collatz.cr

ZERO = Int64.new(0)
ONE = Int64.new(1)
TWO = Int64.new(2)
THREE = Int64.new(3)
ONE_MILLION = Int64.new(1_000_000)

class Collatz
  def initialize(min : Int64, max : Int64)
    @min = min
    @max = max
    @cache = Array(Int64).new(@max, ZERO)
  end

  def longest_length
    n = @min
    max_n, max_length = ZERO, ZERO

    while true
      break if n == @max

      length = length_for(n)

      if length > max_length
        max_n, max_length = n, length
      end

      n += 1
    end

    [max_n, max_length]
  end

  private def length_for(n : Int64)
    @cache[n - ONE] = next_for(n)
  end

  private def next_for(n : Int64)
    return ONE if n == ONE
    return @cache[n - ONE] if n <= ONE_MILLION && @cache[n - ONE] != ZERO

    if n % TWO == ZERO
      ONE + next_for(n / TWO)
    else
      ONE + next_for(n * THREE + ONE)
    end
  end
end

collatz = Collatz.new(ONE, ONE_MILLION)
max_n, max_length = collatz.longest_length

puts "N that generates biggest length: #{max_n}"
puts "Biggest length: #{max_length}"

Why do you not put ZERO, ONE, TWO, THREE, ONE_MILLION inside Collatz class?