tommath/bn_mp_mod_2d.c

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#include <tommath_private.h>
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#ifdef BN_MP_MOD_2D_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.
*
* Tom St Denis, tstdenis82@gmail.com, http://libtom.org
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*/
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/* calc a value mod 2**b */
int mp_mod_2d(const mp_int *a, int b, mp_int *c)
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{
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int x, res;
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/* if b is <= 0 then zero the int */
if (b <= 0) {
mp_zero(c);
return MP_OKAY;
}
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/* if the modulus is larger than the value than return */
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if (b >= (a->used * DIGIT_BIT)) {
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res = mp_copy(a, c);
return res;
}
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/* copy */
if ((res = mp_copy(a, c)) != MP_OKAY) {
return res;
}
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/* zero digits above the last digit of the modulus */
for (x = (b / DIGIT_BIT) + (((b % DIGIT_BIT) == 0) ? 0 : 1); x < c->used; x++) {
c->dp[x] = 0;
}
/* clear the digit that is not completely outside/inside the modulus */
c->dp[b / DIGIT_BIT] &=
((mp_digit)1 << (mp_digit)(b % DIGIT_BIT)) - (mp_digit)1;
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mp_clamp(c);
return MP_OKAY;
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}
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#endif
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/* ref: $Format:%D$ */
/* git commit: $Format:%H$ */
/* commit time: $Format:%ai$ */