class Pawn

Pawn piece for a game of chess

Attributes

direction[R]
move_count[RW]

Public Class Methods

new(color, position) click to toggle source

Initializes a new pawn piece with color and position.

@param [String] color A string denoting the color of the piece. @param [Array<Integer>] position An integer array of length 2 denoting the location of the piece on the board.

Calls superclass method ChessPiece::new 
# File lib/chess_pieces/pawn.rb, line 20
def initialize(color, position)
  @move_count = 0
  @direction = color == 'white' ? 1 : -1
  @back_row = color == 'white' ? 7 : 0
  @move_tree_template = build_pawn_move_tree
  super(color == 'white' ? '♙'.white : '♙', color, position, 1)
end

Public Instance Methods

can_capture?(other_piece) click to toggle source

Returns whether the pawn can capture a piece at a given location based on its current position. This only possible if the opposing piece is in front of and diagonal to the current space.

@param [ChessPiece] other_piece The ChessPiece that is the proposed target. @return [true] if pawn can capture the piece.

# File lib/chess_pieces/pawn.rb, line 72
def can_capture?(other_piece)
  return false unless other_piece.is_a? ChessPiece

  return false if other_piece.color == @color

  occupied_position = other_piece.position
  in_front = occupied_position[0] == (@position[0] + @direction)
  diagonal = [@position[1] + 1, @position[1] - 1].include?(occupied_position[1])
  in_front && diagonal || en_passant(other_piece)
end
first_move?() click to toggle source

Returns whether or not the move the Pawn is on or just finished its first ever move.

@return [true] if the pawn is on or just finished its first move.

# File lib/chess_pieces/pawn.rb, line 61
def first_move?
  @move_count <= 1
end
move(to, player_type = 'human') click to toggle source

Moves the Pawn and updates @move_count If the pawn reaches the back row of the opposing side, it returns a queen.

@param [Array<Integer>] An integer array of length 2 denoting the new location of the pawn. A pawn if the pawn moves or a Queen if the pawn reaches the end of the board. @param [String] The type of player whose pawn it is. Used mainly for pawn promotion.

@return [ChessPiece] A pawn if the pawn moves or a Queen if the pawn reaches the end of the board.

Calls superclass method ChessPiece#move 
# File lib/chess_pieces/pawn.rb, line 44
def move(to, player_type = 'human')
  @move_count += 1

  if to[0] == @back_row
    piece_type = new_piece_type(player_type)
    # Pawn becomes a new piece
    return piece_type.new(@color, to)
  end

  super(to)
end
possible_moves() click to toggle source

Possible moves rewrite for the pawn

Calls superclass method ChessPiece#possible_moves 
# File lib/chess_pieces/pawn.rb, line 30
def possible_moves
  # Remove the two space move on the Pawn's first move
  @move_tree_template = @move_count.zero? ? build_pawn_move_tree_first_move : build_pawn_move_tree
  super
end

Protected Instance Methods

build_pawn_move_tree() click to toggle source

Builds the Pawn move tree without the two-space forward move. The pawn can move forward 2 spaces on its first move, but otherwise can only move one space forward. The Pawn may also move diagonally to capture an enemy piece.

@return [MoveTree] move_tree_template A move tree template for the pawn.

# File lib/chess_pieces/pawn.rb, line 180
def build_pawn_move_tree
  move_tree = MoveTree.new([0, 0])

  # Create changes based on @direction because pawns can only move one
  # direction.
  move_tree.root.add_child([@direction, 0])
  move_tree.root.add_child([@direction, 1])
  move_tree.root.add_child([@direction, -1])

  move_tree
end
build_pawn_move_tree_first_move() click to toggle source

Builds the Pawn move tree. The pawn can move forward 2 spaces on its first move, but otherwise can only move one space forward. The Pawn may also move diagonally to capture an enemy piece.

@return [MoveTree] move_tree_template A move tree template for the pawn.

# File lib/chess_pieces/pawn.rb, line 160
def build_pawn_move_tree_first_move
  move_tree = MoveTree.new([0, 0])

  # Create changes based on @direction because pawns can only move one
  # direction.
  move_tree.root.add_child([@direction, 0])
  move_tree.root.children[0].add_child([2 * @direction, 0])
  move_tree.root.add_child([@direction, 1])
  move_tree.root.add_child([@direction, -1])

  move_tree
end
en_passant(other_piece) click to toggle source

Returns whether the Pawn can capture another Pawn using the rule “En passant.” See the Wikipeida entry.

@param [ChessPiece] other_piece The ChessPiece that is the proposed target. @return [true] if the other piece is a capturable pawn using En Passant.

# File lib/chess_pieces/pawn.rb, line 141
def en_passant(other_piece)
  # Only applies to other Pawns
  return false unless other_piece.is_a? Pawn

  # Only applies when the other Pawn just finished its first move
  return false unless other_piece.first_move?

  # Check beside on the right
  beside = other_piece.position == [@position[0], @position[1] + 1]
  # Check beside on the left
  beside || (other_piece.position == [@position[0], @position[1] - 1])
end
new_piece_type(player_type = 'human') click to toggle source

Prompt the user for input to determine what piece to return when the pawn advances to the last square

@param [String] The type of player whose pawn it is. Used mainly for pawn promotion.

@return [Class] The type of piece the pawn is turning into.

# File lib/chess_pieces/pawn.rb, line 92
def new_piece_type(player_type = 'human')
  case player_type
  when 'human'
    puts 'Your pawn is being promoted! What peice would you like it to become?'
    print_piece_types
    player_input = $stdin.gets
    player_input = player_input&.chomp
    player_input = player_input&.downcase
  when 'random'
    player_input = %w[1 2 3 4].sample
  else
    player_input = '1'
  end

  return Queen if %w[1 queen q].include?(player_input)

  return Rook if %w[2 rook r].include?(player_input)

  return Bishop if %w[3 bishop b].include?(player_input)

  return Knight if %w[4 knight k].include?(player_input)

  raise(StandardError, "'#{player_input}' is not a valid input.")
rescue StandardError => e
  puts e.message
  puts 'Please select an option from the menu.'
  retry
end
print_piece_types() click to toggle source

Prints the menu for pieces the pawn can become.