Introduce media::AudioPushFifo and a couple of use cases (and clean-ups).

media/base/audio_push_fifo.h

Index: media/base/audio_push_fifo.h
diff --git a/media/base/audio_push_fifo.h b/media/base/audio_push_fifo.h
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+++ b/media/base/audio_push_fifo.h
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+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef MEDIA_BASE_AUDIO_PUSH_FIFO_H_
+#define MEDIA_BASE_AUDIO_PUSH_FIFO_H_
+
+#include "base/callback.h"
+#include "base/macros.h"
+#include "base/memory/scoped_ptr.h"
+#include "media/base/audio_bus.h"
+#include "media/base/media_export.h"
+
+namespace media {
+
+// Yet another FIFO for audio data that re-buffers audio to a desired buffer
+// size.  Unlike AudioFifo and AudioBlockFifo, this FIFO cannot overflow: The
+// client is required to provide a callback that is called synchronously during
+// a push whenever enough data becomes available.  This implementation
+// eliminates redundant memory copies when the input buffer size always matches
+// the desired buffer size.
+class MEDIA_EXPORT AudioPushFifo final {
+ public:
+  // Called synchronously zero, one, or multiple times during a call to Push()
+  // to deliver re-buffered audio.  |frame_delay| refers to the position of the
+  // first frame in |output| relative to the first frame in the Push() call.  If
+  // negative, this indicates the output contains some data from a prior call to
+  // Push().  If zero or positive, the output contains data from the current
+  // call to Push().  Clients can use this to adjust timestamps.
+  using OutputCallback =
+      base::Callback<void(const AudioBus& output_bus, int frame_delay)>;
+
+  // Creates a new AudioPushFifo which delivers re-buffered audio by running
+  // |callback|.
+  explicit AudioPushFifo(const OutputCallback& callback);
+
+  ~AudioPushFifo();
+
+  // Returns the number of frames in each AudioBus delivered to the
+  // OutputCallback.
+  int frames_per_buffer() const { return frames_per_buffer_; }
+
+  // Must be called at least once before the first call to Push().  May be
+  // called later (e.g., to support an audio format change).
+  void Reset(int frames_per_buffer);
+
+  // Pushes all audio channel data from |input_bus| through the FIFO.  This will
+  // result in zero, one, or multiple synchronous calls to the OutputCallback
+  // provided in the constructor.  If the |input_bus| has a different number of
+  // channels than the prior Push() call, any currently-queued frames will be
+  // dropped.
+  void Push(const AudioBus& input_bus);
+
+  // Flushes any enqueued frames by invoking the OutputCallback with those
+  // frames plus padded zero samples.  If there are no frames currently
+  // enqueued, OutputCallback is not run.
+  void Flush();
+
+ private:
+  const OutputCallback callback_;
+
+  int frames_per_buffer_;
+
+  // Queue of frames pending for delivery.
+  scoped_ptr<AudioBus> audio_queue_;
+  int queued_frames_;
+
+  DISALLOW_COPY_AND_ASSIGN(AudioPushFifo);
+};
+
+}  // namespace media
+
+#endif  // MEDIA_BASE_AUDIO_PUSH_FIFO_H_