1af0de1f91
* same behavior for positive numbers * generalisation for negative numbers, treating them as two complement * improve algorithm, iterate once over the digits, manually perform two complement * simplify mp_add_d, mp_sub_d * functions are safe in case of a==c or b==c * renamed mp_tc_div_2d to mp_signed_rsh (signed right shift)
75 lines
1.6 KiB
C
75 lines
1.6 KiB
C
#include "tommath_private.h"
|
|
#ifdef BN_MP_SUB_D_C
|
|
/* LibTomMath, multiple-precision integer library -- Tom St Denis */
|
|
/* SPDX-License-Identifier: Unlicense */
|
|
|
|
/* single digit subtraction */
|
|
mp_err mp_sub_d(const mp_int *a, mp_digit b, mp_int *c)
|
|
{
|
|
mp_digit *tmpa, *tmpc;
|
|
mp_err err;
|
|
int ix, oldused;
|
|
|
|
/* grow c as required */
|
|
if (c->alloc < (a->used + 1)) {
|
|
if ((err = mp_grow(c, a->used + 1)) != MP_OKAY) {
|
|
return err;
|
|
}
|
|
}
|
|
|
|
/* 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;
|
|
err = mp_add_d(&a_, b, c);
|
|
c->sign = MP_NEG;
|
|
|
|
/* clamp */
|
|
mp_clamp(c);
|
|
|
|
return err;
|
|
}
|
|
|
|
/* 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 {
|
|
mp_digit mu = b;
|
|
|
|
/* positive/size */
|
|
c->sign = MP_ZPOS;
|
|
c->used = a->used;
|
|
|
|
/* subtract digits, mu is carry */
|
|
for (ix = 0; ix < a->used; ix++) {
|
|
*tmpc = *tmpa++ - mu;
|
|
mu = *tmpc >> (MP_SIZEOF_BITS(mp_digit) - 1u);
|
|
*tmpc++ &= MP_MASK;
|
|
}
|
|
}
|
|
|
|
/* zero excess digits */
|
|
MP_ZERO_DIGITS(tmpc, oldused - ix);
|
|
|
|
mp_clamp(c);
|
|
return MP_OKAY;
|
|
}
|
|
|
|
#endif
|