Miller Rabin primality test

I had exercise to implement Miller Rabin primality test

According to my text book, the test algorithms was as following

Test (n)

1. Find integers k,q,d with K>0, q odd, so that (n-1 = 2^k * q);

2. Select a random integer a,1<a<n-1;

3. if a^q mod n = 1 then return probably prime;

4. for j = 0 to k-1 do;

5. if a^2jq mod n = n-1 then return probably prime;

6. return not prime;

I also found another way of the algorithm in http://rosettacode.org/wiki/Miller-Rabin_primality_test as following

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

When I tried both in  C#, both worked as expected

//textbook
public static bool IsProbabilyPrime(BigInteger n, int k)
{
    bool result = false;
    if (n < 2)
        return false;
    if (n == 2)
        return true;
    // return false if n is even -> divisbla by 2
    if (n % 2 == 0)
        return false;
    //writing n-1 as 2^s.d
    BigInteger d = n - 1;
    BigInteger s = 0;
    while (d % 2 == 0)
    {
        d >>= 1;
        s = s + 1;
    }
    for (int i = 0; i < k; i++)
    {
        BigInteger a;
        do
        {
            a = RandomIntegerBelow(n - 2);
        }
        while (a < 2 || a >= n - 2);

        if ( BigInteger.ModPow(a,d,n) == 1) return true;
        for (int j = 0; j < s - 1; j++)
        {
            if (BigInteger.ModPow(a,2*j*d,n) == n - 1)
                return true;
        }
        result = false;
    }
    return result;
}

//rosettacode.org
public static bool IsProbablePrime(this BigInteger source, int certainty)
{
    if (source == 2 || source == 3)
        return true;
    if (source < 2 || source % 2 == 0)
        return false;

    BigInteger d = source - 1;
    int s = 0;

    while (d % 2 == 0)
    {
        d /= 2;
        s += 1;
    }

    RandomNumberGenerator rng = RandomNumberGenerator.Create();
    byte[] bytes = new byte[source.ToByteArray().LongLength];
    BigInteger a;

    for (int i = 0; i < certainty; i++)
    {
        do
        {
            rng.GetBytes(bytes);
            a = new BigInteger(bytes);
        }
        while (a < 2 || a >= source - 2);

        BigInteger x = BigInteger.ModPow(a, d, source);
        if (x == 1 || x == source - 1)
            continue;

        for (int r = 1; r < s; r++)
        {
            x = BigInteger.ModPow(x, 2, source);
            if (x == 1)
                return false;
            if (x == source - 1)
                break;
        }

        if (x != source - 1)
            return false;
    }

    return true;
}