tommath/bn_mp_div_d.c

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/* LibTomMath, multiple-precision integer library -- Tom St Denis
*
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* LibTomMath is a library that provides multiple-precision
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* integer arithmetic as well as number theoretic functionality.
*
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* The library was designed directly after the MPI library by
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* 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.
*
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* Tom St Denis, tomstdenis@iahu.ca, http://math.libtomcrypt.org
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*/
#include <tommath.h>
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static int s_is_power_of_two(mp_digit b, int *p)
{
int x;
for (x = 1; x < DIGIT_BIT; x++) {
if (b == (((mp_digit)1)<<x)) {
*p = x;
return 1;
}
}
return 0;
}
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/* single digit division (based on routine from MPI) */
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int
mp_div_d (mp_int * a, mp_digit b, mp_int * c, mp_digit * d)
{
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mp_int q;
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mp_word w;
mp_digit t;
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int res, ix;
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/* cannot divide by zero */
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if (b == 0) {
return MP_VAL;
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}
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/* quick outs */
if (b == 1 || mp_iszero(a) == 1) {
if (d != NULL) {
*d = 0;
}
if (c != NULL) {
return mp_copy(a, c);
}
return MP_OKAY;
}
/* power of two ? */
if (s_is_power_of_two(b, &ix) == 1) {
if (d != NULL) {
*d = a->dp[0] & ((1<<ix) - 1);
}
if (c != NULL) {
return mp_div_2d(a, ix, c, NULL);
}
return MP_OKAY;
}
/* three? */
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if (b == 3) {
return mp_div_3(a, c, d);
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}
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/* no easy answer [c'est la vie]. Just division */
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if ((res = mp_init_size(&q, a->used)) != MP_OKAY) {
return res;
}
q.used = a->used;
q.sign = a->sign;
w = 0;
for (ix = a->used - 1; ix >= 0; ix--) {
w = (w << ((mp_word)DIGIT_BIT)) | ((mp_word)a->dp[ix]);
if (w >= b) {
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t = (mp_digit)(w / b);
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w -= ((mp_word)t) * ((mp_word)b);
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} else {
t = 0;
}
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q.dp[ix] = (mp_digit)t;
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}
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if (d != NULL) {
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*d = (mp_digit)w;
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}
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if (c != NULL) {
mp_clamp(&q);
mp_exch(&q, c);
}
mp_clear(&q);
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return res;
}
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