forego memory sample buffer and only write samples that need writing

Rather than writing the entire sequence audio to disk every change, now we
just write the parts we need to write.

app/render/backend/audio/audiobackend.cpp

@@ -19,6 +19,19 @@ QIODevice *AudioBackend::GetAudioPullDevice()
   return &pull_device_;
 }
 
+void AudioBackend::InvalidateCache(const rational &start_range, const rational &end_range)
+{
+  /*
+  // Truncate to length if necessary
+  int max_length_in_bytes = params().time_to_bytes(viewer_node()->Length());
+  if (pcm_data_.size() > max_length_in_bytes) {
+    pcm_data_.resize(max_length_in_bytes);
+  }
+  */
+
+  AudioRenderBackend::InvalidateCache(start_range, end_range);
+}
+
 bool AudioBackend::InitInternal()
 {
   // Initiate one thread per CPU core
@@ -34,7 +47,6 @@ bool AudioBackend::InitInternal()
 
 void AudioBackend::CloseInternal()
 {
-  // This backend doesn't init anything yet
 }
 
 bool AudioBackend::CompileInternal()
@@ -63,23 +75,26 @@ void AudioBackend::ThreadCompletedCache(NodeDependency dep)
   int length = params().time_to_bytes(dep.range().length());
   int out_point = offset + length;
 
-  if (pcm_data_.size() < out_point) {
-    pcm_data_.resize(out_point);
-  }
-
-  // Replace data with this data
-  int copy_length = qMin(length, cached_samples.size());
-
-  memcpy(pcm_data_.data() + offset, cached_samples.data(), static_cast<size_t>(copy_length));
-
-  if (copy_length < length) {
-    // Fill in remainder with silence
-    memset(pcm_data_.data() + offset + copy_length, 0, static_cast<size_t>(length - copy_length));
-  }
-
   QFile f(CachePathName());
-  if (f.open(QFile::WriteOnly)) {
-    f.write(pcm_data_);
+  if (f.open(QFile::WriteOnly | QFile::Append)) {
+
+    if (f.size() < out_point) {
+      f.resize(out_point);
+    }
+
+    f.seek(offset);
+
+    // Replace data with this data
+    int copy_length = qMin(length, cached_samples.size());
+
+    f.write(cached_samples.data(), copy_length);
+
+    if (copy_length < length) {
+      // Fill in remainder with silence
+      QByteArray empty_space(length - copy_length, 0);
+      f.write(empty_space);
+    }
+
     f.close();
   }
 

app/render/backend/audio/audiobackend.h

@@ -15,6 +15,9 @@ public:
 
   virtual QIODevice* GetAudioPullDevice() override;
 
+public slots:
+  virtual void InvalidateCache(const rational &start_range, const rational &end_range) override;
+
 protected:
   virtual bool InitInternal() override;
 

app/render/backend/audiorenderbackend.cpp

@@ -28,12 +28,6 @@ void AudioRenderBackend::InvalidateCache(const rational &start_range, const rati
   rational start_range_adj = qMax(rational(0), start_range);
   rational end_range_adj = qMin(viewer_node()->Length(), end_range);
 
-  // Truncate to length if necessary
-  int max_length_in_bytes = params().time_to_bytes(viewer_node()->Length());
-  if (pcm_data_.size() > max_length_in_bytes) {
-    pcm_data_.resize(max_length_in_bytes);
-  }
-
   // Add the range to the list
   cache_queue_.append(TimeRange(start_range_adj, end_range_adj));
 

app/render/backend/audiorenderbackend.h

@@ -39,8 +39,6 @@ protected:
 
   QString CachePathName();
 
-  QByteArray pcm_data_;
-
 private:
   void ValidateRanges();