s_fp_sub.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 /* unsigned subtraction ||a|| >= ||b|| ALWAYS! */
00013 void s_fp_sub(fp_int *a, fp_int *b, fp_int *c)
00014 {
00015   int      x, oldbused, oldused;
00016   fp_word  t;
00017 
00018   oldused  = c->used;
00019   oldbused = b->used;
00020   c->used  = a->used;
00021   t       = 0;
00022   for (x = 0; x < oldbused; x++) {
00023      t         = ((fp_word)a->dp[x]) - (((fp_word)b->dp[x]) + t);
00024      c->dp[x]  = (fp_digit)t;
00025      t         = (t >> DIGIT_BIT)&1;
00026   }
00027   for (; x < a->used; x++) {
00028      t         = ((fp_word)a->dp[x]) - t;
00029      c->dp[x]  = (fp_digit)t;
00030      t         = (t >> DIGIT_BIT);
00031    }
00032   for (; x < oldused; x++) {
00033      c->dp[x] = 0;
00034   }
00035   fp_clamp(c);
00036 }
00037 
00038 /* $Source: /cvs/libtom/tomsfastmath/src/addsub/s_fp_sub.c,v $ */
00039 /* $Revision: 1.1 $ */
00040 /* $Date: 2006/12/31 21:25:53 $ */