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