fp_mul_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 void fp_mul_2(fp_int * a, fp_int * b)
00013 {
00014   int     x, oldused;
00015    
00016   oldused = b->used;
00017   b->used = a->used;
00018 
00019   {
00020     register fp_digit r, rr, *tmpa, *tmpb;
00021 
00022     /* alias for source */
00023     tmpa = a->dp;
00024     
00025     /* alias for dest */
00026     tmpb = b->dp;
00027 
00028     /* carry */
00029     r = 0;
00030     for (x = 0; x < a->used; x++) {
00031     
00032       /* get what will be the *next* carry bit from the 
00033        * MSB of the current digit 
00034        */
00035       rr = *tmpa >> ((fp_digit)(DIGIT_BIT - 1));
00036       
00037       /* now shift up this digit, add in the carry [from the previous] */
00038       *tmpb++ = ((*tmpa++ << ((fp_digit)1)) | r);
00039       
00040       /* copy the carry that would be from the source 
00041        * digit into the next iteration 
00042        */
00043       r = rr;
00044     }
00045 
00046     /* new leading digit? */
00047     if (r != 0 && b->used != (FP_SIZE-1)) {
00048       /* add a MSB which is always 1 at this point */
00049       *tmpb = 1;
00050       ++(b->used);
00051     }
00052 
00053     /* now zero any excess digits on the destination 
00054      * that we didn't write to 
00055      */
00056     tmpb = b->dp + b->used;
00057     for (x = b->used; x < oldused; x++) {
00058       *tmpb++ = 0;
00059     }
00060   }
00061   b->sign = a->sign;
00062 }
00063 
00064 
00065 /* $Source: /cvs/libtom/tomsfastmath/src/mul/fp_mul_2.c,v $ */
00066 /* $Revision: 1.1 $ */
00067 /* $Date: 2006/12/31 21:25:53 $ */