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git-svn-id: file:///srv/svn/repos/haiku/buildtools/trunk@29042 a95241bf-73f2-0310-859d-f6bbb57e9c96
137 lines
3.8 KiB
C
137 lines
3.8 KiB
C
/* mpfr_log2 -- log base 2
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Copyright 2001, 2002, 2003, 2004, 2005, 2006, 2007 Free Software Foundation, Inc.
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Contributed by the Arenaire and Cacao projects, INRIA.
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This file is part of the MPFR Library.
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The MPFR Library is free software; you can redistribute it and/or modify
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it under the terms of the GNU Lesser General Public License as published by
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the Free Software Foundation; either version 2.1 of the License, or (at your
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option) any later version.
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The MPFR Library is distributed in the hope that it will be useful, but
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WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
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or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public
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License for more details.
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You should have received a copy of the GNU Lesser General Public License
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along with the MPFR Library; see the file COPYING.LIB. If not, write to
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the Free Software Foundation, Inc., 51 Franklin St, Fifth Floor, Boston,
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MA 02110-1301, USA. */
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#define MPFR_NEED_LONGLONG_H
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#include "mpfr-impl.h"
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/* The computation of r=log2(a)
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r=log2(a)=log(a)/log(2) */
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int
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mpfr_log2 (mpfr_ptr r, mpfr_srcptr a, mp_rnd_t rnd_mode)
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{
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int inexact;
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MPFR_SAVE_EXPO_DECL (expo);
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if (MPFR_UNLIKELY (MPFR_IS_SINGULAR (a)))
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{
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/* If a is NaN, the result is NaN */
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if (MPFR_IS_NAN (a))
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{
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MPFR_SET_NAN (r);
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MPFR_RET_NAN;
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}
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/* check for infinity before zero */
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else if (MPFR_IS_INF (a))
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{
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if (MPFR_IS_NEG (a))
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/* log(-Inf) = NaN */
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{
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MPFR_SET_NAN (r);
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MPFR_RET_NAN;
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}
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else /* log(+Inf) = +Inf */
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{
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MPFR_SET_INF (r);
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MPFR_SET_POS (r);
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MPFR_RET (0);
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}
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}
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else /* a is zero */
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{
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MPFR_ASSERTD (MPFR_IS_ZERO (a));
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MPFR_SET_INF (r);
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MPFR_SET_NEG (r);
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MPFR_RET (0); /* log2(0) is an exact -infinity */
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}
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}
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/* If a is negative, the result is NaN */
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if (MPFR_UNLIKELY (MPFR_IS_NEG (a)))
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{
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MPFR_SET_NAN (r);
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MPFR_RET_NAN;
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}
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/* If a is 1, the result is 0 */
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if (MPFR_UNLIKELY (mpfr_cmp_ui (a, 1) == 0))
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{
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MPFR_SET_ZERO (r);
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MPFR_SET_POS (r);
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MPFR_RET (0); /* only "normal" case where the result is exact */
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}
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/* If a is 2^N, log2(a) is exact*/
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if (MPFR_UNLIKELY (mpfr_cmp_ui_2exp (a, 1, MPFR_GET_EXP (a) - 1) == 0))
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return mpfr_set_si(r, MPFR_GET_EXP (a) - 1, rnd_mode);
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MPFR_SAVE_EXPO_MARK (expo);
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/* General case */
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{
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/* Declaration of the intermediary variable */
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mpfr_t t, tt;
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/* Declaration of the size variable */
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mp_prec_t Ny = MPFR_PREC(r); /* target precision */
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mp_prec_t Nt; /* working precision */
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mp_exp_t err; /* error */
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MPFR_ZIV_DECL (loop);
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/* compute the precision of intermediary variable */
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/* the optimal number of bits : see algorithms.tex */
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Nt = Ny + 3 + MPFR_INT_CEIL_LOG2 (Ny);
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/* initialise of intermediary variable */
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mpfr_init2 (t, Nt);
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mpfr_init2 (tt, Nt);
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/* First computation of log2 */
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MPFR_ZIV_INIT (loop, Nt);
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for (;;)
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{
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/* compute log2 */
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mpfr_const_log2(t,GMP_RNDD); /* log(2) */
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mpfr_log(tt,a,GMP_RNDN); /* log(a) */
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mpfr_div(t,tt,t,GMP_RNDN); /* log(a)/log(2) */
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/* estimation of the error */
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err = Nt-3;
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if (MPFR_LIKELY (MPFR_CAN_ROUND (t, err, Ny, rnd_mode)))
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break;
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/* actualisation of the precision */
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MPFR_ZIV_NEXT (loop, Nt);
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mpfr_set_prec (t, Nt);
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mpfr_set_prec (tt, Nt);
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}
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MPFR_ZIV_FREE (loop);
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inexact = mpfr_set (r, t, rnd_mode);
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mpfr_clear (t);
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mpfr_clear (tt);
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}
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MPFR_SAVE_EXPO_FREE (expo);
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return mpfr_check_range (r, inexact, rnd_mode);
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}
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