collatz.rb

#!/usr/bin/env ruby

def longest_collatz(n)
  longest_sequence = 0
  longest_n = 0
  collatz_map = {}
  until n == 1
    n -= 1
    if collatz_map.key? n
      length = collatz_map[n]
    else
      length = collatz(n)
      collatz_map[n] = length
    end
    if length > longest_sequence
      longest_sequence = length
      longest_n = n
    end
  end
  longest_n
end

def collatz(n)
  length = 1
  until n == 1
    length += 1
    n = n.even? ? (n / 2) : (3 * n + 1)
  end
  length
end

puts longest_collatz(ARGV[0].to_i)