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
25extern "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)
60typedef uint64_t bit_t;
61#else
62typedef uint32_t bit_t;
63#endif
64
65typedef uint32_t range_t;
66
67//------------------------------------------------------------------------------
68// Bitreader
69
70typedef struct VP8BitReader VP8BitReader;
71struct 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.
84void VP8InitBitReader(VP8BitReader* const br,
85 const uint8_t* const start, size_t size);
86// Sets the working read buffer.
87void 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'.
92void VP8RemapBitReader(VP8BitReader* const br, ptrdiff_t offset);
93
94// return the next value made of 'num_bits' bits
95uint32_t VP8GetValue(VP8BitReader* const br, int num_bits);
96static WEBP_INLINE uint32_t VP8Get(VP8BitReader* const br) {
97 return VP8GetValue(br, 1);
98}
99
100// return the next value with sign-extension.
101int32_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
119typedef uint64_t vp8l_val_t; // right now, this bit-reader can only use 64bit.
120
121typedef 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
130void VP8LInitBitReader(VP8LBitReader* const br,
131 const uint8_t* const start,
132 size_t length);
133
134// Sets a new data buffer.
135void 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.
142uint32_t VP8LReadBits(VP8LBitReader* const br, int n_bits);
143
144// Return the prefetched bits, so they can be looked up.
145static 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.
151static 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.
158static 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.
165extern void VP8LDoFillBitWindow(VP8LBitReader* const br);
166static 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