1 #include <tommath.h> 2 #ifdef BN_MP_SUB_C 3 /* LibTomMath, multiple-precision integer library -- Tom St Denis 4 * 5 * LibTomMath is a library that provides multiple-precision 6 * integer arithmetic as well as number theoretic functionality. 7 * 8 * The library was designed directly after the MPI library by 9 * Michael Fromberger but has been written from scratch with 10 * additional optimizations in place. 11 * 12 * The library is free for all purposes without any express 13 * guarantee it works. 14 * 15 * Tom St Denis, tomstdenis (at) gmail.com, http://math.libtomcrypt.com 16 */ 17 18 /* high level subtraction (handles signs) */ 19 int 20 mp_sub (mp_int * a, mp_int * b, mp_int * c) 21 { 22 int sa, sb, res; 23 24 sa = a->sign; 25 sb = b->sign; 26 27 if (sa != sb) { 28 /* subtract a negative from a positive, OR */ 29 /* subtract a positive from a negative. */ 30 /* In either case, ADD their magnitudes, */ 31 /* and use the sign of the first number. */ 32 c->sign = sa; 33 res = s_mp_add (a, b, c); 34 } else { 35 /* subtract a positive from a positive, OR */ 36 /* subtract a negative from a negative. */ 37 /* First, take the difference between their */ 38 /* magnitudes, then... */ 39 if (mp_cmp_mag (a, b) != MP_LT) { 40 /* Copy the sign from the first */ 41 c->sign = sa; 42 /* The first has a larger or equal magnitude */ 43 res = s_mp_sub (a, b, c); 44 } else { 45 /* The result has the *opposite* sign from */ 46 /* the first number. */ 47 c->sign = (sa == MP_ZPOS) ? MP_NEG : MP_ZPOS; 48 /* The second has a larger magnitude */ 49 res = s_mp_sub (b, a, c); 50 } 51 } 52 return res; 53 } 54 55 #endif 56 57 /* $Source: /cvs/libtom/libtommath/bn_mp_sub.c,v $ */ 58 /* $Revision: 1.3 $ */ 59 /* $Date: 2006/03/31 14:18:44 $ */ 60
