/* mpfr_ui_sub -- subtract a floating-point number from an integer

Copyright 2000-2019 Free Software Foundation, Inc.
Contributed by the AriC and Caramba projects, INRIA.

This file is part of the GNU MPFR Library.

The GNU 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 3 of the License, or (at your
option) any later version.

The GNU 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 GNU MPFR Library; see the file COPYING.LESSER.  If not, see
https://www.gnu.org/licenses/ or write to the Free Software Foundation, Inc.,
51 Franklin St, Fifth Floor, Boston, MA 02110-1301, USA. */

#define MPFR_NEED_LONGLONG_H
#include "mpfr-impl.h"

int
mpfr_ui_sub (mpfr_ptr y, unsigned long int u, mpfr_srcptr x, mpfr_rnd_t rnd_mode)
{
  MPFR_LOG_FUNC
    (("u=%lu x[%Pu]=%.*Rg rnd=%d",
      u, mpfr_get_prec(x), mpfr_log_prec, x, rnd_mode),
     ("y[%Pu]=%.*Rg", mpfr_get_prec(y), mpfr_log_prec, y));

  /* (unsigned long) 0 is assumed to be a real 0 (unsigned) */
  if (MPFR_UNLIKELY (u == 0))
    return mpfr_neg (y, x, rnd_mode);

  if (MPFR_UNLIKELY (MPFR_IS_SINGULAR (x)))
    {
      if (MPFR_IS_NAN (x))
        {
          MPFR_SET_NAN (y);
          MPFR_RET_NAN;
        }
      if (MPFR_IS_INF (x))
        {
          /*  u - Inf = -Inf and u - -Inf = +Inf  */
          MPFR_SET_INF (y);
          MPFR_SET_OPPOSITE_SIGN (y, x);
          MPFR_RET (0); /* +/-infinity is exact */
        }
      MPFR_ASSERTD (MPFR_IS_ZERO (x) && u != 0);
      /* Note: the fact that u != 0 is important due to signed zeros. */
      /* u - 0 = u */
      return mpfr_set_ui (y, u, rnd_mode);
    }

  /* Main code */
  {
    mpfr_t uu;
    mp_limb_t up[1];
    int cnt;
    int inex;
    MPFR_SAVE_EXPO_DECL (expo);

    MPFR_TMP_INIT1 (up, uu, GMP_NUMB_BITS);
    MPFR_STAT_STATIC_ASSERT (MPFR_LIMB_MAX >= ULONG_MAX);
    /* So, u fits in a mp_limb_t, which justifies the casts below. */
    MPFR_ASSERTD (u != 0);
    count_leading_zeros (cnt, (mp_limb_t) u);
    up[0] = (mp_limb_t) u << cnt;

    /* Optimization note: Exponent save/restore operations may be
       removed if mpfr_sub works even when uu is out-of-range. */
    MPFR_SAVE_EXPO_MARK (expo);
    MPFR_SET_EXP (uu, GMP_NUMB_BITS - cnt);
    inex = mpfr_sub (y, uu, x, rnd_mode);
    MPFR_SAVE_EXPO_UPDATE_FLAGS (expo, __gmpfr_flags);
    MPFR_SAVE_EXPO_FREE (expo);
    return mpfr_check_range (y, inex, rnd_mode);
  }
}