/* mpz_sub_ui -- Subtract an unsigned one-word integer from an MP_INT.

Copyright (C) 1991, 1993, 1994, 1996, 1999 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"

void
#if __STDC__
mpz_sub_ui (mpz_ptr w, mpz_srcptr u, unsigned long int v)
#else
mpz_sub_ui (w, u, v)
     mpz_ptr w;
     mpz_srcptr u;
     unsigned long int v;
#endif
{
  mp_srcptr up;
  mp_ptr wp;
  mp_size_t usize, wsize;
  mp_size_t abs_usize;

  usize = u->_mp_size;
  abs_usize = ABS (usize);

  /* If not space for W (and possible carry), increase space.  */
  wsize = abs_usize + 1;
  if (w->_mp_alloc < wsize)
    _mpz_realloc (w, wsize);

  /* These must be after realloc (U may be the same as W).  */
  up = u->_mp_d;
  wp = w->_mp_d;

  if (abs_usize == 0)
    {
      wp[0] = v;
      w->_mp_size = -(v != 0);
      return;
    }

  if (usize < 0)
    {
      mp_limb_t cy;
      cy = mpn_add_1 (wp, up, abs_usize, (mp_limb_t) v);
      wp[abs_usize] = cy;
      wsize = -(abs_usize + cy);
    }
  else
    {
      /* The signs are different.  Need exact comparison to determine
	 which operand to subtract from which.  */
      if (abs_usize == 1 && up[0] < v)
	{
	  wp[0] = v - up[0];
	  wsize = -1;
	}
      else
	{
	  mpn_sub_1 (wp, up, abs_usize, (mp_limb_t) v);
	  /* Size can decrease with at most one limb.  */
	  wsize = abs_usize - (wp[abs_usize - 1] == 0);
	}
    }

  w->_mp_size = wsize;
}
