| /* mpfr_acos -- arc-cosinus of a floating-point number |
| |
| Copyright 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009 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_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; |
| |
| /* VL: The following change concerning prec comes from r3145 |
| "Optimize mpfr_acos by choosing a better initial precision." |
| but it doesn't seem to be correct and leads to problems (assertion |
| failure or very important inefficiency) with tiny arguments. |
| Therefore, I've disabled it. */ |
| /* 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 0 |
| if (MPFR_PREC (acos) >= MPFR_PREC (x) && MPFR_GET_EXP (x) < 0) |
| { |
| mpfr_uexp_t pmin = (mpfr_uexp_t) (-2 * MPFR_GET_EXP (x)) + 5; |
| MPFR_ASSERTN (pmin <= MPFR_PREC_MAX); |
| if (prec < pmin) |
| prec = pmin; |
| } |
| #endif |
| |
| 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); |
| } |