libtommath/bn_mp_div_d.c

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#include "tommath_private.h"
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#ifdef 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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static int s_is_power_of_two(mp_digit b, int *p)
{
int x;
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/* fast return if no power of two */
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if ((b == 0u) || ((b & (b-1u)) != 0u)) {
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return 0;
}
for (x = 0; x < DIGIT_BIT; x++) {
if (b == ((mp_digit)1<<(mp_digit)x)) {
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*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(const mp_int *a, mp_digit b, mp_int *c, mp_digit *d)
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{
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mp_int q;
mp_word w;
mp_digit t;
int res, ix;
/* cannot divide by zero */
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if (b == 0u) {
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return MP_VAL;
}
/* quick outs */
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if ((b == 1u) || (mp_iszero(a) == MP_YES)) {
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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] & (((mp_digit)1<<(mp_digit)ix) - 1uL);
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}
if (c != NULL) {
return mp_div_2d(a, ix, c, NULL);
}
return MP_OKAY;
}
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#ifdef BN_MP_DIV_3_C
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/* three? */
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if (b == 3u) {
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return mp_div_3(a, c, d);
}
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#endif
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/* no easy answer [c'est la vie]. Just division */
if ((res = mp_init_size(&q, a->used)) != MP_OKAY) {
return res;
}
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q.used = a->used;
q.sign = a->sign;
w = 0;
for (ix = a->used - 1; ix >= 0; ix--) {
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w = (w << (mp_word)DIGIT_BIT) | (mp_word)a->dp[ix];
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if (w >= b) {
t = (mp_digit)(w / b);
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w -= (mp_word)t * (mp_word)b;
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} else {
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t = 0;
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}
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q.dp[ix] = t;
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}
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if (d != NULL) {
*d = (mp_digit)w;
}
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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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}
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#endif
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/* ref: $Format:%D$ */
/* git commit: $Format:%H$ */
/* commit time: $Format:%ai$ */