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(__mips__) // MIPS 47 #define BITS 24 48 #else // reasonable default 49 #define BITS 24 // TODO(skal): test aarch64 and find the proper BITS value. 50 #endif 51 52 //------------------------------------------------------------------------------ 53 // Derived types and constants: 54 // bit_t = natural register type for storing 'value_' (which is BITS+8 bits) 55 // range_t = register for 'range_' (which is 8bits only) 56 57 #if (BITS > 24) 58 typedef uint64_t bit_t; 59 #else 60 typedef uint32_t bit_t; 61 #endif 62 63 typedef uint32_t range_t; 64 65 //------------------------------------------------------------------------------ 66 // Bitreader 67 68 typedef struct VP8BitReader VP8BitReader; 69 struct VP8BitReader { 70 // boolean decoder (keep the field ordering as is!) 71 bit_t value_; // current value 72 range_t range_; // current range minus 1. In [127, 254] interval. 73 int bits_; // number of valid bits left 74 // read buffer 75 const uint8_t* buf_; // next byte to be read 76 const uint8_t* buf_end_; // end of read buffer 77 int eof_; // true if input is exhausted 78 }; 79 80 // Initialize the bit reader and the boolean decoder. 81 void VP8InitBitReader(VP8BitReader* const br, 82 const uint8_t* const start, const uint8_t* const end); 83 84 // Update internal pointers to displace the byte buffer by the 85 // relative offset 'offset'. 86 void VP8RemapBitReader(VP8BitReader* const br, ptrdiff_t offset); 87 88 // return the next value made of 'num_bits' bits 89 uint32_t VP8GetValue(VP8BitReader* const br, int num_bits); 90 static WEBP_INLINE uint32_t VP8Get(VP8BitReader* const br) { 91 return VP8GetValue(br, 1); 92 } 93 94 // return the next value with sign-extension. 95 int32_t VP8GetSignedValue(VP8BitReader* const br, int num_bits); 96 97 // bit_reader_inl.h will implement the following methods: 98 // static WEBP_INLINE int VP8GetBit(VP8BitReader* const br, int prob) 99 // static WEBP_INLINE int VP8GetSigned(VP8BitReader* const br, int v) 100 // and should be included by the .c files that actually need them. 101 // This is to avoid recompiling the whole library whenever this file is touched, 102 // and also allowing platform-specific ad-hoc hacks. 103 104 // ----------------------------------------------------------------------------- 105 // Bitreader for lossless format 106 107 // maximum number of bits (inclusive) the bit-reader can handle: 108 #define VP8L_MAX_NUM_BIT_READ 24 109 110 #define VP8L_LBITS 64 // Number of bits prefetched. 111 #define VP8L_WBITS 32 // Minimum number of bytes ready after VP8LFillBitWindow. 112 113 typedef uint64_t vp8l_val_t; // right now, this bit-reader can only use 64bit. 114 115 typedef struct { 116 vp8l_val_t val_; // pre-fetched bits 117 const uint8_t* buf_; // input byte buffer 118 size_t len_; // buffer length 119 size_t pos_; // byte position in buf_ 120 int bit_pos_; // current bit-reading position in val_ 121 int eos_; // bitstream is finished 122 int error_; // an error occurred (buffer overflow attempt...) 123 } VP8LBitReader; 124 125 void VP8LInitBitReader(VP8LBitReader* const br, 126 const uint8_t* const start, 127 size_t length); 128 129 // Sets a new data buffer. 130 void VP8LBitReaderSetBuffer(VP8LBitReader* const br, 131 const uint8_t* const buffer, size_t length); 132 133 // Reads the specified number of bits from read buffer. 134 // Flags an error in case end_of_stream or n_bits is more than the allowed limit 135 // of VP8L_MAX_NUM_BIT_READ (inclusive). 136 // Flags eos_ if this read attempt is going to cross the read buffer. 137 uint32_t VP8LReadBits(VP8LBitReader* const br, int n_bits); 138 139 // Return the prefetched bits, so they can be looked up. 140 static WEBP_INLINE uint32_t VP8LPrefetchBits(VP8LBitReader* const br) { 141 return (uint32_t)(br->val_ >> br->bit_pos_); 142 } 143 144 // Returns true if there was an attempt at reading bit past the end of 145 // the buffer. Doesn't set br->eos_ flag. 146 static WEBP_INLINE int VP8LIsEndOfStream(const VP8LBitReader* const br) { 147 assert(br->pos_ <= br->len_); 148 return (br->pos_ == br->len_) && (br->bit_pos_ > VP8L_LBITS); 149 } 150 151 // For jumping over a number of bits in the bit stream when accessed with 152 // VP8LPrefetchBits and VP8LFillBitWindow. 153 static WEBP_INLINE void VP8LSetBitPos(VP8LBitReader* const br, int val) { 154 br->bit_pos_ = val; 155 br->eos_ = VP8LIsEndOfStream(br); 156 } 157 158 // Advances the read buffer by 4 bytes to make room for reading next 32 bits. 159 // Speed critical, but infrequent part of the code can be non-inlined. 160 extern void VP8LDoFillBitWindow(VP8LBitReader* const br); 161 static WEBP_INLINE void VP8LFillBitWindow(VP8LBitReader* const br) { 162 if (br->bit_pos_ >= VP8L_WBITS) VP8LDoFillBitWindow(br); 163 } 164 165 #ifdef __cplusplus 166 } // extern "C" 167 #endif 168 169 #endif /* WEBP_UTILS_BIT_READER_H_ */ 170
