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      1 #include <tommath.h>
      2 #ifdef BN_S_MP_MUL_HIGH_DIGS_C
      3 /* LibTomMath, multiple-precision integer library -- Tom St Denis
      4  *
      5  * LibTomMath is a library that provides multiple-precision
      6  * integer arithmetic as well as number theoretic functionality.
      7  *
      8  * The library was designed directly after the MPI library by
      9  * Michael Fromberger but has been written from scratch with
     10  * additional optimizations in place.
     11  *
     12  * The library is free for all purposes without any express
     13  * guarantee it works.
     14  *
     15  * Tom St Denis, tomstdenis (at) gmail.com, http://math.libtomcrypt.com
     16  */
     17 
     18 /* multiplies |a| * |b| and does not compute the lower digs digits
     19  * [meant to get the higher part of the product]
     20  */
     21 int
     22 s_mp_mul_high_digs (mp_int * a, mp_int * b, mp_int * c, int digs)
     23 {
     24   mp_int  t;
     25   int     res, pa, pb, ix, iy;
     26   mp_digit u;
     27   mp_word r;
     28   mp_digit tmpx, *tmpt, *tmpy;
     29 
     30   /* can we use the fast multiplier? */
     31 #ifdef BN_FAST_S_MP_MUL_HIGH_DIGS_C
     32   if (((a->used + b->used + 1) < MP_WARRAY)
     33       && MIN (a->used, b->used) < (1 << ((CHAR_BIT * sizeof (mp_word)) - (2 * DIGIT_BIT)))) {
     34     return fast_s_mp_mul_high_digs (a, b, c, digs);
     35   }
     36 #endif
     37 
     38   if ((res = mp_init_size (&t, a->used + b->used + 1)) != MP_OKAY) {
     39     return res;
     40   }
     41   t.used = a->used + b->used + 1;
     42 
     43   pa = a->used;
     44   pb = b->used;
     45   for (ix = 0; ix < pa; ix++) {
     46     /* clear the carry */
     47     u = 0;
     48 
     49     /* left hand side of A[ix] * B[iy] */
     50     tmpx = a->dp[ix];
     51 
     52     /* alias to the address of where the digits will be stored */
     53     tmpt = &(t.dp[digs]);
     54 
     55     /* alias for where to read the right hand side from */
     56     tmpy = b->dp + (digs - ix);
     57 
     58     for (iy = digs - ix; iy < pb; iy++) {
     59       /* calculate the double precision result */
     60       r       = ((mp_word)*tmpt) +
     61                 ((mp_word)tmpx) * ((mp_word)*tmpy++) +
     62                 ((mp_word) u);
     63 
     64       /* get the lower part */
     65       *tmpt++ = (mp_digit) (r & ((mp_word) MP_MASK));
     66 
     67       /* carry the carry */
     68       u       = (mp_digit) (r >> ((mp_word) DIGIT_BIT));
     69     }
     70     *tmpt = u;
     71   }
     72   mp_clamp (&t);
     73   mp_exch (&t, c);
     74   mp_clear (&t);
     75   return MP_OKAY;
     76 }
     77 #endif
     78 
     79 /* $Source: /cvs/libtom/libtommath/bn_s_mp_mul_high_digs.c,v $ */
     80 /* $Revision: 1.3 $ */
     81 /* $Date: 2006/03/31 14:18:44 $ */
     82