pollard-brent_rho_factor.sf

#!/usr/bin/ruby

# Pollard-Brent rho integer factorization algorithm.

# See also:
#   https://facthacks.cr.yp.to/rho.html
#   https://en.wikipedia.org/wiki/Pollard%27s_rho_algorithm
#   https://en.algorithmica.org/hpc/algorithms/factorization/

func pollard_brent_factor(n, tries = (1 << 20), M = 2048) {

    var x = 1

    for (var l = M; l < tries; l *= 2) {
        var (y,p) = (x,1)
        for i in (^l `by` M) {
            for j in (^M) {
                x = (powmod(x, 2, n) + 1)
                p = mulmod(p, x-y, n)
            }
            is_coprime(n, p) || return gcd(p, n)
        }
    }

    return 1
}

say pollard_brent_factor(503*863)                   #=> 863
say pollard_brent_factor(33670570905491953)         #=> 36169843
say pollard_brent_factor(314159265358979323)        #=> 317213509
say pollard_brent_factor(242363923520394591022973)  #=> 786757556719