diveby3.c

a very popular packet of cryptography tools,it encloses the most common used algorithm and protocols

/* mpn_divexact_by3 -- mpn division by 3, expecting no remainder. */

/*
Copyright 2000, 2001, 2002 Free Software Foundation, Inc.

This file is part of the GNU MP Library.

The GNU MP Library is free software; you can redistribute it and/or modify
it under the terms of the GNU Lesser General Public License as published by
the Free Software Foundation; either version 2.1 of the License, or (at your
option) any later version.

The GNU MP Library is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
License for more details.

You should have received a copy of the GNU Lesser General Public License
along with the GNU MP Library; see the file COPYING.LIB.  If not, write to
the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
MA 02111-1307, USA.
*/


#include "gmp.h"
#include "gmp-impl.h"


/* The "c += ..."s are adding the high limb of 3*l to c.  That high limb
   will be 0, 1 or 2.  Doing two separate "+="s seems to turn out better
   code on gcc (as of 2.95.2 at least).

   When a subtraction of a 0,1,2 carry value causes a borrow, that leaves a
   limb value of either 0xFF...FF or 0xFF...FE and the multiply by
   MODLIMB_INVERSE_3 gives 0x55...55 or 0xAA...AA respectively, producing a
   further borrow of only 0 or 1 respectively.  Hence the carry out of each
   stage and for the return value is always only 0, 1 or 2.

   This implementation has each multiply successively dependent due to the
   "l=s-c", but the multiply src[i]*MODLIMB_INVERSE_3 could be scheduled
   back as far as desired, and the effect of the "-c" applied by subtracting
   0, 1 or 2 copies or MODLIMB_INVERSE_3 from the product.  With some good
   scheduling in assembler it might come down to a dependent chain of maybe
   5 simple operations per limb.  */

mp_limb_t
mpn_divexact_by3c (mp_ptr dst, mp_srcptr src, mp_size_t size, mp_limb_t c)
{
  mp_size_t  i;

  ASSERT (size >= 1);
  ASSERT (c == 0 || c == 1 || c == 2);
  ASSERT (MPN_SAME_OR_SEPARATE_P (dst, src, size));

  i = 0;
  do
    {
      mp_limb_t  l, s;

      s = src[i];
      l = s - c;
      c = (l > s);

      l = (l * MODLIMB_INVERSE_3) & GMP_NUMB_MASK;
      dst[i] = l;

      c += (l > GMP_NUMB_MAX/3);
      c += (l > (GMP_NUMB_MAX/3)*2);
    }
  while (++i < size);

  return c;
}