91 lines
2.1 KiB
C
91 lines
2.1 KiB
C
#include "tommath_private.h"
|
|
#ifdef BN_MP_SUB_D_C
|
|
/* LibTomMath, multiple-precision integer library -- Tom St Denis
|
|
*
|
|
* LibTomMath is a library that provides multiple-precision
|
|
* integer arithmetic as well as number theoretic functionality.
|
|
*
|
|
* The library was designed directly after the MPI library by
|
|
* Michael Fromberger but has been written from scratch with
|
|
* additional optimizations in place.
|
|
*
|
|
* SPDX-License-Identifier: Unlicense
|
|
*/
|
|
|
|
/* single digit subtraction */
|
|
int mp_sub_d(const mp_int *a, mp_digit b, mp_int *c)
|
|
{
|
|
mp_digit *tmpa, *tmpc, mu;
|
|
int res, ix, oldused;
|
|
|
|
/* grow c as required */
|
|
if (c->alloc < (a->used + 1)) {
|
|
if ((res = mp_grow(c, a->used + 1)) != MP_OKAY) {
|
|
return res;
|
|
}
|
|
}
|
|
|
|
/* if a is negative just do an unsigned
|
|
* addition [with fudged signs]
|
|
*/
|
|
if (a->sign == MP_NEG) {
|
|
mp_int a_ = *a;
|
|
a_.sign = MP_ZPOS;
|
|
res = mp_add_d(&a_, b, c);
|
|
c->sign = MP_NEG;
|
|
|
|
/* clamp */
|
|
mp_clamp(c);
|
|
|
|
return res;
|
|
}
|
|
|
|
/* setup regs */
|
|
oldused = c->used;
|
|
tmpa = a->dp;
|
|
tmpc = c->dp;
|
|
|
|
/* if a <= b simply fix the single digit */
|
|
if (((a->used == 1) && (a->dp[0] <= b)) || (a->used == 0)) {
|
|
if (a->used == 1) {
|
|
*tmpc++ = b - *tmpa;
|
|
} else {
|
|
*tmpc++ = b;
|
|
}
|
|
ix = 1;
|
|
|
|
/* negative/1digit */
|
|
c->sign = MP_NEG;
|
|
c->used = 1;
|
|
} else {
|
|
/* positive/size */
|
|
c->sign = MP_ZPOS;
|
|
c->used = a->used;
|
|
|
|
/* subtract first digit */
|
|
*tmpc = *tmpa++ - b;
|
|
mu = *tmpc >> ((sizeof(mp_digit) * (size_t)CHAR_BIT) - 1u);
|
|
*tmpc++ &= MP_MASK;
|
|
|
|
/* handle rest of the digits */
|
|
for (ix = 1; ix < a->used; ix++) {
|
|
*tmpc = *tmpa++ - mu;
|
|
mu = *tmpc >> ((sizeof(mp_digit) * (size_t)CHAR_BIT) - 1u);
|
|
*tmpc++ &= MP_MASK;
|
|
}
|
|
}
|
|
|
|
/* zero excess digits */
|
|
while (ix++ < oldused) {
|
|
*tmpc++ = 0;
|
|
}
|
|
mp_clamp(c);
|
|
return MP_OKAY;
|
|
}
|
|
|
|
#endif
|
|
|
|
/* ref: $Format:%D$ */
|
|
/* git commit: $Format:%H$ */
|
|
/* commit time: $Format:%ai$ */
|