Miller-Rabin test: error in my code

I wrote a Miller-Rabin reconciliation criterion based on the following pseudocode:

Input: n > 2, an odd integer to be tested for primality;
       k, a parameter that determines the accuracy of the test
Output: composite if n is composite, otherwise probably prime
write n − 1 as 2s·d with d odd by factoring powers of 2 from n − 1
LOOP: repeat k times:
   pick a randomly in the range [2, n − 1]
   x ← ad mod n
   if x = 1 or x = n − 1 then do next LOOP
   for r = 1 .. s − 1
      x ← x2 mod n
      if x = 1 then return composite
      if x = n − 1 then do next LOOP
   return composite
return probably prime

The code I rarely get for 31 (if I put it in a loop to check numbers from 2 to 100). There must be something wrong, but I don’t see what it is.

bool isProbablePrime(ulong n, int k) {
    if (n < 2 || n % 2 == 0) 
        return n == 2;

    ulong d = n - 1;
    ulong s = 0;
    while (d % 2 == 0) {
        d /= 2;
        s++;
    }
    assert(2 ^^ s * d == n - 1); 
    outer:
    foreach (_; 0 .. k) {
        ulong a = uniform(2, n);
        ulong x = (a ^^ d) % n;
        if (x == 1 || x == n - 1)
            continue;
        foreach (__; 1 .. s) {
            x = (x ^^ 2) % n;
            if (x == 1) return false;
            if (x == n - 1) continue outer;
        }
        return false;
    }
    return true;
}

I also tried option

    ...

    foreach (__; 1 .. s) {
        x = (x ^^ 2) % n;
        if (x == 1) return false;
        if (x == n - 1) continue outer;
    }
    if ( x !=  n - 1) return false;  // this is different

    ...

I have another version of the test that works correctly, but uses modpow. I would like to have a version that is closer to the pseudo-code that is part of the description of rossetta.org .

: Re: . - . , Ruby . , - . BigInt, , , modpow. , . , modpow, , , .

ulong x = ((BigInt(a) ^^ d) % BigInt(n)).toLong();