59 lines
1.5 KiB
C
59 lines
1.5 KiB
C
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/* LibTomMath, multiple-precision integer library -- Tom St Denis
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*
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* LibTomMath is library that provides for multiple-precision
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* integer arithmetic as well as number theoretic functionality.
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*
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* The library is designed directly after the MPI library by
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* Michael Fromberger but has been written from scratch with
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* additional optimizations in place.
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*
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* The library is free for all purposes without any express
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* guarantee it works.
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*
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* Tom St Denis, tomstdenis@iahu.ca, http://libtommath.iahu.ca
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*/
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#include <tommath.h>
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/* high level subtraction (handles signs) */
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int
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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;
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sb = b->sign;
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/* handle four cases */
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if (sa == MP_ZPOS && sb == MP_ZPOS) {
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/* both positive, a - b, but if b>a then we do -(b - a) */
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if (mp_cmp_mag (a, b) == MP_LT) {
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/* b>a */
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res = s_mp_sub (b, a, c);
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c->sign = MP_NEG;
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} else {
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res = s_mp_sub (a, b, c);
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c->sign = MP_ZPOS;
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}
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} else if (sa == MP_ZPOS && sb == MP_NEG) {
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/* a - -b == a + b */
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res = s_mp_add (a, b, c);
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c->sign = MP_ZPOS;
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} else if (sa == MP_NEG && sb == MP_ZPOS) {
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/* -a - b == -(a + b) */
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res = s_mp_add (a, b, c);
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c->sign = MP_NEG;
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} else {
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/* -a - -b == b - a, but if a>b == -(a - b) */
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if (mp_cmp_mag (a, b) == MP_GT) {
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res = s_mp_sub (a, b, c);
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c->sign = MP_NEG;
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} else {
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res = s_mp_sub (b, a, c);
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c->sign = MP_ZPOS;
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}
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}
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return res;
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}
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