sequence.rb

class Sequence
  
  class << self 
    
    def sequences   
      Enumerator.new do |y|
        sequence = Sequence.new
        while sequence
          sequence = sequence.next
          break unless sequence
          y << sequence
        end
      end
    end
    
  end
  
  attr_accessor :elements, :status
  
  def initialize(*elements)
    @elements = elements
    @status = ""
  end
  
  def sum
    elements.inject { |sum, n| sum + n }
  end
  
  def histogram 
    @histogram ||= begin
      @histogram = Hash.new(0)
      elements.each do |digit|
        @histogram[digit] += 1
      end
      @histogram
    end
  end
  
  def has_digit_pair?
    histogram.values.any? { |x| x == 2 }
  end
  
  def has_digit_trio?
    histogram.values.any? { |x| x > 2 }
  end
  
  def valid?
    !has_digit_trio? && has_digit_pair? && sum == 15
  end
  
  def next
    return Sequence.new(1) if elements.empty? 
    if has_digit_trio?
      elements[-1] += 1
      return Sequence.new(*elements)
    elsif sum < 15
      return Sequence.new(*(elements << elements[-1]))
    elsif sum >= 15
      new_elements = elements[0 .. -2]
      if new_elements[-1] == 9
        return nil if new_elements.size == 1
        new_elements = new_elements[0 .. -2]
      end
      new_elements[-1] += 1
      return Sequence.new(*new_elements)
    end
  end
  
end

if __FILE__ == $0
  result = Sequence.sequences.select { |s| s.valid? }
  p result.size
  p result.map(&:elements)
end