Mix to an intermediate buffer.

services/media/audio/platform/generic/mixer.cc

 // 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.
 
 #include "base/logging.h"
 #include "mojo/services/media/common/cpp/linear_transform.h"
 #include "services/media/audio/platform/generic/mixer.h"
 #include "services/media/audio/platform/generic/mixers/linear_sampler.h"
 #include "services/media/audio/platform/generic/mixers/no_op.h"
 #include "services/media/audio/platform/generic/mixers/point_sampler.h"
 
 namespace mojo {
 namespace media {
 namespace audio {
 
 constexpr uint32_t Mixer::FRAC_ONE;
 constexpr uint32_t Mixer::FRAC_MASK;
 
 Mixer::~Mixer() {}
 
 Mixer::Mixer(uint32_t pos_filter_width,
              uint32_t neg_filter_width)
   : pos_filter_width_(pos_filter_width),
     neg_filter_width_(neg_filter_width) {
 }
 
 MixerPtr Mixer::Select(const LpcmMediaTypeDetailsPtr& src_format,
-                       const LpcmMediaTypeDetailsPtr& dst_format) {
+                       const LpcmMediaTypeDetailsPtr* optional_dst_format) {
   // We should always have a source format.
   DCHECK(src_format);
 
   // If we don't have a destination format, just stick with no-op.  This is
   // probably the ThrottleOutput we are picking a mixer for.
-  if (!dst_format) { return MixerPtr(new mixers::NoOp()); }
+  if (!optional_dst_format) { return MixerPtr(new mixers::NoOp()); }
+
+  const LpcmMediaTypeDetailsPtr& dst_format = *optional_dst_format;
+  DCHECK(dst_format);
 
   // If the source sample rate is an integer multiple of the destination sample
   // rate, just use the point sampler.  Otherwise, use the linear re-sampler.
   LinearTransform::Ratio src_to_dst(src_format->frames_per_second,
                                     dst_format->frames_per_second);
   if (src_to_dst.numerator == 1) {
     return mixers::PointSampler::Select(src_format, dst_format);
   } else {
     return mixers::LinearSampler::Select(src_format, dst_format);
   }
 }
 
 }  // namespace audio
 }  // namespace media
 }  // namespace mojo