PixelSampleMapper.cpp

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/*  SPDX-License-Identifier: GPL-2.0-or-later */
/*!********************************************************************
 
  Audacity: A Digital Audio Editor
 
  PixelSampleMapper.h
 
  Dmitry Vedenko
 
**********************************************************************/
#include "PixelSampleMapper.h"
 
#include <cassert>
#include <cmath>
 
#include "SampleCount.h"
#include "Variant.h"
 
PixelSampleMapper::PixelSampleMapper(
   double t0, double rate, double samplesPerPixel) noexcept
    : mMapper(LinearMapper { (0.5 + t0 * rate), samplesPerPixel })
{
   assert((0.5 + t0 * rate) >= 0.0);
}
 
void PixelSampleMapper::applyBias(double bias) noexcept
{
   auto mapper = std::get_if<LinearMapper>(&mMapper);
 
   if (mapper != nullptr)
      mapper->mInitialValue += bias;
}
 
double PixelSampleMapper::applyCorrection(
   const PixelSampleMapper& oldMapper, size_t oldLen, size_t newLen)
{
   assert(mMapper.index() == 0);
   assert(oldMapper.mMapper.index() == 0);
 
   LinearMapper* currentMapper = std::get_if<LinearMapper>(&mMapper);
 
   if (currentMapper == nullptr)
      return {};
 
   const LinearMapper* oldLinearMapper =
      std::get_if<LinearMapper>(&oldMapper.mMapper);
 
   if (oldLinearMapper == nullptr)
      return {};
 
   // Find the sample position that is the origin in the old cache.
   const double oldWhere0 =
      (*oldLinearMapper)(1).as_double() - currentMapper->mSamplesPerPixel;
   const double oldWhereLast =
      oldWhere0 + oldLen * currentMapper->mSamplesPerPixel;
   // Find the length in samples of the old cache.
   const double denom = oldWhereLast - oldWhere0;
 
   // What sample would go in where[0] with no correction?
   const double guessWhere0 =
      currentMapper->mInitialValue - 0.5; // Why do we ignore initial bias?
 
   if ( // Skip if old and NEW are disjoint:
      oldWhereLast <= guessWhere0 ||
      guessWhere0 + newLen * currentMapper->mSamplesPerPixel <= oldWhere0 ||
      // Skip unless denom rounds off to at least 1.
      denom < 0.5)
   {
      // The computation of oldX0 in the other branch
      // may underflow and the assertion would be violated.
      return oldLen;
   }
   else
   {
      // What integer position in the old cache array does that map to?
      // (even if it is out of bounds)
      const auto oldX0 =
         std::floor(0.5 + oldLen * (guessWhere0 - oldWhere0) / denom);
      // What sample count would the old cache have put there?
      const double where0 =
         oldWhere0 + double(oldX0) * currentMapper->mSamplesPerPixel;
      // What correction is needed to align the NEW cache with the old?
      const double correction0 = where0 - guessWhere0;
      const double correction = std::max(
         -currentMapper->mSamplesPerPixel,
         std::min(currentMapper->mSamplesPerPixel, correction0));
 
      assert(correction == correction0);
 
      currentMapper->mInitialValue += correction;
 
      return oldX0;
   }
}
 
sampleCount PixelSampleMapper::GetFirstSample(uint32_t column) const
{
   return Variant::Visit(
      [column](const auto& mapper) { return mapper(column); }, mMapper);
}
 
sampleCount PixelSampleMapper::GetLastSample(uint32_t column) const
{
   return GetFirstSample(column + 1);
}
 
std::pair<sampleCount, sampleCount>
PixelSampleMapper::GetSampleRange(uint32_t column) const
{
   return { GetFirstSample(column), GetLastSample(column) };
}
 
void PixelSampleMapper::setCustomMapper(CustomMapper mapper)
{
   mMapper = std::move(mapper);
}
 
bool PixelSampleMapper::IsValid() const
{
   return Variant::Visit([](const auto& mapper) { return !!mapper; }, mMapper);
}
 
bool PixelSampleMapper::IsLinear() const noexcept
{
   return std::get_if<LinearMapper>(&mMapper) != nullptr;
}
 
sampleCount
PixelSampleMapper::LinearMapper::operator()(uint32_t column) const noexcept
{
   // Previous code used floor, but "required" mInitialValue to be positive.
   // For positive values, let's just trunc the value, as it is at least twice
   // as fast.
   return sampleCount(mInitialValue + column * mSamplesPerPixel);
}
 
PixelSampleMapper::LinearMapper::operator bool() const noexcept
{
   return mSamplesPerPixel > 0.0;
}