project-euler/Sidef/058 Spiral primes.sf

#!/usr/bin/ruby

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

# What is the side length of the square spiral for which the ratio of primes along both diagonals first falls below 10%?

# https://projecteuler.net/problem=58

# Runtime: 1.439s

var f1 = func(n) { 3 + n*(4*n - 6) }
var f2 = func(n) { 5 + n*(4*n - 8) }
var f3 = func(n) { 7 + n*(4*n - 10) }

var primes = 0
Inf.times { |i|

    var total = (4*i + 1)
    var side  = (2*i + 1)

    primes += [f1(i+1), f2(i+1), f3(i+1)].count { .is_prime }

    if (side > 7 && (primes/total < 0.1)) {
        say side
        break
    }
}