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(__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);
VP8Get(VP8BitReader * const br)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.
VP8LPrefetchBits(VP8LBitReader * const br)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.
VP8LIsEndOfStream(const VP8LBitReader * const br)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.
VP8LSetBitPos(VP8LBitReader * const br,int val)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);
VP8LFillBitWindow(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