1 // Copyright 2016 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 // Note: ported from Chromium commit head: 1323b9c 5 6 #include "vp9_bool_decoder.h" 7 8 #include <algorithm> 9 10 #include "base/logging.h" 11 #include "bit_reader.h" 12 13 namespace media { 14 15 namespace { 16 17 // This is an optimization lookup table for the loop in spec 9.2.2. 18 // while BoolRange <= 128: 19 // read 1 bit 20 // BoolRange *= 2 21 // This table indicates how many iterations to run for a given BoolRange. So 22 // the loop could be reduced to 23 // read (kCountToShiftTo128[BoolRange]) bits 24 const int kCountToShiftTo128[256] = { 25 0, 7, 6, 6, 5, 5, 5, 5, 4, 4, 4, 4, 4, 4, 4, 4, 3, 3, 3, 3, 3, 3, 3, 3, 26 3, 3, 3, 3, 3, 3, 3, 3, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 27 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 1, 1, 1, 1, 1, 1, 1, 1, 28 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 29 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 30 1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 31 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 32 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 33 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 34 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 35 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 36 }; 37 } // namespace 38 39 Vp9BoolDecoder::Vp9BoolDecoder() = default; 40 41 Vp9BoolDecoder::~Vp9BoolDecoder() = default; 42 43 // 9.2.1 Initialization process for Boolean decoder 44 bool Vp9BoolDecoder::Initialize(const uint8_t* data, size_t size) { 45 DCHECK(data); 46 if (size < 1) { 47 DVLOG(1) << "input size of bool decoder shall be at least 1"; 48 valid_ = false; 49 return false; 50 } 51 52 reader_.reset(new BitReader(data, size)); 53 valid_ = true; 54 55 bool_value_ = 0; 56 count_to_fill_ = 8; 57 bool_range_ = 255; 58 if (ReadLiteral(1) != 0) { 59 DVLOG(1) << "marker bit should be 0"; 60 valid_ = false; 61 return false; 62 } 63 return true; 64 } 65 66 // Fill at least |count_to_fill_| bits and prefill remain bits of |bool_value_| 67 // if data is enough. 68 bool Vp9BoolDecoder::Fill() { 69 DCHECK_GE(count_to_fill_, 0); 70 71 int bits_left = reader_->bits_available(); 72 if (bits_left < count_to_fill_) { 73 valid_ = false; 74 DVLOG(1) << "Vp9BoolDecoder reads beyond the end of stream"; 75 return false; 76 } 77 78 DCHECK_LE(count_to_fill_, kBoolSize); 79 int max_bits_to_read = kBigBoolBitSize - kBoolSize + count_to_fill_; 80 int bits_to_read = std::min(max_bits_to_read, bits_left); 81 82 BigBool data; 83 reader_->ReadBits(bits_to_read, &data); 84 bool_value_ |= data << (max_bits_to_read - bits_to_read); 85 count_to_fill_ -= bits_to_read; 86 87 return true; 88 } 89 90 // 9.2.2 Boolean decoding process 91 bool Vp9BoolDecoder::ReadBool(int prob) { 92 DCHECK(reader_); 93 94 if (count_to_fill_ > 0) { 95 if (!Fill()) 96 return false; 97 } 98 99 unsigned int split = (bool_range_ * prob + (256 - prob)) >> kBoolSize; 100 BigBool big_split = static_cast<BigBool>(split) 101 << (kBigBoolBitSize - kBoolSize); 102 103 bool bit; 104 if (bool_value_ < big_split) { 105 bool_range_ = split; 106 bit = false; 107 } else { 108 bool_range_ -= split; 109 bool_value_ -= big_split; 110 bit = true; 111 } 112 113 // Need to fill |count| bits next time in order to make |bool_range_| >= 114 // 128. 115 DCHECK_LT(bool_range_, arraysize(kCountToShiftTo128)); 116 DCHECK_GT(bool_range_, 0u); 117 int count = kCountToShiftTo128[bool_range_]; 118 bool_range_ <<= count; 119 bool_value_ <<= count; 120 count_to_fill_ += count; 121 122 return bit; 123 } 124 125 // 9.2.4 Parsing process for read_literal 126 uint8_t Vp9BoolDecoder::ReadLiteral(int bits) { 127 DCHECK_LT(static_cast<size_t>(bits), sizeof(uint8_t) * 8); 128 DCHECK(reader_); 129 130 uint8_t x = 0; 131 for (int i = 0; i < bits; i++) 132 x = 2 * x + ReadBool(128); 133 134 return x; 135 } 136 137 bool Vp9BoolDecoder::ConsumePaddingBits() { 138 DCHECK(reader_); 139 140 if (count_to_fill_ > reader_->bits_available()) { 141 // 9.2.2 Boolean decoding process 142 // Although we actually don't used the value, spec says the bitstream 143 // should have enough bits to fill bool range, this should never happen. 144 DVLOG(2) << "not enough bits in bitstream to fill bool range"; 145 return false; 146 } 147 148 if (bool_value_ != 0) { 149 DVLOG(1) << "prefilled padding bits are not zero"; 150 return false; 151 } 152 while (reader_->bits_available() > 0) { 153 int data; 154 int size_to_read = 155 std::min(reader_->bits_available(), static_cast<int>(sizeof(data) * 8)); 156 reader_->ReadBits(size_to_read, &data); 157 if (data != 0) { 158 DVLOG(1) << "padding bits are not zero"; 159 return false; 160 } 161 } 162 return true; 163 } 164 165 } // namespace media 166
