fermat_strong_primality_test.sf

#!/usr/bin/ruby

# Strong Fermat primality test (Miller–Rabin primality test).

# See also:
#   https://oeis.org/A001262 -- Strong pseudoprimes to base 2.
#   https://oeis.org/A020229 -- Strong pseudoprimes to base 3.
#   https://en.wikipedia.org/wiki/Miller%E2%80%93Rabin_primality_test

func is_strong_fermat_pseudoprime(n, base=2) {

    return false if (n <= 1)
    return true  if (n == 2)
    return false if (n.is_even)

    var d = n-1
    var v = d.valuation(2)
    var q = d>>v

    var x = powmod(base, q, n)

    if (x ~~ [1, n-1]) {
        return true
    }

    (v-1).times {
        x = powmod(x, 2, n)

        if (x == 1) {
            return false
        }

        if (x == n-1) {
            return true
        }
    }

    return false
}

say ("Primes below 100: ", is_strong_fermat_pseudoprime.grep(1..100))
say ("First 5 strong pseudoprimes to base 2: ", { !.is_prime && is_strong_fermat_pseudoprime(_, 2) }.first(5))

__END__
Primes below 100: [2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97]
First 5 strong pseudoprimes to base 2: [2047, 3277, 4033, 4681, 8321]