project-euler/Sidef/134 Prime pair connection.sf

#!/usr/bin/ruby

# Author: Daniel "Trizen" Șuteu
# Date: 24 August 2016
# License: GPLv3
# Website: https://github.com/trizen

# https://projecteuler.net/problem=134

# Runtime: 2.189s (previously: 2.855s)

# k = 10^(1+floor(log10(p1)))
# g = (invmod(k, p2) * -p1) % p2
# S = k*g + p1

var p1 = 5
var p2 = p1.next_prime

var sum = 0
var len = p1.len
var pow = 10**len

loop {
    if (p1.len != len) {
        len += 1
        pow *= 10
    }

    sum += (((invmod(pow, p2) * -p1) % p2) * pow + p1)

    p1 = p2
    p2.next_prime!

    p1 > 1000000 && break
}

say sum