euler14.rb

def collatz(i)
  Enumerator.new do |y|
    n = i
    while n != 1 do
      n = n.even? ? n/2 : 3*n + 1
      y << n
    end
  end
end

max = (1...1000000).each.map do |i|
  { input: i, count: collatz(i).count }
end.max {|i, j| i[:count] <=> j[:count] }
puts max[:input]

# 837799
# time 105s CPU: Core i5 1.6GHz, MEM: 4GB