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