Four fives: Method C

rot90c.rb

#!/usr/bin/env ruby
# -*- coding: utf-8 -*-

class Rot90c
  def initialize
    @field_s = (<<EOS).gsub(/\n/, "")
  **  
  **  
******
******
  **  
  **  
EOS
    reset_field
    @width = @height = 6
    @field_size = @width * @height
    setup_coord
    @pos_1 = @width * (@height / 2 - 1) + @width / 2 - 1
  end

  def start
    print_field
    @qstar = count_star / 4
    @stack = [] # array of position
    @num_answer = 0
    search
  end

  def setup_coord
    @coord1 = {} # {0, 1, ..., @width-1} => {-@width/2, ..., -1, 1, ..., @width/2}
    @coord2 = {} # inverse mapping of @coord1
    half = @width / 2
    @width.times do |i|
      j = i - half
      j += 1 if i >= half
      @coord1[i] = j
      @coord2[j] = i
    end
  end

  def search(pos_start = 0)
    pos_start.upto(@field_size - 1) do |i|
      if @field[i, 1] == "*"
        @stack << i
        if @stack.length == @qstar
          if @stack.index(@pos_1) && examine_field
            @num_answer += 1
            puts "Answer No.#{@num_answer}"
            print_field
          end
          reset_field
        else
          search(i + 1)
        end
        @stack.pop
      end
    end
  end

  def examine_field
    reset_field
    @stack.each do |pos|
      @field[pos, 1] = "1"
    end
    rotate_superpose("1", "2")
    rotate_superpose("2", "3")
    rotate_superpose("3", "4")

    # If there is no "*" in the field, then that is a solution!
    count_star == 0
  end

  def reset_field
    @field = @field_s.dup
  end

  def count_star
    @field.scan(/\*/).length
  end

  def print_field
    @height.times do |i|
      puts @field[@height * i, @width]
    end
  end

  def xy_rot90(x_from, y_from)
    x1 = @coord1[x_from]
    y1 = @coord1[y_from]
    x2 = y1
    y2 = -x1
    x_to = @coord2[x2]
    y_to = @coord2[y2]
    [x_to, y_to]
  end

  def rotate_superpose(c_from, c_to)
    @field_size.times do |pos|
      if @field[pos, 1] == c_from
        y, x = pos.divmod(@width)
        x_to, y_to = xy_rot90(x, y)
        @field[@width * y_to + x_to] = c_to
      end
    end
  end
end

if __FILE__ == $0
  Rot90c.new.start
end