RingBuffer.cpp

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/**********************************************************************
 
  Audacity: A Digital Audio Editor
 
  RingBuffer.cpp
 
  Dominic Mazzoni
  Paul Licameli
 
*******************************************************************//*******************************************************************/
 
 
#include "RingBuffer.h"
#include "Dither.h"
 
RingBuffer::RingBuffer(sampleFormat format, size_t size)
   : mBufferSize{ std::max<size_t>(size, 64) }
   , mFormat{ format }
   , mBuffer{ mBufferSize, mFormat }
{
}
 
RingBuffer::~RingBuffer()
{
}
 
// Calculations of free and filled space, given snapshots taken of the start
// and end values
 
size_t RingBuffer::Filled( size_t start, size_t end )
{
   return (end + mBufferSize - start) % mBufferSize;
}
 
size_t RingBuffer::Free( size_t start, size_t end )
{
   return std::max<size_t>(mBufferSize - Filled( start, end ), 4) - 4;
}
 
//
// For the writer only:
// Only writer reads or writes mWritten
// Only writer writes the end, so it can read it again relaxed
// And it reads the start written by reader, with acquire order,
// so that any reading done in Get() happens-before any reuse of the space.
//
 
size_t RingBuffer::AvailForPut()
{
   auto start = mStart.load( std::memory_order_relaxed );
   return Free( start, mWritten );
 
   // Reader might increase the available free space after return, but will
   // never decrease it, so writer can safely assume this much at least
}
 
size_t RingBuffer::Put(constSamplePtr buffer, sampleFormat format,
                    size_t samplesToCopy, size_t padding)
{
   auto start = mStart.load( std::memory_order_acquire );
   auto end = mWritten;
   const auto free = Free( start, end );
   samplesToCopy = std::min( samplesToCopy, free );
   padding = std::min( padding, free - samplesToCopy );
   auto src = buffer;
   size_t copied = 0;
   auto pos = end;
 
   while ( samplesToCopy ) {
      auto block = std::min( samplesToCopy, mBufferSize - pos );
 
      CopySamples(src, format,
                  mBuffer.ptr() + pos * SAMPLE_SIZE(mFormat), mFormat,
                  block, DitherType::none);
 
      src += block * SAMPLE_SIZE(format);
      pos = (pos + block) % mBufferSize;
      samplesToCopy -= block;
      copied += block;
   }
 
   while ( padding ) {
      const auto block = std::min( padding, mBufferSize - pos );
      ClearSamples( mBuffer.ptr(), mFormat, pos, block );
      pos = (pos + block) % mBufferSize;
      padding -= block;
      copied += block;
   }
 
   mWritten = pos;
 
   return copied;
}
 
size_t RingBuffer::Clear(sampleFormat format, size_t samplesToClear)
{
   auto start = mStart.load( std::memory_order_acquire );
   auto end = mWritten;
   samplesToClear = std::min( samplesToClear, Free( start, end ) );
   size_t cleared = 0;
   auto pos = end;
 
   while(samplesToClear) {
      auto block = std::min( samplesToClear, mBufferSize - pos );
 
      ClearSamples(mBuffer.ptr(), format, pos, block);
 
      pos = (pos + block) % mBufferSize;
      samplesToClear -= block;
      cleared += block;
   }
 
   mWritten = pos;
 
   return cleared;
}
 
std::pair<samplePtr, size_t> RingBuffer::GetUnflushed(unsigned iBlock)
{
   // This function is called by the writer
 
   // Find total number of samples unflushed:
   auto end = mEnd.load(std::memory_order_relaxed);
   const size_t size = Filled(end, mWritten);
 
   // How many in the first part:
   const size_t size0 = std::min(size, mBufferSize - end);
   // How many wrap around the ring buffer:
   const size_t size1 = size - size0;
 
   if (iBlock == 0)
      return {
         size0 ? mBuffer.ptr() + end * SAMPLE_SIZE(mFormat) : nullptr,
         size0 };
   else
      return {
         size1 ? mBuffer.ptr() : nullptr,
         size1 };
}
 
void RingBuffer::Flush()
{
   // Atomically update the end pointer with release, so the nonatomic writes
   // just done to the buffer don't get reordered after
   mEnd.store(mWritten, std::memory_order_release);
}
 
//
// For the reader only:
// Only reader writes the start, so it can read it again relaxed
// But it reads the end written by the writer, who also sends sample data
// with the changes of end; therefore that must be read with acquire order
// if we do more than merely query the size or throw samples away
//
 
size_t RingBuffer::AvailForGet()
{
   auto end = mEnd.load( std::memory_order_relaxed ); // get away with it here
   auto start = mStart.load( std::memory_order_relaxed );
   return Filled( start, end );
 
   // Writer might increase the available samples after return, but will
   // never decrease them, so reader can safely assume this much at least
}
 
size_t RingBuffer::Get(samplePtr buffer, sampleFormat format,
                       size_t samplesToCopy)
{
   // Must match the writer's release with acquire for well defined reads of
   // the buffer
   auto end = mEnd.load( std::memory_order_acquire );
   auto start = mStart.load( std::memory_order_relaxed );
   samplesToCopy = std::min( samplesToCopy, Filled( start, end ) );
   auto dest = buffer;
   size_t copied = 0;
 
   while(samplesToCopy) {
      auto block = std::min( samplesToCopy, mBufferSize - start );
 
      CopySamples(mBuffer.ptr() + start * SAMPLE_SIZE(mFormat), mFormat,
                  dest, format,
                  block, DitherType::none);
 
      dest += block * SAMPLE_SIZE(format);
      start = (start + block) % mBufferSize;
      samplesToCopy -= block;
      copied += block;
   }
 
   // Communicate to writer that we have consumed some data,
   // with nonrelaxed ordering
   mStart.store( start, std::memory_order_release );
 
   return copied;
}
 
size_t RingBuffer::Discard(size_t samplesToDiscard)
{
   auto end = mEnd.load( std::memory_order_relaxed ); // get away with it here
   auto start = mStart.load( std::memory_order_relaxed );
   samplesToDiscard = std::min( samplesToDiscard, Filled( start, end ) );
 
   // Communicate to writer that we have skipped some data, and that's all
   mStart.store((start + samplesToDiscard) % mBufferSize,
                std::memory_order_relaxed);
 
   return samplesToDiscard;
}