def piece_at(square)
    square && board.at(square[0], square[1])
  end

  # If the move is a capture and there is no piece on the
  # destination square, it must be an en passant capture.
  #
  def en_passant_capture
    return nil if move.castle

    move.destination[0] + origin[1] if !board.at(move.destination) && move.capture
  end

  def king_position
    king = move.white? ? 'K' : 'k'

    0.upto(7) do |file|
      0.upto(7) do |rank|
        return [file, rank] if board.at(file, rank) == king
      end
    end

    nil
  end

  def valid_square?(file, rank)
    file >= 0 && file < 8 && rank >= 0 && rank < 8
  end

  def destination_coords
    @destination_coords ||= board.coordinates_for(move.destination)
  end
end

#origin ⇒ String?

Returns the origin square in SAN.

Returns:

  • (String, nil) —

    the origin square in SAN



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# File 'lib/pgn/move_calculator.rb', line 18

class MoveCalculator
  # Specifies the movement of pieces who are allowed to move in a
  # given direction until they reach an obstacle or the end of the
  # board.
  #
  DIRECTIONS = {
    'b' => [[1, 1], [-1,  1], [-1, -1], [1, -1]],
    'r' => [[-1, 0], [1,  0], [0, -1], [0,  1]],
    'q' => [[1,  1], [-1,  1], [-1, -1], [1, -1],
            [-1, 0], [1,  0], [0, -1], [0, 1]]
  }.freeze

  # Specifies the movement of pieces that have a limited set of moves
  # they are allowed to make.
  #
  MOVES = {
    'k' => [[-1, -1], [0, -1], [1, -1], [1,  0],
            [1,  1], [0,  1], [-1,  1], [-1, 0]],
    'n' => [[-1, -2], [-1, 2], [1, -2], [1,  2],
            [-2, -1], [2, -1], [-2, 1], [2,  1]]
  }.freeze

  # Specifies possible pawn movements. It may seem backwards since it is
  # used to compute the origin square and not the destination.
  #
  PAWN_MOVES = {
    'P' => {
      capture: [[-1, -1], [1, -1]],
      normal: [[0, -1]],
      double: [[0, -2]]
    },
    'p' => {
      capture: [[-1, 1], [1, 1]],
      normal: [[0,  1]],
      double: [[0,  2]]
    }
  }.freeze

  # The squares to update for each possible castling move.
  #
  CASTLING = {
    'Q' => {
      'a1' => nil,
      'c1' => 'K',
      'd1' => 'R',
      'e1' => nil
    },
    'K' => {
      'e1' => nil,
      'f1' => 'R',
      'g1' => 'K',
      'h1' => nil
    },
    'q' => {
      'a8' => nil,
      'c8' => 'k',
      'd8' => 'r',
      'e8' => nil
    },
    'k' => {
      'e8' => nil,
      'f8' => 'r',
      'g8' => 'k',
      'h8' => nil
    }
  }.freeze

  # Frozen rook-origin -> castling-restriction lookup, shared by both
  # white ('R') and black ('r') since their rook origins (a1/h1, a8/h8)
  # are distinct keys. Replaces a per-call hash literal.
  ROOK_RESTRICTIONS = { 'a1' => 'Q', 'h1' => 'K', 'a8' => 'q', 'h8' => 'k' }.freeze

  attr_accessor :board, :move, :origin

  # @param board [PGN::Board] the current board
  # @param move [PGN::Move] the current move
  #
  def initialize(board, move)
    self.board = board
    self.move  = move
    self.origin = compute_origin
  end

  # @return [PGN::Board] the board after the move is made
  #
  def result_board
    new_board = board.dup
    new_board.change!(changes)

    new_board
  end

  # @return [Array<String>] which castling moves are no longer available
  #
  def castling_restrictions
    restrict = []

    # when a king or rook is moved
    case move.piece
    when 'K'
      restrict += %w[K Q]
    when 'k'
      restrict += %w[k q]
    when 'R', 'r'
      restrict << ROOK_RESTRICTIONS[origin]
    end

    # when castling occurs
    restrict += %w[K Q] if %w[K Q].include?(move.castle)
    restrict += %w[k q] if %w[k q].include?(move.castle)

