1 // Copyright 2015 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 // Note: ported from Chromium commit head: 1323b9c 6 7 /* 8 * Copyright (c) 2010, The WebM Project authors. All rights reserved. 9 * 10 * Redistribution and use in source and binary forms, with or without 11 * modification, are permitted provided that the following conditions are 12 * met: 13 * 14 * * Redistributions of source code must retain the above copyright 15 * notice, this list of conditions and the following disclaimer. 16 * 17 * * Redistributions in binary form must reproduce the above copyright 18 * notice, this list of conditions and the following disclaimer in 19 * the documentation and/or other materials provided with the 20 * distribution. 21 * 22 * * Neither the name of Google, nor the WebM Project, nor the names 23 * of its contributors may be used to endorse or promote products 24 * derived from this software without specific prior written 25 * permission. 26 * 27 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS 28 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT 29 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR 30 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT 31 * HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, 32 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT 33 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 34 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 35 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 36 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE 37 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 38 */ 39 40 // This file is modified from the dboolhuff.{c,h} from the WebM's libvpx 41 // project. (http://www.webmproject.org/code) 42 // It is used to decode bits from a vp8 stream. 43 44 #ifndef VP8_BOOL_DECODER_H_ 45 #define VP8_BOOL_DECODER_H_ 46 47 #include <stddef.h> 48 #include <stdint.h> 49 #include <sys/types.h> 50 51 #include "base/logging.h" 52 #include "base/macros.h" 53 54 namespace media { 55 56 // A class to decode the VP8's boolean entropy coded stream. It's a variant of 57 // arithmetic coding. See RFC 6386 - Chapter 7. Boolean Entropy Decoder. 58 class Vp8BoolDecoder { 59 public: 60 Vp8BoolDecoder(); 61 62 // Initializes the decoder to start decoding |data|, |size| being size 63 // of |data| in bytes. Returns false if |data| is NULL or empty. 64 bool Initialize(const uint8_t* data, size_t size); 65 66 // Reads a boolean from the coded stream. Returns false if it has reached the 67 // end of |data| and failed to read the boolean. The probability of |out| to 68 // be true is |probability| / 256, e.g., when |probability| is 0x80, the 69 // chance is 1/2 (i.e., 0x80 / 256). 70 bool ReadBool(bool* out, uint8_t probability); 71 72 // Reads a boolean from the coded stream with the default probability 1/2. 73 // Returns false if it has reached the end of |data| and failed to read the 74 // boolean. 75 bool ReadBool(bool* out); 76 77 // Reads a "literal", that is, a "num_bits"-wide unsigned value whose bits 78 // come high- to low-order, with each bit encoded at probability 1/2. 79 // Returns false if it has reached the end of |data| and failed to read the 80 // literal. 81 bool ReadLiteral(size_t num_bits, int* out); 82 83 // Reads a literal with sign from the coded stream. This is similar to 84 // the ReadListeral(), it first read a "num_bits"-wide unsigned value, and 85 // then read an extra bit as the sign of the literal. Returns false if it has 86 // reached the end of |data| and failed to read the literal or the sign. 87 // This is different from the "read_signed_literal(d, n)" defined in RFC 6386. 88 bool ReadLiteralWithSign(size_t num_bits, int* out); 89 90 // The following methods are used to get the internal states of the decoder. 91 92 // Returns the bit offset to the current top bit of the coded stream. It is 93 // also the number of bits that have been written in the corresponding 94 // encoding state. More specifically, we have the following constraint: 95 // w + (bottom * S) <= v < w + (bottom + range) * S, 96 // where "w" is for the bits already written, 97 // "v" is for the possible values of the coded number. 98 // "S" is the scale for the current bit position, 99 // i.e., S = pow(2, -(n + 8)), where "n" is the bit number of "w". 100 // BitOffset() returns the bit count of "w", i.e., "n". 101 size_t BitOffset(); 102 103 // Gets the "bottom" of the current coded value. See BitOffset() for 104 // more details. 105 uint8_t GetBottom(); 106 107 // Gets the "range" of the current coded value. See BitOffset() for 108 // more details. 109 uint8_t GetRange(); 110 111 private: 112 // Reads the next bit from the coded stream. The probability of the bit to 113 // be one is |probability| / 256. 114 int ReadBit(int probability); 115 116 // Fills more bits from |user_buffer_| to |value_|. We shall keep at least 8 117 // bits of the current |user_buffer_| in |value_|. 118 void FillDecoder(); 119 120 // Returns true iff we have ran out of bits. 121 bool OutOfBuffer(); 122 123 const uint8_t* user_buffer_; 124 const uint8_t* user_buffer_start_; 125 const uint8_t* user_buffer_end_; 126 size_t value_; 127 int count_; 128 size_t range_; 129 130 DISALLOW_COPY_AND_ASSIGN(Vp8BoolDecoder); 131 }; 132 133 } // namespace media 134 135 #endif // VP8_BOOL_DECODER_H_ 136
