source4/heimdal/lib/hcrypto/libtommath/bn_s_mp_mul_high_digs.c

   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@gmail.com, http://libtom.org
  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: 1.4 $ */
  81 /* $Date: 2006/12/28 01:25:13 $ */