    # when a rook is taken
    restrict << 'Q' if move.destination == 'a1'
    restrict << 'q' if move.destination == 'a8'
    restrict << 'K' if move.destination == 'h1'
    restrict << 'k' if move.destination == 'h8'

    restrict.empty? ? restrict : restrict.compact.uniq
  end

  # @return [Boolean] whether to increment the halfmove clock
  #
  def increment_halfmove?
    !(move.capture || move.pawn?)
  end

  # @return [Boolean] whether to increment the fullmove counter
  #
  def increment_fullmove?
    move.black?
  end

  # @return [String, nil] the en passant square if applicable
  #
  def en_passant_square
    return nil if move.castle

    return unless move.pawn? && (origin[1].to_i - move.destination[1].to_i).abs == 2

    if move.white?
      "#{origin[0]}3"
    else
      "#{origin[0]}6"
    end
  end

  private

  def changes
    changes = {}
    changes.merge!(CASTLING[move.castle]) if move.castle
    changes.merge!(
      origin => nil,
      move.destination => move.piece,
      en_passant_capture => nil
    )
    changes[move.destination] = move.promotion if move.promotion

    changes.reject! { |key, _| key.nil? or key.empty? }

    changes
  end

  # Using the current position and move, figure out where the piece
  # came from.
  #
  def compute_origin
    return nil if move.castle

    possibilities = case move.piece
                    when 'B', 'R', 'Q', 'b', 'r', 'q' then direction_origins
                    when 'K', 'N', 'k', 'n' then move_origins
                    when 'P', 'p' then pawn_origins
                    else # don't care move, used in variations
                      return nil
                    end

    possibilities = disambiguate(possibilities) if possibilities.length > 1

    board.position_for(possibilities.first)
  end

  # From the destination square, move in each direction stopping if we
  # reach the end of the board. If we encounter a piece, add it to the
  # list of origin possibilities if it is the moving piece, or else
  # check the next direction.
  #
  def direction_origins
    directions    = DIRECTIONS[move.piece.downcase]
    possibilities = []

    directions.each do |dir|
      square = first_piece(destination_coords, dir)
      possibilities << square if piece_at(square) == move.piece
    end

    possibilities
  end

  # From the destination square, make each move. If it is a valid
  # square and matches the moving piece, add it to the list of origin
  # possibilities.
  #
  def move_origins(moves = nil)
    moves         ||= MOVES[move.piece.downcase]
    possibilities   = []
    file, rank      = destination_coords

    moves.each do |i, j|
      f = file + i
      r = rank + j

      possibilities << [f, r] if valid_square?(f, r) && board.at(f, r) == move.piece
    end

    possibilities
  end

  # Computes the possbile pawn origins based on the destination square
  # and whether or not the move is a capture.
  #
  def pawn_origins
    _, rank     = destination_coords
    double_rank = (rank == 3 && move.white?) || (rank == 4 && move.black?)

    pawn_moves = PAWN_MOVES[move.piece]

    moves = move.capture ? pawn_moves[:capture] : pawn_moves[:normal]
    moves += pawn_moves[:double] if double_rank

    move_origins(moves)
  end

  def disambiguate(possibilities)
    possibilities = disambiguate_san(possibilities)
    possibilities = disambiguate_pawns(possibilities)            if possibilities.length > 1
    possibilities = disambiguate_discovered_check(possibilities) if possibilities.length > 1

    possibilities
  end

  # Try to disambiguate based on the standard algebraic notation.
  #
  def disambiguate_san(possibilities)
    if move.disambiguation
      possibilities.select { |p| board.position_for(p).match(move.disambiguation) }
    else
      possibilities
    end
  end

  # A pawn can't move two spaces if there is a pawn in front of it.
  #
  def disambiguate_pawns(possibilities)
    if move.piece.match(/p/i) && !move.capture
      possibilities.reject { |p| board.position_for(p).match(/2|7/) }
    else
      possibilities
    end
  end

  # A piece can't move if it would result in a discovered check.
  #
  def disambiguate_discovered_check(possibilities)
    king_pos = king_position

    DIRECTIONS.each do |attacking_piece, directions|
      attacking_piece = attacking_piece.upcase if move.black?

