project-euler/Sidef/035 Circular primes -- v2.sf

#!/usr/bin/ruby

# Author: Trizen
# Date: 15 March 2023
# https://github.com/trizen

# How many circular primes are there below one million?

# https://projecteuler.net/problem=35

# Runtime: 0.405s

func primes_from_digits(upto, digits, base = 10) {

    upto = prev_prime(upto+1)

    var list = []
    var end_digits = digits.grep { .is_coprime(base) }

    for k in (1 .. upto.ilog(base)) {
        digits.variations_with_repetition(k, {|*a|
            var v = a.digits2num(base)
            end_digits.each {|d|
                var n = (v*base + d)
                list << n if (n.is_prime && (n <= upto))
            }
        })
    }

    list.sort
}

var count = 4      # number of circular primes < 10
var limit = 1e6

primes_from_digits(limit, [1,3,7,9]).each {|p|
    var d = Str(p).chars
    if (d.range.all {|k| Num(d.rotate(k).join).is_prime }) {
        ++count
    }
}

say count