71 lines
1.7 KiB
C
71 lines
1.7 KiB
C
#include "tommath_private.h"
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#ifdef BN_MP_DIV_2D_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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/* shift right by a certain bit count (store quotient in c, optional remainder in d) */
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int mp_div_2d(const mp_int *a, int b, mp_int *c, mp_int *d)
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{
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mp_digit D, r, rr;
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int x, res;
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/* if the shift count is <= 0 then we do no work */
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if (b <= 0) {
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res = mp_copy(a, c);
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if (d != NULL) {
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mp_zero(d);
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}
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return res;
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}
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/* copy */
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if ((res = mp_copy(a, c)) != MP_OKAY) {
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return res;
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}
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/* 'a' should not be used after here - it might be the same as d */
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/* get the remainder */
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if (d != NULL) {
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if ((res = mp_mod_2d(a, b, d)) != MP_OKAY) {
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return res;
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}
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}
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/* shift by as many digits in the bit count */
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if (b >= MP_DIGIT_BIT) {
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mp_rshd(c, b / MP_DIGIT_BIT);
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}
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/* shift any bit count < MP_DIGIT_BIT */
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D = (mp_digit)(b % MP_DIGIT_BIT);
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if (D != 0u) {
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mp_digit *tmpc, mask, shift;
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/* mask */
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mask = ((mp_digit)1 << D) - 1uL;
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/* shift for lsb */
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shift = (mp_digit)MP_DIGIT_BIT - D;
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/* alias */
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tmpc = c->dp + (c->used - 1);
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/* carry */
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r = 0;
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for (x = c->used - 1; x >= 0; x--) {
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/* get the lower bits of this word in a temp */
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rr = *tmpc & mask;
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/* shift the current word and mix in the carry bits from the previous word */
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*tmpc = (*tmpc >> D) | (r << shift);
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--tmpc;
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/* set the carry to the carry bits of the current word found above */
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r = rr;
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}
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}
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mp_clamp(c);
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return MP_OKAY;
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}
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#endif
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