Improved readability and DCHECKing in AudioVector::[]

webrtc/modules/audio_coding/neteq/audio_vector.h

 /*
  *  Copyright (c) 2012 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 109 matching lines @@
   }
 
  private:
   static const size_t kDefaultInitialSize = 10;
 
   // This method is used by the [] operators to calculate an index within the
   // capacity of the array, but without using the modulo operation (%).
   static inline size_t WrapIndex(size_t index,
                                  size_t begin_index,
                                  size_t capacity) {
-    RTC_DCHECK_GE(begin_index + index, index);  // Check for overflow.
-    const size_t ix =
-        begin_index + index - (begin_index + index >= capacity ? capacity : 0);
+    RTC_DCHECK_LT(index, capacity);
+    RTC_DCHECK_LT(begin_index, capacity);
+    size_t ix = begin_index + index;
+    RTC_DCHECK_GE(ix, index);  // Check for overflow.

[nisse-webrtc, 2017/02/22 07:48:40]

Given above DCHECKS, this could fail only if capacity > (maximum size_t) / 2.
Which is unlikely to happen (we'd run out of memory), but could be handled
correctly by changing the below condition to 

  if (ix < index || ix >= capacity)
    ix -= capacity;

[kwiberg-webrtc, 2017/02/22 08:25:48]

Aha. The two DCHECKs on line 130 and 131 guarantee that even if we do have
overflow, subtracting capacity will make everything work.

Doing what you suggest will however (1) cost extra at run-time (the ix < index
check) and (2) make the code harder to read, so given that it's very unlikely to
matter and that this piece of code is performance critical enough that we're
counting instructions for various alternatives, not acting on this clever idea
is probably the right thing to do.

+    if (ix >= capacity) {
+      ix -= capacity;
+    }
     RTC_DCHECK_LT(ix, capacity);
     return ix;
   }
 
   void Reserve(size_t n);
 
   void InsertByPushBack(const int16_t* insert_this, size_t length,
                         size_t position);
 
   void InsertByPushFront(const int16_t* insert_this, size_t length,
@@ skipping 12 matching lines @@
   size_t begin_index_;
 
   // The index of the sample after the last sample in |array_|.
   size_t end_index_;
 
   RTC_DISALLOW_COPY_AND_ASSIGN(AudioVector);
 };
 
 }  // namespace webrtc
 #endif  // WEBRTC_MODULES_AUDIO_CODING_NETEQ_AUDIO_VECTOR_H_