tommath/bn_mp_div_3.c

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#include <tommath_private.h>
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#ifdef BN_MP_DIV_3_C
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/* LibTomMath, multiple-precision integer library -- Tom St Denis
*
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* LibTomMath is a library that provides multiple-precision
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* integer arithmetic as well as number theoretic functionality.
*
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* The library was designed directly after the MPI library by
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* Michael Fromberger but has been written from scratch with
* additional optimizations in place.
*
* The library is free for all purposes without any express
* guarantee it works.
*/
/* divide by three (based on routine from MPI and the GMP manual) */
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int mp_div_3(const mp_int *a, mp_int *c, mp_digit *d)
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{
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mp_int q;
mp_word w, t;
mp_digit b;
int res, ix;
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/* b = 2**DIGIT_BIT / 3 */
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b = ((mp_word)1 << (mp_word)DIGIT_BIT) / (mp_word)3;
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if ((res = mp_init_size(&q, a->used)) != MP_OKAY) {
return res;
}
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q.used = a->used;
q.sign = a->sign;
w = 0;
for (ix = a->used - 1; ix >= 0; ix--) {
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w = (w << (mp_word)DIGIT_BIT) | (mp_word)a->dp[ix];
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if (w >= 3u) {
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/* multiply w by [1/3] */
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t = (w * (mp_word)b) >> (mp_word)DIGIT_BIT;
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/* now subtract 3 * [w/3] from w, to get the remainder */
w -= t+t+t;
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/* fixup the remainder as required since
* the optimization is not exact.
*/
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while (w >= 3u) {
t += 1u;
w -= 3u;
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}
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} else {
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t = 0;
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}
q.dp[ix] = (mp_digit)t;
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}
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/* [optional] store the remainder */
if (d != NULL) {
*d = (mp_digit)w;
}
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/* [optional] store the quotient */
if (c != NULL) {
mp_clamp(&q);
mp_exch(&q, c);
}
mp_clear(&q);
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return res;
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}
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#endif
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/* ref: $Format:%D$ */
/* git commit: $Format:%H$ */
/* commit time: $Format:%ai$ */