277 A Modified Collatz sequence.sf

#!/usr/bin/ruby

# Author: Daniel "Trizen" Șuteu
# Date: 25 August 2016
# License: GPLv3
# Website: https://github.com/trizen

# https://projecteuler.net/problem=277

# Runtime: 0.271s

func collatz(n) {
    gather {
        loop {
            n.is_one && break

            if (n % 3 == 0) {
                n /= 3
                take('D')
            }
            elsif (n % 3 == 1) {
                n = ((4*n + 2) / 3)
                take('U')
            }
            elsif (n % 3 == 2) {
                n = ((2*n - 1) / 3)
                take('d')
            }
        }
    }.join
}

var step = 1
var from = 10**15

var j = 1
var s = 'UDDDUdddDDUDDddDdDddDDUDDdUUDd'
var l = s.len

loop {
    for i in RangeNum(from, Inf, step) {

        var c = collatz(i)
        if (c.begins_with(s.substr(0, j))) {

            if (j == l) {
                say i;
                goto :OUTER
            }

            from = i
            step *= 3
            j += 1

            break
        }
    }
}; @:OUTER