1// Copyright 2013 The Chromium Authors. All rights reserved. 2// Use of this source code is governed by a BSD-style license that can be 3// found in the LICENSE file. 4 5#ifndef MEDIA_BASE_AUDIO_HASH_H_ 6#define MEDIA_BASE_AUDIO_HASH_H_ 7 8#include <string> 9 10#include "base/basictypes.h" 11#include "base/strings/string_piece.h" 12#include "media/base/media_export.h" 13 14namespace media { 15 16class AudioBus; 17 18// Computes a running hash for a series of AudioBus objects. The hash is the 19// sum of each sample bucketed based on the frame index, channel number, and 20// current hash count. The hash was designed with two properties in mind: 21// 22// 1. Uniform error distribution across the input sample. 23// 2. Resilience to error below a certain threshold. 24// 25// The first is achieved by using a simple summing approach and moving position 26// weighting into the bucket choice. The second is handled during conversion to 27// string by rounding out values to only two decimal places. 28// 29// Using only two decimal places allows for roughly -40 dBFS of error. For 30// reference, SincResampler produces an RMS error of around -15 dBFS. See 31// http://en.wikipedia.org/wiki/DBFS and http://crbug.com/168204 for more info. 32class MEDIA_EXPORT AudioHash { 33 public: 34 AudioHash(); 35 ~AudioHash(); 36 37 // Update current hash with the contents of the provided AudioBus. 38 void Update(const AudioBus* audio_bus, int frames); 39 40 // Return a string representation of the current hash. 41 std::string ToString() const; 42 43 private: 44 // Storage for the audio hash. The number of buckets controls the importance 45 // of position in the hash. A higher number reduces the chance of false 46 // positives related to incorrect sample position. Value chosen by dice roll. 47 float audio_hash_[6]; 48 49 // The total number of samples processed per channel. Uses a uint32 instead 50 // of size_t so overflows on 64-bit and 32-bit machines are equivalent. 51 uint32 sample_count_; 52 53 DISALLOW_COPY_AND_ASSIGN(AudioHash); 54}; 55 56} // namespace media 57 58#endif // MEDIA_BASE_AUDIO_HASH_H_ 59