project-euler/Sidef/050 Consecutive prime sum.sf

#!/usr/bin/ruby

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

# Which prime, below one-million, can be written as the sum of the most consecutive primes?

# https://projecteuler.net/problem=50

# Runtime: 1.345s

define limit = 1e6

var sum = 0
var primes = []

var p = 2

loop {
    (sum += p)/1.2 > limit && break;        # /1.2 is a lucky optimization?
    primes << p
    p.next_prime!
}

var l = primes.len
var arr = []

for (var i = 0; i <= l; ++i) {
    var sum = primes[i]
    for (var j = i+1; (j <= l-i) && ((sum += primes[j]) < limit); ++j) {
        if (sum.is_prime) {
            arr << [j-i + 1, sum, i, j]
        }
    }
}

var res = arr.max_by { .head }
say "#{res[1]} is the sum of #{res[0]} starting with #{prime(res[2]+1)}"