media/base/audio_hash.h

   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
  14 namespace media {
  15
  16 class 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.
  32 class 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   enum { kHashBuckets = 6 };
  48   float audio_hash_[kHashBuckets];
  49
  50   // The total number of samples processed per channel.  Uses a uint32 instead
  51   // of size_t so overflows on 64-bit and 32-bit machines are equivalent.
  52   uint32 sample_count_;
  53
  54   DISALLOW_COPY_AND_ASSIGN(AudioHash);
  55 };
  56
  57 }  // namespace media
  58
  59 #endif  // MEDIA_BASE_AUDIO_HASH_H_