project-euler/Sidef/007 10001st prime -- v2.sf

#!/usr/bin/ruby

# Daniel "Trizen" Șuteu
# Date: 09 October 2017
# https://github.com/trizen

# https://projecteuler.net/problem=7

# Runtime: 0.757s (previously: 1.226s)

var n = 10_001
var upper = int(n*(n.log + n.log.log))

var sieve = [true, true]
for i in (2..upper.isqrt) {
    if (!sieve[i]) {
        for j in (i.sqr .. upper -> by(i)) {
            sieve[j] = true
        }
    }
}

sieve.map_kv! {|k,v| v ? () : k }

say sieve[n-1]