bn_s_mp_sub.c
Go to the documentation of this file.00001 #include <tommath.h> 00002 #ifdef BN_S_MP_SUB_C 00003 /* LibTomMath, multiple-precision integer library -- Tom St Denis 00004 * 00005 * LibTomMath is a library that provides multiple-precision 00006 * integer arithmetic as well as number theoretic functionality. 00007 * 00008 * The library was designed directly after the MPI library by 00009 * Michael Fromberger but has been written from scratch with 00010 * additional optimizations in place. 00011 * 00012 * The library is free for all purposes without any express 00013 * guarantee it works. 00014 * 00015 * Tom St Denis, tomstdenis@gmail.com, http://math.libtomcrypt.com 00016 */ 00017 00018 /* low level subtraction (assumes |a| > |b|), HAC pp.595 Algorithm 14.9 */ 00019 int 00020 s_mp_sub (mp_int * a, mp_int * b, mp_int * c) 00021 { 00022 int olduse, res, min, max; 00023 00024 /* find sizes */ 00025 min = b->used; 00026 max = a->used; 00027 00028 /* init result */ 00029 if (c->alloc < max) { 00030 if ((res = mp_grow (c, max)) != MP_OKAY) { 00031 return res; 00032 } 00033 } 00034 olduse = c->used; 00035 c->used = max; 00036 00037 { 00038 register mp_digit u, *tmpa, *tmpb, *tmpc; 00039 register int i; 00040 00041 /* alias for digit pointers */ 00042 tmpa = a->dp; 00043 tmpb = b->dp; 00044 tmpc = c->dp; 00045 00046 /* set carry to zero */ 00047 u = 0; 00048 for (i = 0; i < min; i++) { 00049 /* T[i] = A[i] - B[i] - U */ 00050 *tmpc = *tmpa++ - *tmpb++ - u; 00051 00052 /* U = carry bit of T[i] 00053 * Note this saves performing an AND operation since 00054 * if a carry does occur it will propagate all the way to the 00055 * MSB. As a result a single shift is enough to get the carry 00056 */ 00057 u = *tmpc >> ((mp_digit)(CHAR_BIT * sizeof (mp_digit) - 1)); 00058 00059 /* Clear carry from T[i] */ 00060 *tmpc++ &= MP_MASK; 00061 } 00062 00063 /* now copy higher words if any, e.g. if A has more digits than B */ 00064 for (; i < max; i++) { 00065 /* T[i] = A[i] - U */ 00066 *tmpc = *tmpa++ - u; 00067 00068 /* U = carry bit of T[i] */ 00069 u = *tmpc >> ((mp_digit)(CHAR_BIT * sizeof (mp_digit) - 1)); 00070 00071 /* Clear carry from T[i] */ 00072 *tmpc++ &= MP_MASK; 00073 } 00074 00075 /* clear digits above used (since we may not have grown result above) */ 00076 for (i = c->used; i < olduse; i++) { 00077 *tmpc++ = 0; 00078 } 00079 } 00080 00081 mp_clamp (c); 00082 return MP_OKAY; 00083 } 00084 00085 #endif 00086 00087 /* $Source: /cvsroot/tcl/libtommath/bn_s_mp_sub.c,v $ */ 00088 /* $Revision: 1.1.1.3 $ */ 00089 /* $Date: 2006/12/01 00:08:11 $ */
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