/* mpfr_add_one_ulp, mpfr_sub_one_ulp -- add/subtract one unit in last place

Copyright (C) 1999, 2001 Free Software Foundation, Inc.

This file is part of the MPFR Library.

The MPFR 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 MPFR 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 MPFR 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 "mpfr.h"
#include "mpfr-impl.h"
#include "gmp-impl.h"

/* sets x to x+sign(x)*2^(MPFR_EXP(x)-MPFR_PREC(x)) */
void
mpfr_add_one_ulp (mpfr_ptr x)
{
  int xn, sh;
  mp_limb_t *xp;

  if (MPFR_IS_INF(x))
    return;

  xn = 1 + (MPFR_PREC(x) - 1) / BITS_PER_MP_LIMB;
  sh = xn * BITS_PER_MP_LIMB - MPFR_PREC(x);
  xp = MPFR_MANT(x);
  if (mpn_add_1 (xp, xp, xn, MP_LIMB_T_ONE << sh)) /* got 1.0000... */
    {
      MPFR_EXP(x)++;
      xp[xn-1] = MP_LIMB_T_HIGHBIT;
  }
  return;
}

/* sets x to x-sign(x)*ulp(x) */
void
mpfr_sub_one_ulp(mpfr_ptr x)
{
  int xn, sh;
  mp_limb_t *xp;

  if (MPFR_IS_INF(x))
    return;

  xn = 1 + (MPFR_PREC(x) - 1) / BITS_PER_MP_LIMB;
  sh = xn * BITS_PER_MP_LIMB - MPFR_PREC(x);
  xp = MPFR_MANT(x);
  mpn_sub_1 (xp, xp, xn, MP_LIMB_T_ONE << sh);
  if (xp[xn-1] >> (BITS_PER_MP_LIMB - 1) == 0) {
    /* was an exact power of two: not normalized any more */
    MPFR_EXP(x)--;
    mpn_lshift (xp, xp, xn, 1);
    *xp |= MP_LIMB_T_ONE << sh;
  }
  return;
}