Four fives: Method B

rot90b.rb

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

class Rot90b
  def initialize
    @field_s = <<EOS
  **  
  **  
******
******
  **  
  **  
EOS
    init_field
  end

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

  def search(pos_start = 0)
    pos_start.upto(@width * @height - 1) do |i|
      x = i % @width
      y = i / @width
      if @field[y][x] == "*"
        @stack << [x, y]
        if @stack.length == @qstar
          if 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 |x, y|
      @field[y][x] = "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 = []
    s = @field_s.split(/\n/)
    s.each do |line|
      @field << line.split(//)
    end
  end

  def init_field
    reset_field
    @width = @field.map {|line_a| line_a.length}.max
    @height = @field.length
    if @width == 0 || @width != @height
      puts "The field is not square. Abort."
      exit(1)
    end
    if @width % 2 == 1 || @height % 2 == 1
      puts "The width and/or the height is odd. Abort."
      exit(1)
    end
    s = count_star
    if s == 0 || s % 4 > 0
      puts "The number of asterisk is #{s} and wrong. Abort."
      exit(1)
    end
    @x_center = (@width - 1) * 0.5
    @y_center = (@height - 1) * 0.5
  end

  def count_star
    @field.map {|line_a| line_a.join}.join.scan(/\*/).length
  end

  def print_field
    print *(@field.map {|line_a| line_a.join + "\n"})
  end

  def xy_rot90(x_from, y_from)
    x1 = x_from - @x_center
    y1 = y_from - @y_center
    x2 = y1
    y2 = -x1
    x_to = (x2 + @x_center).to_i
    y_to = (y2 + @y_center).to_i
    [x_to, y_to]
  end

  def rotate_superpose(c_from, c_to)
    @height.times do |y|
      @width.times do |x|
        if @field[y][x] == c_from
          x_to, y_to = xy_rot90(x, y)
          @field[y_to][x_to] = c_to
        end
      end
    end
  end
end

if __FILE__ == $0
  Rot90b.new.start
end