euler_problem_14

euler_problem_14.rb

$chain_lengths = {:largest_num => 0, :largest_chain => 0, 1 => 1, 2 => 2}

def get_chain_length_for(n)
  return $chain_lengths[n] if $chain_lengths.has_key?(n)
  next_n = 3 * n + 1 if n.odd?
  next_n = n / 2 if n.even?
  return ($chain_lengths[next_n] = get_chain_length_for(next_n)) + 1
end

(2..999999).each do |x|
  len = get_chain_length_for(x)
  $chain_lengths[x] = len
  if len > $chain_lengths[:largest_chain]
    $chain_lengths[:largest_num]   = x
    $chain_lengths[:largest_chain] = len
  end
end
puts $chain_lengths[:largest_num]
puts $chain_lengths[:largest_chain]