bn_mp_div_2.c
Go to the documentation of this file.00001 #include <tommath.h> 00002 #ifdef BN_MP_DIV_2_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 /* b = a/2 */ 00019 int mp_div_2(mp_int * a, mp_int * b) 00020 { 00021 int x, res, oldused; 00022 00023 /* copy */ 00024 if (b->alloc < a->used) { 00025 if ((res = mp_grow (b, a->used)) != MP_OKAY) { 00026 return res; 00027 } 00028 } 00029 00030 oldused = b->used; 00031 b->used = a->used; 00032 { 00033 register mp_digit r, rr, *tmpa, *tmpb; 00034 00035 /* source alias */ 00036 tmpa = a->dp + b->used - 1; 00037 00038 /* dest alias */ 00039 tmpb = b->dp + b->used - 1; 00040 00041 /* carry */ 00042 r = 0; 00043 for (x = b->used - 1; x >= 0; x--) { 00044 /* get the carry for the next iteration */ 00045 rr = *tmpa & 1; 00046 00047 /* shift the current digit, add in carry and store */ 00048 *tmpb-- = (*tmpa-- >> 1) | (r << (DIGIT_BIT - 1)); 00049 00050 /* forward carry to next iteration */ 00051 r = rr; 00052 } 00053 00054 /* zero excess digits */ 00055 tmpb = b->dp + b->used; 00056 for (x = b->used; x < oldused; x++) { 00057 *tmpb++ = 0; 00058 } 00059 } 00060 b->sign = a->sign; 00061 mp_clamp (b); 00062 return MP_OKAY; 00063 } 00064 #endif 00065 00066 /* $Source: /cvsroot/tcl/libtommath/bn_mp_div_2.c,v $ */ 00067 /* $Revision: 1.1.1.3 $ */ 00068 /* $Date: 2006/12/01 00:08:11 $ */
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