/* mpfr_asin -- arc-sinus of a floating-point number

Copyright 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008 Free Software Foundation, Inc.
Contributed by the Arenaire and Cacao projects, INRIA.

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

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 2.1 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.LIB.  If not, write to
the Free Software Foundation, Inc., 51 Franklin St, Fifth Floor, Boston,
MA 02110-1301, USA. */

#include "mpfr-impl.h"

int
mpfr_asin (mpfr_ptr asin, mpfr_srcptr x, mp_rnd_t rnd_mode)
{
  mpfr_t xp;
  int compared, inexact;
  mp_prec_t prec;
  mp_exp_t xp_exp;
  MPFR_SAVE_EXPO_DECL (expo);
  MPFR_ZIV_DECL (loop);

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

  /* Special cases */
  if (MPFR_UNLIKELY (MPFR_IS_SINGULAR (x)))
    {
      if (MPFR_IS_NAN (x) || MPFR_IS_INF (x))
        {
          MPFR_SET_NAN (asin);
          MPFR_RET_NAN;
        }
      else /* x = 0 */
        {
          MPFR_ASSERTD (MPFR_IS_ZERO (x));
          MPFR_SET_ZERO (asin);
          MPFR_SET_SAME_SIGN (asin, x);
          MPFR_RET (0); /* exact result */
        }
    }

  /* asin(x) = x + x^3/6 + ... so the error is < 2^(3*EXP(x)-2) */
  MPFR_FAST_COMPUTE_IF_SMALL_INPUT (asin, x, -2 * MPFR_GET_EXP (x), 2, 1,
                                    rnd_mode, {});

  /* Set x_p=|x| (x is a normal number) */
  mpfr_init2 (xp, MPFR_PREC (x));
  inexact = mpfr_abs (xp, x, GMP_RNDN);
  MPFR_ASSERTD (inexact == 0);

  compared = mpfr_cmp_ui (xp, 1);

  if (MPFR_UNLIKELY (compared >= 0))
    {
      mpfr_clear (xp);
      if (compared > 0)                  /* asin(x) = NaN for |x| > 1 */
        {
          MPFR_SET_NAN (asin);
          MPFR_RET_NAN;
        }
      else                              /* x = 1 or x = -1 */
        {
          if (MPFR_IS_POS (x)) /* asin(+1) = Pi/2 */
            inexact = mpfr_const_pi (asin, rnd_mode);
          else /* asin(-1) = -Pi/2 */
            {
              inexact = -mpfr_const_pi (asin, MPFR_INVERT_RND(rnd_mode));
              MPFR_CHANGE_SIGN (asin);
            }
          mpfr_div_2ui (asin, asin, 1, rnd_mode); /* May underflow */
          return inexact;
        }
    }

  MPFR_SAVE_EXPO_MARK (expo);

  /* Compute exponent of 1 - ABS(x) */
  mpfr_ui_sub (xp, 1, xp, GMP_RNDD);
  MPFR_ASSERTD (MPFR_GET_EXP (xp) <= 0);
  MPFR_ASSERTD (MPFR_GET_EXP (x) <= 0);
  xp_exp = 2 - MPFR_GET_EXP (xp);

  /* Set up initial prec */
  prec = MPFR_PREC (asin) + 10 + xp_exp;

  /* use asin(x) = atan(x/sqrt(1-x^2)) */
  MPFR_ZIV_INIT (loop, prec);
  for (;;)
    {
      mpfr_set_prec (xp, prec);
      mpfr_sqr (xp, x, GMP_RNDN);
      mpfr_ui_sub (xp, 1, xp, GMP_RNDN);
      mpfr_sqrt (xp, xp, GMP_RNDN);
      mpfr_div (xp, x, xp, GMP_RNDN);
      mpfr_atan (xp, xp, GMP_RNDN);
      if (MPFR_LIKELY (MPFR_CAN_ROUND (xp, prec - xp_exp,
                                       MPFR_PREC (asin), rnd_mode)))
        break;
      MPFR_ZIV_NEXT (loop, prec);
    }
  MPFR_ZIV_FREE (loop);
  inexact = mpfr_set (asin, xp, rnd_mode);

  mpfr_clear (xp);

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