1 // Copyright 2010 Google Inc. All Rights Reserved. 2 // 3 // Use of this source code is governed by a BSD-style license 4 // that can be found in the COPYING file in the root of the source 5 // tree. An additional intellectual property rights grant can be found 6 // in the file PATENTS. All contributing project authors may 7 // be found in the AUTHORS file in the root of the source tree. 8 // ----------------------------------------------------------------------------- 9 // 10 // Boolean decoder 11 // 12 // Author: Skal (pascal.massimino (at) gmail.com) 13 // Vikas Arora (vikaas.arora (at) gmail.com) 14 15 #ifndef WEBP_UTILS_BIT_READER_H_ 16 #define WEBP_UTILS_BIT_READER_H_ 17 18 #include <assert.h> 19 #ifdef _MSC_VER 20 #include <stdlib.h> // _byteswap_ulong 21 #endif 22 #include "../webp/types.h" 23 24 #ifdef __cplusplus 25 extern "C" { 26 #endif 27 28 // The Boolean decoder needs to maintain infinite precision on the value_ field. 29 // However, since range_ is only 8bit, we only need an active window of 8 bits 30 // for value_. Left bits (MSB) gets zeroed and shifted away when value_ falls 31 // below 128, range_ is updated, and fresh bits read from the bitstream are 32 // brought in as LSB. To avoid reading the fresh bits one by one (slow), we 33 // cache BITS of them ahead. The total of (BITS + 8) bits must fit into a 34 // natural register (with type bit_t). To fetch BITS bits from bitstream we 35 // use a type lbit_t. 36 // 37 // BITS can be any multiple of 8 from 8 to 56 (inclusive). 38 // Pick values that fit natural register size. 39 40 #if defined(__i386__) || defined(_M_IX86) // x86 32bit 41 #define BITS 24 42 #elif defined(__x86_64__) || defined(_M_X64) // x86 64bit 43 #define BITS 56 44 #elif defined(__arm__) || defined(_M_ARM) // ARM 45 #define BITS 24 46 #elif defined(__aarch64__) // ARM 64bit 47 #define BITS 56 48 #elif defined(__mips__) // MIPS 49 #define BITS 24 50 #else // reasonable default 51 #define BITS 24 52 #endif 53 54 //------------------------------------------------------------------------------ 55 // Derived types and constants: 56 // bit_t = natural register type for storing 'value_' (which is BITS+8 bits) 57 // range_t = register for 'range_' (which is 8bits only) 58 59 #if (BITS > 24) 60 typedef uint64_t bit_t; 61 #else 62 typedef uint32_t bit_t; 63 #endif 64 65 typedef uint32_t range_t; 66 67 //------------------------------------------------------------------------------ 68 // Bitreader 69 70 typedef struct VP8BitReader VP8BitReader; 71 struct VP8BitReader { 72 // boolean decoder (keep the field ordering as is!) 73 bit_t value_; // current value 74 range_t range_; // current range minus 1. In [127, 254] interval. 75 int bits_; // number of valid bits left 76 // read buffer 77 const uint8_t* buf_; // next byte to be read 78 const uint8_t* buf_end_; // end of read buffer 79 const uint8_t* buf_max_; // max packed-read position on buffer 80 int eof_; // true if input is exhausted 81 }; 82 83 // Initialize the bit reader and the boolean decoder. 84 void VP8InitBitReader(VP8BitReader* const br, 85 const uint8_t* const start, size_t size); 86 // Sets the working read buffer. 87 void VP8BitReaderSetBuffer(VP8BitReader* const br, 88 const uint8_t* const start, size_t size); 89 90 // Update internal pointers to displace the byte buffer by the 91 // relative offset 'offset'. 