Ugly checkers move rule method

assert_legal_direction.rb

def assert_legal_direction(dir)
  unless dir.all?{|coord| coord.abs == 1}
    raise InvalidMoveError.new "tried to move too many squares"
  end

  unless (dir[0] + dir[1]).even?
    raise InvalidMoveError.new "did not move diagonally"
  end

  unless king
    if wrong_way?(dir[0])
      raise InvalidMoveError.new "pawns can't move in that direction"
    end
  end

  nil
end