Mix to an intermediate buffer.

services/media/audio/platform/generic/output_formatter.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 <limits>
+#include <type_traits>
+
+#include "base/logging.h"
+#include "mojo/services/media/common/cpp/linear_transform.h"
+#include "services/media/audio/platform/generic/output_formatter.h"
+
+namespace mojo {
+namespace media {
+namespace audio {
+
+// Template to produce destination samples from normalized samples.
+template <typename DType, typename Enable = void> class DstConverter;
+
+template <typename DType>
+class DstConverter<DType,
+      typename std::enable_if<
+        std::is_same<DType, int16_t>::value,
+      void>::type> {
+ public:
+  static inline constexpr DType Convert(int32_t sample) {
+    return static_cast<DType>(sample);
+  }
+};
+

[dalesat, 2016/01/24 19:07:39]

Can we just define a template function and specialize? Why is the Enable type
parameter needed?

template <typename DType> inline DType DstConvert(int32_t sample);

inline uint8_t DstConvert(in32_t sample) {
  return static_cast<uint8_t>((sample >> 8) + 0x80);
}
 

[johngro, 2016/01/25 18:12:32]

Its been a while, but this can fail in some cases when using type traits to do
the specialization.  IIRC, its something about not referencing all of the
template parameters in the specialization list directly which then causes the
compiler to complain with

"""
warning: class template partial specialization contains a template parameter
that cannot be deduced; this partial specialization will never be used
"""

I'd have to get back into the problem to really understand it and provide a
proper explanation; but the "Add one last parameter to the template" pattern (in
this case, the Enable parameter) is a common way of getting around the trouble. 
There are others, but whether or not they work depend on the specifics of the
situation.

I found this pattern to be reliable, and generally more readable than the
alternatives (not that C++ partial specialization syntax lends itself to
read-ability), so I apply it pretty much everywhere here.  It may not be needed
in this specific case, I can remove it if it offends, or at least page the
problem back into my brain and provide a more precise answer as to the details
about partial template parameter deduction which would explain why this needed a
little bit better.

+template <typename DType>
+class DstConverter<DType,
+      typename std::enable_if<
+        std::is_same<DType, uint8_t>::value,
+      void>::type> {
+ public:
+  static inline constexpr DType Convert(int32_t sample) {
+    return static_cast<DType>((sample >> 8) + 0x80);
+  }
+};
+
+// Template to fill samples with silence based on sample type.
+template <typename DType, typename Enable = void> class SilenceMaker;
+
+template <typename DType>
+class SilenceMaker<DType,
+      typename std::enable_if<
+        std::is_same<DType, int16_t>::value,
+      void>::type> {
+ public:
+  static inline void Fill(void* dest, size_t samples) {
+    ::memset(dest, 0, samples * sizeof(DType));
+  }
+};
+
+template <typename DType>
+class SilenceMaker<DType,
+      typename std::enable_if<
+        std::is_same<DType, uint8_t>::value,
+      void>::type> {
+ public:
+  static inline void Fill(void* dest, size_t samples) {
+    ::memset(dest, 0x80, samples * sizeof(DType));
+  }
+};
+
+// A templated class which implements the ProduceOutput and FillWithSilence
+// methods of OutputFormatter
+template <typename DType, uint32_t DChCount>
+class OutputFormatterImpl : public OutputFormatter {
+ public:
+  explicit OutputFormatterImpl(const LpcmMediaTypeDetailsPtr& format)
+    : OutputFormatter(format, sizeof(DType), DChCount) {}
+
+  void ProduceOutput(const int32_t* source,
+                     void*          dest_void,
+                     uint32_t       frames) const override {
+    using DC = DstConverter<DType>;
+    DType* dest = static_cast<DType*>(dest_void);
+
+    for (size_t i = 0; i < (static_cast<size_t>(frames) * DChCount); ++i) {
+      register int32_t val = source[i];

[jeffbrown, 2015/12/01 22:13:30]

Does the register keyword even do anything these days?

[johngro, 2016/01/25 18:12:32]

probably not, but it does not hurt either.  It has always been just a suggestion
of intent to the compiler, I can remove it if it bugs you or is confusing.

+      if (val > std::numeric_limits<int16_t>::max()) {
+        dest[i] = DC::Convert(std::numeric_limits<int16_t>::max());
+      } else if (val < std::numeric_limits<int16_t>::min()) {
+        dest[i] = DC::Convert(std::numeric_limits<int16_t>::min());
+      } else {
+        dest[i] = DC::Convert(val);
+      }
+    }
+  }
+
+  void FillWithSilence(void* dest, uint32_t frames) const override {
+    SilenceMaker<DType>::Fill(dest, frames * DChCount);
+  }
+};
+
+// Constructor/destructor for the bommon OutputFormatter base class.

[jeffbrown, 2015/12/01 22:13:30]

common

[johngro, 2016/01/25 18:12:32]

Done.

+OutputFormatter::OutputFormatter(const LpcmMediaTypeDetailsPtr& format,
+                                 uint32_t bytes_per_sample,
+                                 uint32_t channels)
+  : format_(format.Clone()),
+    channels_(channels),
+    bytes_per_sample_(bytes_per_sample),
+    bytes_per_frame_(bytes_per_sample * channels) {}
+
+OutputFormatter::~OutputFormatter() {}
+
+// Selection routines which will instantiate a particular templatized version of
+// the output formatter.
+template <typename DType>
+static inline OutputFormatterPtr SelectOF(
+    const LpcmMediaTypeDetailsPtr& format) {
+  switch (format->channels) {
+  case 1:
+    return OutputFormatterPtr(new OutputFormatterImpl<DType, 1>(format));
+  case 2:
+    return OutputFormatterPtr(new OutputFormatterImpl<DType, 2>(format));
+  default:
+    LOG(ERROR) << "Unsupported output channels "
+               << format->channels;
+    return nullptr;
+  }
+}
+
+OutputFormatterPtr OutputFormatter::Select(
+    const LpcmMediaTypeDetailsPtr& format) {
+  DCHECK(format);
+
+  switch (format->sample_format) {
+  case LpcmSampleFormat::UNSIGNED_8:
+    return SelectOF<uint8_t>(format);
+  case LpcmSampleFormat::SIGNED_16:
+    return SelectOF<int16_t>(format);
+  default:
+    LOG(ERROR) << "Unsupported output sample format "
+               << format->sample_format;
+    return nullptr;
+  }
+}
+
+}  // namespace audio
+}  // namespace media
+}  // namespace mojo