h2-mod/deps/libtommath/s_mp_sub.c
2024-03-07 03:33:59 -05:00

57 lines
1.5 KiB
C

#include "tommath_private.h"
#ifdef S_MP_SUB_C
/* LibTomMath, multiple-precision integer library -- Tom St Denis */
/* SPDX-License-Identifier: Unlicense */
/* low level subtraction (assumes |a| > |b|), HAC pp.595 Algorithm 14.9 */
mp_err s_mp_sub(const mp_int *a, const mp_int *b, mp_int *c)
{
int oldused = c->used, min = b->used, max = a->used, i;
mp_digit u;
mp_err err;
/* init result */
if ((err = mp_grow(c, max)) != MP_OKAY) {
return err;
}
c->used = max;
/* set carry to zero */
u = 0;
for (i = 0; i < min; i++) {
/* T[i] = A[i] - B[i] - U */
c->dp[i] = (a->dp[i] - b->dp[i]) - u;
/* U = carry bit of T[i]
* Note this saves performing an AND operation since
* if a carry does occur it will propagate all the way to the
* MSB. As a result a single shift is enough to get the carry
*/
u = c->dp[i] >> (MP_SIZEOF_BITS(mp_digit) - 1u);
/* Clear carry from T[i] */
c->dp[i] &= MP_MASK;
}
/* now copy higher words if any, e.g. if A has more digits than B */
for (; i < max; i++) {
/* T[i] = A[i] - U */
c->dp[i] = a->dp[i] - u;
/* U = carry bit of T[i] */
u = c->dp[i] >> (MP_SIZEOF_BITS(mp_digit) - 1u);
/* Clear carry from T[i] */
c->dp[i] &= MP_MASK;
}
/* clear digits above used (since we may not have grown result above) */
s_mp_zero_digs(c->dp + c->used, oldused - c->used);
mp_clamp(c);
return MP_OKAY;
}
#endif