ref: 7501b4951959d10ccff953d7a208b12ef22ded42
dir: /mod.c/
#include <u.h>
#include <libc.h>
#include "asif.h"
/* modular exponentiation by repeated binary square and multiply,
* (addition chaining), right-to-left scan.
* assumptions: mod > 0, base >= 0, exp >= 0
* if base ≥ 0x10000, base² in the first iteration will overflow.
* if mod > 0x10000, base can be ≥ 0x10000 in subsequent iterations.
*/
int
modpow(int base, int exp, int mod)
{
int r;
if(base == 0)
return 0;
assert(base < 0x10000);
assert(mod <= 0x10000);
r = 1;
while(exp > 0){
if(exp & 1)
r = r * base % mod;
exp >>= 1;
base = base * base % mod;
}
return r;
}