1/* $NetBSD: bn_mp_sub.c,v 1.2 2017/01/28 21:31:47 christos Exp $ */ 2 3#include <tommath.h> 4#ifdef BN_MP_SUB_C 5/* LibTomMath, multiple-precision integer library -- Tom St Denis 6 * 7 * LibTomMath is a library that provides multiple-precision 8 * integer arithmetic as well as number theoretic functionality. 9 * 10 * The library was designed directly after the MPI library by 11 * Michael Fromberger but has been written from scratch with 12 * additional optimizations in place. 13 * 14 * The library is free for all purposes without any express 15 * guarantee it works. 16 * 17 * Tom St Denis, tomstdenis@gmail.com, http://libtom.org 18 */ 19 20/* high level subtraction (handles signs) */ 21int 22mp_sub (mp_int * a, mp_int * b, mp_int * c) 23{ 24 int sa, sb, res; 25 26 sa = a->sign; 27 sb = b->sign; 28 29 if (sa != sb) { 30 /* subtract a negative from a positive, OR */ 31 /* subtract a positive from a negative. */ 32 /* In either case, ADD their magnitudes, */ 33 /* and use the sign of the first number. */ 34 c->sign = sa; 35 res = s_mp_add (a, b, c); 36 } else { 37 /* subtract a positive from a positive, OR */ 38 /* subtract a negative from a negative. */ 39 /* First, take the difference between their */ 40 /* magnitudes, then... */ 41 if (mp_cmp_mag (a, b) != MP_LT) { 42 /* Copy the sign from the first */ 43 c->sign = sa; 44 /* The first has a larger or equal magnitude */ 45 res = s_mp_sub (a, b, c); 46 } else { 47 /* The result has the *opposite* sign from */ 48 /* the first number. */ 49 c->sign = (sa == MP_ZPOS) ? MP_NEG : MP_ZPOS; 50 /* The second has a larger magnitude */ 51 res = s_mp_sub (b, a, c); 52 } 53 } 54 return res; 55} 56 57#endif 58 59/* Source: /cvs/libtom/libtommath/bn_mp_sub.c,v */ 60/* Revision: 1.4 */ 61/* Date: 2006/12/28 01:25:13 */ 62