92 void VP8RemapBitReader(VP8BitReader* const br, ptrdiff_t offset); 93 94 // return the next value made of 'num_bits' bits 95 uint32_t VP8GetValue(VP8BitReader* const br, int num_bits); 96 static WEBP_INLINE uint32_t VP8Get(VP8BitReader* const br) { 97 return VP8GetValue(br, 1); 98 } 99 100 // return the next value with sign-extension. 101 int32_t VP8GetSignedValue(VP8BitReader* const br, int num_bits); 102 103 // bit_reader_inl.h will implement the following methods: 104 // static WEBP_INLINE int VP8GetBit(VP8BitReader* const br, int prob) 105 // static WEBP_INLINE int VP8GetSigned(VP8BitReader* const br, int v) 106 // and should be included by the .c files that actually need them. 107 // This is to avoid recompiling the whole library whenever this file is touched, 108 // and also allowing platform-specific ad-hoc hacks. 109 110 // ----------------------------------------------------------------------------- 111 // Bitreader for lossless format 112 113 // maximum number of bits (inclusive) the bit-reader can handle: 114 #define VP8L_MAX_NUM_BIT_READ 24 115 116 #define VP8L_LBITS 64 // Number of bits prefetched (= bit-size of vp8l_val_t). 117 #define VP8L_WBITS 32 // Minimum number of bytes ready after VP8LFillBitWindow. 118 119 typedef uint64_t vp8l_val_t; // right now, this bit-reader can only use 64bit. 120 121 typedef struct { 122 vp8l_val_t val_; // pre-fetched bits 123 const uint8_t* buf_; // input byte buffer 124 size_t len_; // buffer length 125 size_t pos_; // byte position in buf_ 126 int bit_pos_; // current bit-reading position in val_ 127 int eos_; // true if a bit was read past the end of buffer 128 } VP8LBitReader; 129 130 void VP8LInitBitReader(VP8LBitReader* const br, 131 const uint8_t* const start, 132 size_t length); 133 134 // Sets a new data buffer. 135 void VP8LBitReaderSetBuffer(VP8LBitReader* const br, 136 const uint8_t* const buffer, size_t length); 137 138 // Reads the specified number of bits from read buffer. 139 // Flags an error in case end_of_stream or n_bits is more than the allowed limit 140 // of VP8L_MAX_NUM_BIT_READ (inclusive). 141 // Flags eos_ if this read attempt is going to cross the read buffer. 142 uint32_t VP8LReadBits(VP8LBitReader* const br, int n_bits); 143 144 // Return the prefetched bits, so they can be looked up. 145 static WEBP_INLINE uint32_t VP8LPrefetchBits(VP8LBitReader* const br) { 146 return (uint32_t)(br->val_ >> (br->bit_pos_ & (VP8L_LBITS - 1))); 147 } 148 149 // Returns true if there was an attempt at reading bit past the end of 150 // the buffer. Doesn't set br->eos_ flag. 151 static WEBP_INLINE int VP8LIsEndOfStream(const VP8LBitReader* const br) { 152 assert(br->pos_ <= br->len_); 153 return br->eos_ || ((br->pos_ == br->len_) && (br->bit_pos_ > VP8L_LBITS)); 154 } 155 156 // For jumping over a number of bits in the bit stream when accessed with 157 // VP8LPrefetchBits and VP8LFillBitWindow. 158 static WEBP_INLINE void VP8LSetBitPos(VP8LBitReader* const br, int val) { 159 br->bit_pos_ = val; 160 br->eos_ = VP8LIsEndOfStream(br); 161 } 162 163 // Advances the read buffer by 4 bytes to make room for reading next 32 bits. 164 // Speed critical, but infrequent part of the code can be non-inlined. 165 extern void VP8LDoFillBitWindow(VP8LBitReader* const br); 166 static WEBP_INLINE void VP8LFillBitWindow(VP8LBitReader* const br) { 167 if (br->bit_pos_ >= VP8L_WBITS) VP8LDoFillBitWindow(br); 168 } 169 170 #ifdef __cplusplus 171 } // extern "C" 172 #endif 173 174 #endif /* WEBP_UTILS_BIT_READER_H_ */ 175
