Redesign of the render buffering in AEC3

This CL centralizes the render buffering in AEC3 so that all render
buffers are updated and synchronized/aligned with the render alignment
buffer.

Bug: webrtc:8597, chromium:790905
Change-Id: I8a94e5c1f27316b6100b420eec9652ea31c1a91d
Reviewed-on: https://webrtc-review.googlesource.com/25680
Commit-Queue: Per Åhgren <peah@webrtc.org>
Reviewed-by: Gustaf Ullberg <gustaf@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20989}

modules/audio_processing/aec3/downsampled_render_buffer.h

@@ -14,6 +14,7 @@
 #include <vector>
 
 #include "modules/audio_processing/aec3/aec3_common.h"
+#include "rtc_base/checks.h"
 
 namespace webrtc {
 
@@ -21,8 +22,31 @@ namespace webrtc {
 struct DownsampledRenderBuffer {
   explicit DownsampledRenderBuffer(size_t downsampled_buffer_size);
   ~DownsampledRenderBuffer();
+
+  size_t IncIndex(size_t index) {
+    return index < (buffer.size() - 1) ? index + 1 : 0;
+  }
+
+  size_t DecIndex(size_t index) {
+    return index > 0 ? index - 1 : buffer.size() - 1;
+  }
+
+  size_t OffsetIndex(size_t index, int offset) {
+    RTC_DCHECK_GE(buffer.size(), offset);
+    return (buffer.size() + index + offset) % buffer.size();
+  }
+
+  void UpdateWriteIndex(int offset) { write = OffsetIndex(write, offset); }
+  void IncWriteIndex() { write = IncIndex(write); }
+  void DecWriteIndex() { write = DecIndex(write); }
+  void UpdateReadIndex(int offset) { read = OffsetIndex(read, offset); }
+  void IncReadIndex() { read = IncIndex(read); }
+  void DecReadIndex() { read = DecIndex(read); }
+
+  size_t size;
   std::vector<float> buffer;
-  int position = 0;
+  int write = 0;
+  int read = 0;
 };
 
 }  // namespace webrtc