65 lines
1.4 KiB
C
65 lines
1.4 KiB
C
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#include "tommath_private.h"
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#ifdef S_MP_DIV_3_C
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/* LibTomMath, multiple-precision integer library -- Tom St Denis */
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/* SPDX-License-Identifier: Unlicense */
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/* divide by three (based on routine from MPI and the GMP manual) */
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mp_err s_mp_div_3(const mp_int *a, mp_int *c, mp_digit *d)
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{
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mp_int q;
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mp_word w;
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mp_digit b;
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mp_err err;
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int ix;
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/* b = 2**MP_DIGIT_BIT / 3 */
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b = ((mp_word)1 << (mp_word)MP_DIGIT_BIT) / (mp_word)3;
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if ((err = mp_init_size(&q, a->used)) != MP_OKAY) {
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return err;
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}
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q.used = a->used;
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q.sign = a->sign;
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w = 0;
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for (ix = a->used; ix --> 0;) {
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mp_word t;
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w = (w << (mp_word)MP_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)MP_DIGIT_BIT;
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/* now subtract 3 * [w/3] from w, to get the remainder */
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w -= t+t+t;
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/* fixup the remainder as required since
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* the optimization is not exact.
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*/
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while (w >= 3u) {
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t += 1u;
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w -= 3u;
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}
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} else {
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t = 0;
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}
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q.dp[ix] = (mp_digit)t;
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}
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/* [optional] store the remainder */
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if (d != NULL) {
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*d = (mp_digit)w;
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}
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/* [optional] store the quotient */
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if (c != NULL) {
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mp_clamp(&q);
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mp_exch(&q, c);
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}
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mp_clear(&q);
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return MP_OKAY;
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}
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#endif
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