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_UTILS_H_
16 #define WEBP_UTILS_BIT_READER_UTILS_H_
17
18 #include <assert.h>
19 #ifdef _MSC_VER
20 #include <stdlib.h> // _byteswap_ulong
21 #endif
22 #include "src/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.
158 // This function does *not* set br->eos_, since it's speed-critical.
159 // Use with extreme care!
VP8LSetBitPos(VP8LBitReader * const br,int val)160 static WEBP_INLINE void VP8LSetBitPos(VP8LBitReader* const br, int val) {
161 br->bit_pos_ = val;
162 }
163
164 // Advances the read buffer by 4 bytes to make room for reading next 32 bits.
165 // Speed critical, but infrequent part of the code can be non-inlined.
166 extern void VP8LDoFillBitWindow(VP8LBitReader* const br);
VP8LFillBitWindow(VP8LBitReader * const br)167 static WEBP_INLINE void VP8LFillBitWindow(VP8LBitReader* const br) {
168 if (br->bit_pos_ >= VP8L_WBITS) VP8LDoFillBitWindow(br);
169 }
170
171 #ifdef __cplusplus
172 } // extern "C"
173 #endif
174
175 #endif /* WEBP_UTILS_BIT_READER_UTILS_H_ */
176