Rabin Miller Primality test

Meh
// Find a witness for the Rabin-Miller test
// http://en.wikipedia.org/wiki/Miller%E2%80%93Rabin_primality_test
// (variable names following article)

#include <stdio.h>
#include <math.h>

int gcd(int a, int b) {
int c;
while(a != 0) {
c = a;
a = b%a;
b = c;
}
return b;
}

int maxpowerof2(int n) {
int r = 0;
while(n % 2 == 0) {
n /= 2;
r++;
}
return r;
}

int main(void) {
int r, n, a, i, s, d;
printf("Give n: ");
fflush(stdout);
scanf("%d", &n);
s = maxpowerof2(n - 1);
d = (n - 1)/pow(2, s);
printf("n - 1 = 2^s * d\n"
"%d = 2^(%d) * %d\n", n - 1, s, d);
for(a = 1; a < n; a++) {
if(gcd(a, n) == 1) {
if((int)pow(a, d) % n != 1) {
for(r = 0; r < s; r++)
if((int)pow(a, pow(2, r)*d) % n == n - 1) break;
if(r == s) {
printf("Found a witness: %d\n", a);
break;
}
}
}
}
return 0;
}

>>36

while (!a)

is really easy to mistake for

while (a)

I'm so annoyed that I'm not even going to click (Post truncated). If you are that fucking retarded, then don't even bother trying to program.