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#include <string.h>
int timingsafe_memcmp(const void* a,const void* b,size_t n) {
/* slightly harder than bcmp because we want to know which one was
* smaller, not just if they did not match */
register const unsigned char *x=a;
register const unsigned char *y=b;
int r=0,done=0;
/* r is what we return in the end,
* done is there to be 0 or -1 so we can assign to r branchlessly */
size_t i;
for (i=0; i<n; ++i) {
int t=x[i]-y[i];
/* if t is nonzero, t|-t will have the sign bit set */
int minusone = (t|-t)>>(sizeof(int)-1);
/* if there is no mismatch, t is 0. Then r |= t is a no-op. */
/* if this is the first mismatch, we want to do r |= t. */
/* if this is not the first mismatch, we want to leave r alone. */
r |= t & ~done;
done |= minusone;
}
return r;
}
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