sub_n.c

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/* __mpn_sub_n -- Subtract two limb vectors of equal, non-zero length.

Copyright (C) 1992, 1993, 1994 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 Library General Public License as published by
the Free Software Foundation; either version 2 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 Library General Public
License for more details.

You should have received a copy of the GNU Library General Public License
along with the GNU MP Library; see the file COPYING.LIB.  If not, write to
the Free Software Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA. */

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

mp_limb
#if __STDC__
__mpn_sub_n (mp_ptr res_ptr, mp_srcptr s1_ptr, mp_srcptr s2_ptr, mp_size_t size)
#else
__mpn_sub_n (res_ptr, s1_ptr, s2_ptr, size)
     register mp_ptr res_ptr;
     register mp_srcptr s1_ptr;
     register mp_srcptr s2_ptr;
     mp_size_t size;
#endif
{
  register mp_limb x, y, cy;
  register mp_size_t j;

  /* The loop counter and index J goes from -SIZE to -1.  This way
     the loop becomes faster.  */
  j = -size;

  /* Offset the base pointers to compensate for the negative indices.  */
  s1_ptr -= j;
  s2_ptr -= j;
  res_ptr -= j;

  cy = 0;
  do
    {
      y = s2_ptr[j];
      x = s1_ptr[j];
      y += cy;			/* add previous carry to subtrahend */
      cy = (y < cy);		/* get out carry from that addition */
      y = x - y;		/* main subtract */
      cy = (y > x) + cy;	/* get out carry from the subtract, combine */
      res_ptr[j] = y;
    }
  while (++j != 0);

  return cy;
}