Added first layer of the echo canceller 3 functionality

webrtc/modules/audio_processing/aec3/aec3_constants.h

+/*
+ *  Copyright (c) 2016 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.
+ */
+
+#ifndef WEBRTC_MODULES_AUDIO_PROCESSING_AEC3_AEC3_CONSTANTS_H_
+#define WEBRTC_MODULES_AUDIO_PROCESSING_AEC3_AEC3_CONSTANTS_H_
+
+#include <stddef.h>
+
+namespace webrtc {
+
+constexpr size_t kFftLengthBy2 = 64;
+constexpr size_t kFftLengthBy2Plus1 = kFftLengthBy2 + 1;
+constexpr size_t kFftLength = 2 * kFftLengthBy2;
+
+constexpr size_t kMaxNumBands = 3;
+constexpr size_t kSubFrameLength = 80;
+
+constexpr size_t kBlockSize = kFftLengthBy2;
+constexpr size_t kExtendedBlockSize = 2 * kFftLengthBy2;
+constexpr size_t kSubBlockSize = 16;
+
+constexpr size_t NumBandsForRate(int sample_rate_hz) {
+  return static_cast<size_t>(sample_rate_hz == 8000 ? 1
+                                                    : sample_rate_hz / 16000);
+}
+constexpr int LowestBandRate(int sample_rate_hz) {
+  return sample_rate_hz == 8000 ? sample_rate_hz : 16000;
+}
+
+}  // namespace webrtc
+
+#endif  // WEBRTC_MODULES_AUDIO_PROCESSING_AEC3_AEC3_CONSTANTS_H_