Mix to an intermediate buffer.

services/media/audio/platform/generic/standard_output_base.h

 // Copyright 2015 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 SERVICES_MEDIA_AUDIO_PLATFORM_GENERIC_STANDARD_OUTPUT_BASE_H_
 #define SERVICES_MEDIA_AUDIO_PLATFORM_GENERIC_STANDARD_OUTPUT_BASE_H_
 
 #include "base/callback.h"
 #include "mojo/services/media/common/cpp/linear_transform.h"
 #include "mojo/services/media/common/cpp/local_time.h"
 #include "mojo/services/media/common/interfaces/media_common.mojom.h"
 #include "mojo/services/media/common/interfaces/media_types.mojom.h"
 #include "services/media/audio/audio_output.h"
 #include "services/media/audio/audio_track_to_output_link.h"
 #include "services/media/audio/platform/generic/mixer.h"
+#include "services/media/audio/platform/generic/output_formatter.h"
 
 namespace mojo {
 namespace media {
 namespace audio {
 
 class StandardOutputBase : public AudioOutput {
  public:
   ~StandardOutputBase() override;
 
  protected:
@@ skipping 38 matching lines @@
     next_sched_time_known_ = true;
   }
 
   void SetNextSchedDelay(const LocalDuration& next_sched_delay) {
     SetNextSchedTime(LocalClock::now() + next_sched_delay);
   }
 
   virtual bool StartMixJob(MixJob* job, const LocalTime& process_start) = 0;
   virtual bool FinishMixJob(const MixJob& job) = 0;
   virtual TrackBookkeeping* AllocBookkeeping();
+  void SetupMixBuffer(uint32_t max_mix_frames);
 
-  LpcmMediaTypeDetailsPtr output_format_;
-  uint32_t output_bytes_per_frame_;
+  // Details about the final output format
+  OutputFormatterPtr output_formatter_;
 
  private:
   using TrackSetupTask = std::function<bool(const AudioTrackImplPtr& track,
                                             TrackBookkeeping* info)>;
   using TrackProcessTask =
     std::function<bool(const AudioTrackImplPtr& track,
                        TrackBookkeeping* info,
                        const AudioPipe::AudioPacketRefPtr& pkt_ref)>;
 
   void ForeachTrack(const TrackSetupTask& setup,
                     const TrackProcessTask& process);
 
   bool SetupMix(const AudioTrackImplPtr& track, TrackBookkeeping* info);
   bool ProcessMix(const AudioTrackImplPtr& track,
                   TrackBookkeeping* info,
                   const AudioPipe::AudioPacketRefPtr& pkt_ref);
 
   bool SetupTrim(const AudioTrackImplPtr& track, TrackBookkeeping* info);
   bool ProcessTrim(const AudioTrackImplPtr& track,
                    TrackBookkeeping* info,
                    const AudioPipe::AudioPacketRefPtr& pkt_ref);
 
   LocalTime next_sched_time_;
   bool next_sched_time_known_;
 
+  // State for the internal buffer which holds intermediate mix results.
+  //
+  // TODO(johngro): Right now, the cannonical intermediate format is signed 32
+  // bit ints.  As time goes on, we may need to reconsider this.  This will
+  // become more important when...
+  //
+  // 1) We support 24 bit audio.  Right now, with a 16 bit max, we can
+  //    accumulate for up to a maximum of 2^16-1 tracks without needing to do
+  //    anything special about about clipping.  With 24 bit audio, this number
+  //    will drop to only 255 simultanious tracks.  It is unclear if this is a
+  //    reasonable system-wide limitation or not.
+  // 2) We support floating point audio.
+  std::unique_ptr<int32_t> mix_buf_;
+  uint32_t mix_buf_frames_ = 0;
+
   // State used by the mix task.
   TrackSetupTask setup_mix_;
   TrackProcessTask process_mix_;
   MixJob cur_mix_job_;
 
   // State used by the trim task.
   TrackSetupTask setup_trim_;
   TrackProcessTask process_trim_;
   int64_t trim_threshold_;
 };
 
 }  // namespace audio
 }  // namespace media
 }  // namespace mojo
 
 #endif  // SERVICES_MEDIA_AUDIO_PLATFORM_GENERIC_STANDARD_OUTPUT_BASE_H_