Reimplementing NetEq4's AudioVector

The current implementation using std::vector is too slow.
This CL introduces a new implementation, using a regular
array as data container.

In AudioMultiVector::ReadInterleavedFromIndex, a special case for
1 channel was implemented, to further reduce runtime. Finally,
AudioMultiVector::Channels was reimplemented.

The changes in this CL reduces the runtime of neteq4_speed_test
by 33%.

BUG=1363
R=turaj@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/3509004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@5115 4adac7df-926f-26a2-2b94-8c16560cd09d

webrtc/modules/audio_coding/neteq4/audio_vector.h

@@ -13,9 +13,8 @@
 
 #include <string.h>  // Access to size_t.
 
-#include <vector>
-
 #include "webrtc/system_wrappers/interface/constructor_magic.h"
+#include "webrtc/system_wrappers/interface/scoped_ptr.h"
 #include "webrtc/typedefs.h"
 
 namespace webrtc {
@@ -23,11 +22,18 @@ namespace webrtc {
 class AudioVector {
  public:
   // Creates an empty AudioVector.
-  AudioVector() {}
+  AudioVector()
+      : array_(new int16_t[kDefaultInitialSize]),
+        first_free_ix_(0),
+        capacity_(kDefaultInitialSize) {}
 
   // Creates an AudioVector with an initial size.
   explicit AudioVector(size_t initial_size)
-      : vector_(initial_size, 0) {}
+      : array_(new int16_t[initial_size]),
+        first_free_ix_(initial_size),
+        capacity_(initial_size) {
+    memset(array_.get(), 0, initial_size * sizeof(int16_t));
+  }
 
   virtual ~AudioVector() {}
 
@@ -88,17 +94,24 @@ class AudioVector {
   virtual void CrossFade(const AudioVector& append_this, size_t fade_length);
 
   // Returns the number of elements in this AudioVector.
-  virtual size_t Size() const { return vector_.size(); }
+  virtual size_t Size() const { return first_free_ix_; }
 
   // Returns true if this AudioVector is empty.
-  virtual bool Empty() const { return vector_.empty(); }
+  virtual bool Empty() const { return (first_free_ix_ == 0); }
 
   // Accesses and modifies an element of AudioVector.
   const int16_t& operator[](size_t index) const;
   int16_t& operator[](size_t index);
 
  private:
-  std::vector<int16_t> vector_;
+  static const size_t kDefaultInitialSize = 10;
+
+  void Reserve(size_t n);
+
+  scoped_ptr<int16_t[]> array_;
+  size_t first_free_ix_;  // The first index after the last sample in array_.
+                          // Note that this index may point outside of array_.
+  size_t capacity_;  // Allocated number of samples in the array.
 
   DISALLOW_COPY_AND_ASSIGN(AudioVector);
 };