MathApprox.h

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/*  SPDX-License-Identifier: GPL-2.0-or-later */
/*!********************************************************************
 
  Audacity: A Digital Audio Editor
 
  MathApprox.h
 
  Matthieu Hodgkinson
 
**********************************************************************/
#pragma once
 
#include <cstdint>
 
constexpr float FastLog2(float x)
{
   static_assert(sizeof(float) == sizeof(int32_t));
   union
   {
      float val;
      int32_t x;
   } u = { x };
   auto log_2 = (float)(((u.x >> 23) & 255) - 128);
   u.x &= ~(255 << 23);
   u.x += 127 << 23;
   log_2 += ((-0.3358287811f) * u.val + 2.0f) * u.val - 0.65871759316667f;
   return log_2;
}