def collatz(num)
  list = []
  current_num = num
  while current_num > 1
    list << current_num
    if current_num % 2 == 0
      current_num /= 2
    else
      current_num = 3*current_num + 1
    end
  end
  list << 1
end

def longest_chain(num)
  chain_length = {}
  (1..num).each do |starting_num|
    chain_length[starting_num] = collatz(starting_num).count
  end
  chain_length.sort_by { |key, value| value }.last
end

puts longest_chain(999999)