fp_div_2.c

00001 /* TomsFastMath, a fast ISO C bignum library.
00002  * 
00003  * This project is meant to fill in where LibTomMath
00004  * falls short.  That is speed ;-)
00005  *
00006  * This project is public domain and free for all purposes.
00007  * 
00008  * Tom St Denis, tomstdenis@gmail.com
00009  */
00010 #include <tfm.h>
00011 
00012 /* b = a/2 */
00013 void fp_div_2(fp_int * a, fp_int * b)
00014 {
00015   int     x, oldused;
00016 
00017   oldused = b->used;
00018   b->used = a->used;
00019   {
00020     register fp_digit r, rr, *tmpa, *tmpb;
00021 
00022     /* source alias */
00023     tmpa = a->dp + b->used - 1;
00024 
00025     /* dest alias */
00026     tmpb = b->dp + b->used - 1;
00027 
00028     /* carry */
00029     r = 0;
00030     for (x = b->used - 1; x >= 0; x--) {
00031       /* get the carry for the next iteration */
00032       rr = *tmpa & 1;
00033 
00034       /* shift the current digit, add in carry and store */
00035       *tmpb-- = (*tmpa-- >> 1) | (r << (DIGIT_BIT - 1));
00036 
00037       /* forward carry to next iteration */
00038       r = rr;
00039     }
00040 
00041     /* zero excess digits */
00042     tmpb = b->dp + b->used;
00043     for (x = b->used; x < oldused; x++) {
00044       *tmpb++ = 0;
00045     }
00046   }
00047   b->sign = a->sign;
00048   fp_clamp (b);
00049 }
00050 
00051 /* $Source: /cvs/libtom/tomsfastmath/src/bit/fp_div_2.c,v $ */
00052 /* $Revision: 1.1 $ */
00053 /* $Date: 2006/12/31 21:25:53 $ */