      directions.each do |dir|
        square = first_piece(king_pos, dir)
        next unless piece_at(square) == move.piece && possibilities.include?(square)

        next_square = first_piece(square, dir)
        possibilities.reject! { |p| p == square } if piece_at(next_square) == attacking_piece
      end
    end

    possibilities
  end

  # Walks from `from` in `direction` until it reaches the edge of the board
  # or the first occupied square. Returns that square's `[file, rank]`
  # coordinates, or `nil` if no piece was encountered before the edge. The
  # caller reads the piece off the board itself, so this avoids allocating
  # the `[piece, square]` wrapper tuple per direction scan.
  #
  def first_piece(from, direction)
    file, rank = from
    i,    j    = direction

    loop do
      file += i
      rank += j
      return nil if file.negative? || file > 7 || rank.negative? || rank > 7

      square = [file, rank]
      return square if board.at(file, rank)
    end
  end

  # Reads the piece on a square, tolerating a nil square (returned by
  # {#first_piece} when the scan ran off the edge). Kept as a helper so the
  # callers read the piece once instead of unpacking a `[piece, square]`
  # tuple per direction scan.
  #
  def piece_at(square)
    square && board.at(square[0], square[1])
  end

  # If the move is a capture and there is no piece on the
  # destination square, it must be an en passant capture.
  #
  def en_passant_capture
    return nil if move.castle

    move.destination[0] + origin[1] if !board.at(move.destination) && move.capture
  end

  def king_position
    king = move.white? ? 'K' : 'k'

    0.upto(7) do |file|
      0.upto(7) do |rank|
        return [file, rank] if board.at(file, rank) == king
      end
    end

    nil
  end

  def valid_square?(file, rank)
    file >= 0 && file < 8 && rank >= 0 && rank < 8
  end

  def destination_coords
    @destination_coords ||= board.coordinates_for(move.destination)
  end
end

Instance Method Details

#castling_restrictions ⇒ Array<String>

Returns which castling moves are no longer available.

Returns:

  • (Array<String>) —

    which castling moves are no longer available



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# File 'lib/pgn/move_calculator.rb', line 112

def castling_restrictions
  restrict = []

  # when a king or rook is moved
  case move.piece
  when 'K'
    restrict += %w[K Q]
  when 'k'
    restrict += %w[k q]
  when 'R', 'r'
    restrict << ROOK_RESTRICTIONS[origin]
  end

  # when castling occurs
  restrict += %w[K Q] if %w[K Q].include?(move.castle)
  restrict += %w[k q] if %w[k q].include?(move.castle)

  # when a rook is taken
  restrict << 'Q' if move.destination == 'a1'
  restrict << 'q' if move.destination == 'a8'
  restrict << 'K' if move.destination == 'h1'
  restrict << 'k' if move.destination == 'h8'

  restrict.empty? ? restrict : restrict.compact.uniq
end

#en_passant_square ⇒ String?

Returns the en passant square if applicable.

Returns:

  • (String, nil) —

    the en passant square if applicable



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# File 'lib/pgn/move_calculator.rb', line 152

def en_passant_square
  return nil if move.castle

  return unless move.pawn? && (origin[1].to_i - move.destination[1].to_i).abs == 2

  if move.white?
    "#{origin[0]}3"
  else
    "#{origin[0]}6"
  end
end

#increment_fullmove? ⇒ Boolean

Returns whether to increment the fullmove counter.

Returns:

  • (Boolean) —

    whether to increment the fullmove counter



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# File 'lib/pgn/move_calculator.rb', line 146

def increment_fullmove?
  move.black?
end

#increment_halfmove? ⇒ Boolean

Returns whether to increment the halfmove clock.

Returns:

  • (Boolean) —

    whether to increment the halfmove clock



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# File 'lib/pgn/move_calculator.rb', line 140

def increment_halfmove?
  !(move.capture || move.pawn?)
end

#result_board ⇒ PGN::Board

Returns the board after the move is made.

Returns:

  • (PGN::Board) —

    the board after the move is made



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# File 'lib/pgn/move_calculator.rb', line 103

def result_board
  new_board = board.dup
  new_board.change!(changes)

  new_board
end