Sub-sample accurate start of OscillatorNode

third_party/WebKit/Source/modules/webaudio/OscillatorNode.cpp

 /*
  * Copyright (C) 2012, Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1.  Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 221 matching lines @@
   }
 
   // We must access m_periodicWave only inside the lock.
   if (!m_periodicWave.get()) {
     outputBus->zero();
     return;
   }
 
   size_t quantumFrameOffset;
   size_t nonSilentFramesToProcess;
+  double startFrameOffset;
 
   updateSchedulingInfo(framesToProcess, outputBus, quantumFrameOffset,
-                       nonSilentFramesToProcess);
+                       nonSilentFramesToProcess, startFrameOffset);
 
   if (!nonSilentFramesToProcess) {
     outputBus->zero();
     return;
   }
 
   unsigned periodicWaveSize = m_periodicWave->periodicWaveSize();
   double invPeriodicWaveSize = 1.0 / periodicWaveSize;
 
   float* destP = outputBus->channel(0)->mutableData();
@@ skipping 24 matching lines @@
 
   float incr = frequency * rateScale;
   float* phaseIncrements = m_phaseIncrements.data();
 
   unsigned readIndexMask = periodicWaveSize - 1;
 
   // Start rendering at the correct offset.
   destP += quantumFrameOffset;
   int n = nonSilentFramesToProcess;
 
+  // If startFrameOffset is not 0, that means the oscillator doesn't actually
+  // start at quantumFrameOffset, but just past that time.  Adjust destP and n
+  // to reflect that, and adjust virtualReadIndex to start the value at
+  // startFrameOffset.
+  if (startFrameOffset > 0) {
+    ++destP;
+    --n;
+    virtualReadIndex += (1 - startFrameOffset) * frequency * rateScale;
+    DCHECK(virtualReadIndex < periodicWaveSize);
+  } else if (startFrameOffset < 0) {
+    virtualReadIndex = -startFrameOffset * frequency * rateScale;
+  }
+
   while (n--) {
     unsigned readIndex = static_cast<unsigned>(virtualReadIndex);
     unsigned readIndex2 = readIndex + 1;
 
     // Contain within valid range.
     readIndex = readIndex & readIndexMask;
     readIndex2 = readIndex2 & readIndexMask;
 
     if (hasSampleAccurateValues) {
       incr = *phaseIncrements++;
@@ skipping 145 matching lines @@
 
 AudioParam* OscillatorNode::detune() {
   return m_detune;
 }
 
 void OscillatorNode::setPeriodicWave(PeriodicWave* wave) {
   oscillatorHandler().setPeriodicWave(wave);
 }
 
 }  // namespace blink