kc3-lang/gnulib/lib/log2f.c

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• Hash : 32a72f45
  Author :
  Date : 2023-01-01T01:14:21
  

  maint: run 'make update-copyright'
  

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• lib/log2f.c

• /* Base 2 logarithm.
     Copyright (C) 2012-2023 Free Software Foundation, Inc.
  
     This file 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 3 of the
     License, or (at your option) any later version.
  
     This file 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 this program.  If not, see <https://www.gnu.org/licenses/>.  */
  
  #include <config.h>
  
  /* Specification.  */
  #include <math.h>
  
  #if HAVE_LOG2
  
  float
  log2f (float x)
  {
    return (float) log2 ((double) x);
  }
  
  #else
  
  /* Best possible approximation of log(2) as a 'float'.  */
  #define LOG2 0.693147180559945309417232121458176568075f
  
  /* Best possible approximation of 1/log(2) as a 'float'.  */
  #define LOG2_INVERSE 1.44269504088896340735992468100189213743f
  
  /* sqrt(0.5).  */
  #define SQRT_HALF 0.707106781186547524400844362104849039284f
  
  float
  log2f (float x)
  {
    if (isnanf (x))
      return x;
  
    if (x <= 0.0f)
      {
        if (x == 0.0f)
          /* Return -Infinity.  */
          return - HUGE_VALF;
        else
          {
            /* Return NaN.  */
  #if defined _MSC_VER
            static float zero;
            return zero / zero;
  #else
            return 0.0f / 0.0f;
  #endif
          }
      }
  
    /* Decompose x into
         x = 2^e * y
       where
         e is an integer,
         1/2 < y < 2.
       Then log2(x) = e + log2(y) = e + log(y)/log(2).  */
    {
      int e;
      float y;
  
      y = frexpf (x, &e);
      if (y < SQRT_HALF)
        {
          y = 2.0f * y;
          e = e - 1;
        }
  
      return (float) e + logf (y) * LOG2_INVERSE;
    }
  }
  
  #endif
  

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