tommath/bn_mp_sub.c

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#include <tommath_private.h>
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#ifdef BN_MP_SUB_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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*/
/* high level subtraction (handles signs) */
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int mp_sub(mp_int *a, mp_int *b, mp_int *c)
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{
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int sa, sb, res;
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sa = a->sign;
sb = b->sign;
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if (sa != sb) {
/* subtract a negative from a positive, OR */
/* subtract a positive from a negative. */
/* In either case, ADD their magnitudes, */
/* and use the sign of the first number. */
c->sign = sa;
res = s_mp_add(a, b, c);
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} else {
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/* subtract a positive from a positive, OR */
/* subtract a negative from a negative. */
/* First, take the difference between their */
/* magnitudes, then... */
if (mp_cmp_mag(a, b) != MP_LT) {
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/* Copy the sign from the first */
c->sign = sa;
/* The first has a larger or equal magnitude */
res = s_mp_sub(a, b, c);
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} else {
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/* The result has the *opposite* sign from */
/* the first number. */
c->sign = (sa == MP_ZPOS) ? MP_NEG : MP_ZPOS;
/* The second has a larger magnitude */
res = s_mp_sub(b, a, c);
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}
}
return res;
}
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#endif
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/* ref: $Format:%D$ */
/* git commit: $Format:%H$ */
/* commit time: $Format:%ai$ */