1  /* __gmpfr_ceil_log2 - returns ceil(log(d)/log(2))
       2  
       3  Copyright 1999-2004, 2006-2023 Free Software Foundation, Inc.
       4  Contributed by the AriC and Caramba projects, INRIA.
       5  
       6  This file is part of the GNU MPFR Library.
       7  
       8  The GNU MPFR Library is free software; you can redistribute it and/or modify
       9  it under the terms of the GNU Lesser General Public License as published by
      10  the Free Software Foundation; either version 3 of the License, or (at your
      11  option) any later version.
      12  
      13  The GNU MPFR Library is distributed in the hope that it will be useful, but
      14  WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
      15  or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
      16  License for more details.
      17  
      18  You should have received a copy of the GNU Lesser General Public License
      19  along with the GNU MPFR Library; see the file COPYING.LESSER.  If not, see
      20  https://www.gnu.org/licenses/ or write to the Free Software Foundation, Inc.,
      21  51 Franklin St, Fifth Floor, Boston, MA 02110-1301, USA. */
      22  
      23  #include "mpfr-impl.h"
      24  
      25  /* returns ceil(log(d)/log(2)) if d > 0,
      26     -1023 if d = +0,
      27     and floor(log(-d)/log(2))+1 if d < 0
      28  */
      29  long
      30  __gmpfr_ceil_log2 (double d)
      31  {
      32    long exp;
      33  #if _MPFR_IEEE_FLOATS
      34    union mpfr_ieee_double_extract x;
      35  
      36    x.d = d;
      37    /* The cast below is useless in theory, but let us not depend on the
      38       integer promotion rules (for instance, tcc is currently wrong). */
      39    exp = (long) x.s.exp - 1023;
      40    MPFR_ASSERTN (exp < 1023);  /* fail on infinities */
      41    x.s.exp = 1023; /* value for 1 <= d < 2 */
      42    if (x.d != 1.0) /* d: not a power of two? */
      43      exp++;
      44    return exp;
      45  #else /* _MPFR_IEEE_FLOATS */
      46    double m;
      47  
      48    if (d < 0.0)
      49      return __gmpfr_floor_log2 (-d) + 1;
      50    else if (d == 0.0)
      51      return -1023;
      52    else if (d >= 1.0)
      53      {
      54        exp = 0;
      55        for (m = 1.0; m < d; m *= 2.0)
      56          exp++;
      57      }
      58    else
      59      {
      60        exp = 1;
      61        for (m = 1.0; m >= d; m *= 0.5)
      62          exp--;
      63      }
      64  #endif /* _MPFR_IEEE_FLOATS */
      65    return exp;
      66  }
      67