14_collatz_sequence.rb

module Collatz
  class << self
    attr_reader :lengths

    def length_recursive(current, counter=0)
      counter += 1
      if current == 1
        counter
      elsif @lengths.key? current
        @lengths[current] + counter # Take advantage of already working out the length from this number.
      else
        next_num = current.even? ? current / 2 : current * 3 + 1
        length_recursive next_num, counter
      end
    end
    private :length_recursive

    def length(initial)
      @lengths ||= Hash.new {|h, k| h[k] = length_recursive k }
 
      @lengths[initial]
    end

    def longest(range)
      range.max_by {|n| length n }
    end
  end
end

puts "Answer is #{Collatz.length 13}"

t = Time.now
puts "Longest sequence starts with #{Collatz.longest 1...1_000_000}"
puts "Took %.2fs" % (Time.now - t)