shithub: femtolisp

ref: 76a7c66de3702ff0486900f7d4fa626407b953c3
dir: /3rd/mp/mpinvert.c/

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#include "platform.h"
#include "mp.h"

// use extended gcd to find the multiplicative inverse
// res = b**-1 mod m
void
mpinvert(mpint *b, mpint *m, mpint *res)
{
	mpint *v;

	v = mpnew(0);
	mpextendedgcd(b, m, v, res, nil);
	if(mpcmp(v, mpone) != 0){
		fprintf(stderr, "mpinvert: impossible\n");
		exit(2);
	}
	mpfree(v);
	mpmod(res, m, res);
}