NetBSD/external/lgpl3/mpfr/dist/exp.c

/* mpfr_exp -- exponential of a floating-point number

Copyright 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011 Free Software Foundation, Inc.
Contributed by the Arenaire and Cacao 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
http://www.gnu.org/licenses/ or write to the Free Software Foundation, Inc.,
51 Franklin St, Fifth Floor, Boston, MA 02110-1301, USA. */

#include "mpfr-impl.h"

/* #define DEBUG */

int
mpfr_exp (mpfr_ptr y, mpfr_srcptr x, mpfr_rnd_t rnd_mode)
{
  mpfr_exp_t expx;
  mpfr_prec_t precy;
  int inexact;
  MPFR_SAVE_EXPO_DECL (expo);

  MPFR_LOG_FUNC (("x[%#R]=%R rnd=%d", x, x, rnd_mode),
                 ("y[%#R]=%R inexact=%d", y, y, inexact));

  if (MPFR_UNLIKELY( MPFR_IS_SINGULAR(x) ))
    {
      if (MPFR_IS_NAN(x))
        {
          MPFR_SET_NAN(y);
          MPFR_RET_NAN;
        }
      else if (MPFR_IS_INF(x))
        {
          if (MPFR_IS_POS(x))
            MPFR_SET_INF(y);
          else
            MPFR_SET_ZERO(y);
          MPFR_SET_POS(y);
          MPFR_RET(0);
        }
      else
        {
          MPFR_ASSERTD(MPFR_IS_ZERO(x));
          return mpfr_set_ui (y, 1, rnd_mode);
        }
    }

  /* First, let's detect most overflow and underflow cases. */
  {
    mpfr_t e, bound;

    /* We must extended the exponent range and save the flags now. */
    MPFR_SAVE_EXPO_MARK (expo);

    mpfr_init2 (e, sizeof (mpfr_exp_t) * CHAR_BIT);
    mpfr_init2 (bound, 32);

    inexact = mpfr_set_exp_t (e, expo.saved_emax, MPFR_RNDN);
    MPFR_ASSERTD (inexact == 0);
    mpfr_const_log2 (bound, expo.saved_emax < 0 ? MPFR_RNDD : MPFR_RNDU);
    mpfr_mul (bound, bound, e, MPFR_RNDU);
    if (MPFR_UNLIKELY (mpfr_cmp (x, bound) >= 0))
      {
        /* x > log(2^emax), thus exp(x) > 2^emax */
        mpfr_clears (e, bound, (mpfr_ptr) 0);
        MPFR_SAVE_EXPO_FREE (expo);
        return mpfr_overflow (y, rnd_mode, 1);
      }

    inexact = mpfr_set_exp_t (e, expo.saved_emin, MPFR_RNDN);
    MPFR_ASSERTD (inexact == 0);
    inexact = mpfr_sub_ui (e, e, 2, MPFR_RNDN);
    MPFR_ASSERTD (inexact == 0);
    mpfr_const_log2 (bound, expo.saved_emin < 0 ? MPFR_RNDU : MPFR_RNDD);
    mpfr_mul (bound, bound, e, MPFR_RNDD);
    if (MPFR_UNLIKELY (mpfr_cmp (x, bound) <= 0))
      {
        /* x < log(2^(emin - 2)), thus exp(x) < 2^(emin - 2) */
        mpfr_clears (e, bound, (mpfr_ptr) 0);
        MPFR_SAVE_EXPO_FREE (expo);
        return mpfr_underflow (y, rnd_mode == MPFR_RNDN ? MPFR_RNDZ : rnd_mode,
                               1);
      }

    /* Other overflow/underflow cases must be detected
       by the generic routines. */
    mpfr_clears (e, bound, (mpfr_ptr) 0);
    MPFR_SAVE_EXPO_FREE (expo);
  }

  expx  = MPFR_GET_EXP (x);
  precy = MPFR_PREC (y);

  /* if x < 2^(-precy), then exp(x) i.e. gives 1 +/- 1 ulp(1) */
  if (MPFR_UNLIKELY (expx < 0 && (mpfr_uexp_t) (-expx) > precy))
    {
      mpfr_exp_t emin = __gmpfr_emin;
      mpfr_exp_t emax = __gmpfr_emax;
      int signx = MPFR_SIGN (x);

      MPFR_SET_POS (y);
      if (MPFR_IS_NEG_SIGN (signx) && (rnd_mode == MPFR_RNDD ||
                                       rnd_mode == MPFR_RNDZ))
        {
          __gmpfr_emin = 0;
          __gmpfr_emax = 0;
          mpfr_setmax (y, 0);  /* y = 1 - epsilon */
          inexact = -1;
        }
      else
        {
          __gmpfr_emin = 1;
          __gmpfr_emax = 1;
          mpfr_setmin (y, 1);  /* y = 1 */
          if (MPFR_IS_POS_SIGN (signx) && (rnd_mode == MPFR_RNDU ||
                                           rnd_mode == MPFR_RNDA))
            {
              mp_size_t yn;
              int sh;

              yn = 1 + (MPFR_PREC(y) - 1) / GMP_NUMB_BITS;
              sh = (mpfr_prec_t) yn * GMP_NUMB_BITS - MPFR_PREC(y);
              MPFR_MANT(y)[0] += MPFR_LIMB_ONE << sh;
              inexact = 1;
            }
          else
            inexact = -MPFR_FROM_SIGN_TO_INT(signx);
        }

      __gmpfr_emin = emin;
      __gmpfr_emax = emax;
    }
  else  /* General case */
    {
      if (MPFR_UNLIKELY (precy >= MPFR_EXP_THRESHOLD))
        /* mpfr_exp_3 saves the exponent range and flags itself, otherwise
           the flag changes in mpfr_exp_3 are lost */
        inexact = mpfr_exp_3 (y, x, rnd_mode); /* O(M(n) log(n)^2) */
      else
        {
          MPFR_SAVE_EXPO_MARK (expo);
          inexact = mpfr_exp_2 (y, x, rnd_mode); /* O(n^(1/3) M(n)) */
          MPFR_SAVE_EXPO_UPDATE_FLAGS (expo, __gmpfr_flags);
          MPFR_SAVE_EXPO_FREE (expo);
        }
    }

  return mpfr_check_range (y, inexact, rnd_mode);
}