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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@gmail.com)
13 //         Vikas Arora (vikaas.arora@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);
VP8Get(VP8BitReader * const br)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.
VP8LPrefetchBits(VP8LBitReader * const br)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.
VP8LIsEndOfStream(const VP8LBitReader * const br)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.
VP8LSetBitPos(VP8LBitReader * const br,int val)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);
VP8LFillBitWindow(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