1/*
2 * Copyright (C) 2013 Google Inc. All rights reserved.
3 *
4 * Redistribution and use in source and binary forms, with or without
5 * modification, are permitted provided that the following conditions
6 * are met:
7 *
8 * 1.  Redistributions of source code must retain the above copyright
9 *     notice, this list of conditions and the following disclaimer.
10 * 2.  Redistributions in binary form must reproduce the above copyright
11 *     notice, this list of conditions and the following disclaimer in the
12 *     documentation and/or other materials provided with the distribution.
13 * 3.  Neither the name of Apple Inc. ("Apple") nor the names of
14 *     its contributors may be used to endorse or promote products derived
15 *     from this software without specific prior written permission.
16 *
17 * THIS SOFTWARE IS PROVIDED BY APPLE AND ITS CONTRIBUTORS "AS IS" AND ANY
18 * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
19 * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
20 * DISCLAIMED. IN NO EVENT SHALL APPLE OR ITS CONTRIBUTORS BE LIABLE FOR ANY
21 * DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
22 * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
23 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
24 * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
25 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
26 * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
27 */
28
29#ifndef DownSampler_h
30#define DownSampler_h
31
32#include "AudioArray.h"
33#include "DirectConvolver.h"
34
35namespace WebCore {
36
37// DownSampler down-samples the source stream by a factor of 2x.
38
39class DownSampler {
40public:
41    DownSampler(size_t inputBlockSize);
42
43    // The destination buffer |destP| is of size sourceFramesToProcess / 2.
44    void process(const float* sourceP, float* destP, size_t sourceFramesToProcess);
45
46    void reset();
47
48    // Latency based on the destination sample-rate.
49    size_t latencyFrames() const;
50
51private:
52    enum { DefaultKernelSize = 256 };
53
54    size_t m_inputBlockSize;
55
56    // Computes ideal band-limited half-band filter coefficients.
57    // In other words, filter out all frequencies higher than 0.25 * Nyquist.
58    void initializeKernel();
59    AudioFloatArray m_reducedKernel;
60
61    // Half-band filter.
62    DirectConvolver m_convolver;
63
64    AudioFloatArray m_tempBuffer;
65
66    // Used as delay-line (FIR filter history) for the input samples to account for the 0.5 term right in the middle of the kernel.
67    AudioFloatArray m_inputBuffer;
68};
69
70} // namespace WebCore
71
72#endif // DownSampler_h
73