euler14.rb

   1 require 'logger'
   2 log = Logger.new(STDERR)
   3 log.level = Logger::WARN
   4
   5 $chain_lengths = {1 => 1}
   6
   7 def chain_length(n)
   8   unless $chain_lengths.has_key?(n)
   9     if n % 2 == 0
  10       $chain_lengths[n] = 1 + chain_length(n / 2)
  11     else
  12       $chain_lengths[n] = 1 + chain_length(3 * n + 1)
  13     end
  14   end
  15   $chain_lengths[n]
  16 end
  17
  18 longest_chain_start = 1
  19 1.upto(1000000) do |i|
  20   if chain_length(i) > chain_length(longest_chain_start)
  21     longest_chain_start = i
  22     log.info {"Found new longest chain. Starts at #{i}, #{chain_length(i)} elements"}
  23   end
  24   log.debug {"#{i} has chain length #{chain_length(i)}"}
  25 end
  26
  27 puts "Longest chain starts at #{longest_chain_start}, with #{$chain_lengths[longest_chain_start]} elements"
  28
  29 log.close