/* 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.
*
* The library is free for all purposes without any express
* guarantee it works.
*
* Tom St Denis, tomstdenis@iahu.ca, http://math.libtomcrypt.org
*/
#include <tommath.h>
/* single digit subtraction */
int
mp_sub_d (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) {
a->sign = MP_ZPOS;
res = mp_add_d(a, b, c);
a->sign = c->sign = MP_NEG;
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) * CHAR_BIT - 1);
*tmpc++ &= MP_MASK;
/* handle rest of the digits */
for (ix = 1; ix < a->used; ix++) {
*tmpc = *tmpa++ - mu;
mu = *tmpc >> (sizeof(mp_digit) * CHAR_BIT - 1);
*tmpc++ &= MP_MASK;
}
}
/* zero excess digits */
while (ix++ < oldused) {
*tmpc++ = 0;
}
mp_clamp(c);
return MP_OKAY;
}
