### view libtommath/bn_mp_sub_d.c @ 1734:73646de50f13DROPBEAR_2020.80

version 2020.80
author Matt Johnston Fri, 26 Jun 2020 21:45:59 +0800 1051e4eea25a
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```#include "tommath_private.h"
#ifdef BN_MP_SUB_D_C
/* LibTomMath, multiple-precision integer library -- Tom St Denis */

/* 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
*/
if (a->sign == MP_NEG) {
mp_int a_ = *a;
a_.sign = MP_ZPOS;
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);
}
}

/* zero excess digits */
MP_ZERO_DIGITS(tmpc, oldused - ix);

mp_clamp(c);
return MP_OKAY;
}

#endif
```