project-euler/Sidef/808 Reversible prime squares.sf

#!/usr/bin/ruby

# Author: Trizen
# Date: 06 November 2022
# https://github.com/trizen

# Reversible prime squares
# https://projecteuler.net/problem=808

# From the first few terms, it seems that if n = p^2 and reverse(n) = q^2, then p = reverse(q).

# However, this is not always the case, as discovered by neverforget:
#   110092211^2 = 12120294922868521
#   112191011^2 = 12586822949202121

# Runtime: 21.784s (previously: 26 seconds).

func p808(N) {

    var list = []

    for (var p = 2; list.len < N; p.next_prime!) {

        var t = p.sqr

        if (t.flip.is_square && t.flip.isqrt.is_prime && !t.is_palindrome) {
            list << t
            say [list.len, p, list.tail]
        }
    }

    list.sum
}

say p808(50)