checkers_piece.rb

class Piece #red on bottom, black moves first
  attr_accessor  :board, :pos, :king
  attr_reader  :color

  UPDELTAS = [[-1,1],[-1,-1]]
  DOWNDELTAS = [[1,1],[1,-1]]

  def initialize(color, board, pos, king = false)
    @color, @board, @pos, @king = color,board, pos, king
  end

  def possible_moves
    jump_moves.empty? ? slide_moves : jump_moves
  end

  def jump_moves
    my_deltas.map do |delta|
      slide = pos.add_delta(delta)
      next unless board.in_bounds?(slide) && enemy?(slide)
      slide.add_delta(delta)
    end.compact.keep_if { |jump| board.valid_pos?(jump) }
  end

  def slide_moves
    my_deltas.map do |delta|
      pos.add_delta(delta)
    end.keep_if { |pos| board.valid_pos?(pos) }
  end

  def enemy?(pos)#move to board?
    return false if board.empty?(pos)
    board[pos].color != color
  end



  def my_deltas
    return (UPDELTAS + DOWNDELTAS) if king
    color == :r ? UPDELTAS : DOWNDELTAS
  end

  def render
    player = color == :r ? "red" : "bla"
    king ? player.upcase : player
  end

end



class Array
  def add_delta(delta, &prc)
    self.dup.add_delta!(delta, &prc)
  end

  def add_delta!(delta, &prc)
    prc = Proc.new{ |x,y| x + y } unless prc
    self.each_with_index { |item,index| self[index] = prc.call(item,delta[index]) }
  end
end