/* mpfr_acos -- arc-cosinus of a floating-point number

Copyright 2001, 2002, 2003, 2004, 2005, 2006 Free Software Foundation, Inc.

This file is part of the MPFR Library, and was contributed by Mathieu Dutour.

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., 51 Franklin Place, Fifth Floor, Boston,
MA 02110-1301, USA. */

#include "mpfr-impl.h"

int
mpfr_acos (mpfr_ptr acos, mpfr_srcptr x, mp_rnd_t rnd_mode)
{
  mpfr_t xp, arcc, tmp;
  mp_exp_t supplement;
  mp_prec_t prec;
  int sign, compared, inexact;
  MPFR_SAVE_EXPO_DECL (expo);
  MPFR_ZIV_DECL (loop);

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

  /* Singular cases */
  if (MPFR_UNLIKELY (MPFR_IS_SINGULAR (x)))
    {
      if (MPFR_IS_NAN (x) || MPFR_IS_INF (x))
        {
          MPFR_SET_NAN (acos);
          MPFR_RET_NAN;
        }
      else /* necessarily x=0 */
        {
          MPFR_ASSERTD(MPFR_IS_ZERO(x));
          /* acos(0)=Pi/2 */
          inexact = mpfr_const_pi (acos, rnd_mode);
          mpfr_div_2ui (acos, acos, 1, rnd_mode); /* exact */
          MPFR_RET (inexact);
        }
    }

  /* Set x_p=|x| */
  sign = MPFR_SIGN (x);
  mpfr_init2 (xp, MPFR_PREC (x));
  mpfr_abs (xp, x, GMP_RNDN); /* Exact */

  compared = mpfr_cmp_ui (xp, 1);

  if (MPFR_UNLIKELY (compared >= 0))
    {
      mpfr_clear (xp);
      if (compared > 0) /* acos(x) = NaN for x > 1 */
        {
          MPFR_SET_NAN(acos);
          MPFR_RET_NAN;
        }
      else
        {
          if (MPFR_IS_POS_SIGN (sign)) /* acos(+1) = 0 */
            return mpfr_set_ui (acos, 0, rnd_mode);
          else /* acos(-1) = Pi */
            return mpfr_const_pi (acos, rnd_mode);
        }
    }

  MPFR_SAVE_EXPO_MARK (expo);

  /* Compute the supplement */
  mpfr_ui_sub (xp, 1, xp, GMP_RNDD);
  if (MPFR_IS_POS_SIGN (sign))
    supplement = 2 - 2 * MPFR_GET_EXP (xp);
  else
    supplement = 2 - MPFR_GET_EXP (xp);
  mpfr_clear (xp);

  prec = MPFR_PREC (acos) + 10 + supplement;

  /* If x ~ 2^-N, acos(x) ~ PI/2 - x - x^3/6
     If Prec < 2*N, we can't round since x^3/6 won't be counted. */
  if (MPFR_PREC (acos) >= MPFR_PREC (x)
      && (mp_exp_t) prec <= -2*MPFR_GET_EXP (x) + 5)
    prec = (mpfr_uexp_t) (-2*MPFR_GET_EXP (x)) + 5;

  mpfr_init2 (tmp, prec);
  mpfr_init2 (arcc, prec);

  MPFR_ZIV_INIT (loop, prec);
  for (;;)
    {
      /* acos(x) = Pi/2 - asin(x) = Pi/2 - atan(x/sqrt(1-x^2)) */
      mpfr_sqr (tmp, x, GMP_RNDN);
      mpfr_ui_sub (tmp, 1, tmp, GMP_RNDN);
      mpfr_sqrt (tmp, tmp, GMP_RNDN);
      mpfr_div (tmp, x, tmp, GMP_RNDN);
      mpfr_atan (arcc, tmp, GMP_RNDN);
      mpfr_const_pi (tmp, GMP_RNDN);
      mpfr_div_2ui (tmp, tmp, 1, GMP_RNDN);
      mpfr_sub (arcc, tmp, arcc, GMP_RNDN);

      if (MPFR_LIKELY (MPFR_CAN_ROUND (arcc, prec-supplement,
                                       MPFR_PREC (acos), rnd_mode)))
        break;
      MPFR_ZIV_NEXT (loop, prec);
      mpfr_set_prec (tmp, prec);
      mpfr_set_prec (arcc, prec);
    }
  MPFR_ZIV_FREE (loop);

  inexact = mpfr_set (acos, arcc, rnd_mode);
  mpfr_clear (tmp);
  mpfr_clear (arcc);

  MPFR_SAVE_EXPO_FREE (expo);
  return mpfr_check_range (acos, inexact, rnd_mode);
}