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      1 #include <tommath.h>
      2 #ifdef BN_FAST_S_MP_MUL_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 /* Fast (comba) multiplier
     19  *
     20  * This is the fast column-array [comba] multiplier.  It is
     21  * designed to compute the columns of the product first
     22  * then handle the carries afterwards.  This has the effect
     23  * of making the nested loops that compute the columns very
     24  * simple and schedulable on super-scalar processors.
     25  *
     26  * This has been modified to produce a variable number of
     27  * digits of output so if say only a half-product is required
     28  * you don't have to compute the upper half (a feature
     29  * required for fast Barrett reduction).
     30  *
     31  * Based on Algorithm 14.12 on pp.595 of HAC.
     32  *
     33  */
     34 int fast_s_mp_mul_digs (mp_int * a, mp_int * b, mp_int * c, int digs)
     35 {
     36   int     olduse, res, pa, ix, iz;
     37   mp_digit W[MP_WARRAY];
     38   register mp_word  _W;
     39 
     40   /* grow the destination as required */
     41   if (c->alloc < digs) {
     42     if ((res = mp_grow (c, digs)) != MP_OKAY) {
     43       return res;
     44     }
     45   }
     46 
     47   /* number of output digits to produce */
     48   pa = MIN(digs, a->used + b->used);
     49 
     50   /* clear the carry */
     51   _W = 0;
     52   for (ix = 0; ix < pa; ix++) {
     53       int      tx, ty;
     54       int      iy;
     55       mp_digit *tmpx, *tmpy;
     56 
     57       /* get offsets into the two bignums */
     58       ty = MIN(b->used-1, ix);
     59       tx = ix - ty;
     60 
     61       /* setup temp aliases */
     62       tmpx = a->dp + tx;
     63       tmpy = b->dp + ty;
     64 
     65       /* this is the number of times the loop will iterrate, essentially
     66          while (tx++ < a->used && ty-- >= 0) { ... }
     67        */
     68       iy = MIN(a->used-tx, ty+1);
     69 
     70       /* execute loop */
     71       for (iz = 0; iz < iy; ++iz) {
     72          _W += ((mp_word)*tmpx++)*((mp_word)*tmpy--);
     73 
     74       }
     75 
     76       /* store term */
     77       W[ix] = ((mp_digit)_W) & MP_MASK;
     78 
     79       /* make next carry */
     80       _W = _W >> ((mp_word)DIGIT_BIT);
     81  }
     82 
     83   /* setup dest */
     84   olduse  = c->used;
     85   c->used = pa;
     86 
     87   {
     88     register mp_digit *tmpc;
     89     tmpc = c->dp;
     90     for (ix = 0; ix < pa+1; ix++) {
     91       /* now extract the previous digit [below the carry] */
     92       *tmpc++ = W[ix];
     93     }
     94 
     95     /* clear unused digits [that existed in the old copy of c] */
     96     for (; ix < olduse; ix++) {
     97       *tmpc++ = 0;
     98     }
     99   }
    100   mp_clamp (c);
    101   return MP_OKAY;
    102 }
    103 #endif
    104 
    105 /* $Source: /cvs/libtom/libtommath/bn_fast_s_mp_mul_digs.c,v $ */
    106 /* $Revision: 1.7 $ */
    107 /* $Date: 2006/03/31 14:18:44 $ */
    108