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(__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) 58typedef uint64_t bit_t; 59#else 60typedef uint32_t bit_t; 61#endif 62 63typedef uint32_t range_t; 64 65//------------------------------------------------------------------------------ 66// Bitreader 67 68typedef struct VP8BitReader VP8BitReader; 69struct 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. 81void 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'. 86void VP8RemapBitReader(VP8BitReader* const br, ptrdiff_t offset); 87 88// return the next value made of 'num_bits' bits 89uint32_t VP8GetValue(VP8BitReader* const br, int num_bits); 90static WEBP_INLINE uint32_t VP8Get(VP8BitReader* const br) { 91 return VP8GetValue(br, 1); 92} 93 94// return the next value with sign-extension. 95int32_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 110typedef uint64_t vp8l_val_t; // right now, this bit-reader can only use 64bit. 111 112typedef struct { 113 vp8l_val_t val_; // pre-fetched bits 114 const uint8_t* buf_; // input byte buffer 115 size_t len_; // buffer length 116 size_t pos_; // byte position in buf_ 117 int bit_pos_; // current bit-reading position in val_ 118 int eos_; // bitstream is finished 119 int error_; // an error occurred (buffer overflow attempt...) 120} VP8LBitReader; 121 122void VP8LInitBitReader(VP8LBitReader* const br, 123 const uint8_t* const start, 124 size_t length); 125 126// Sets a new data buffer. 127void VP8LBitReaderSetBuffer(VP8LBitReader* const br, 128 const uint8_t* const buffer, size_t length); 129 130// Reads the specified number of bits from read buffer. 131// Flags an error in case end_of_stream or n_bits is more than the allowed limit 132// of VP8L_MAX_NUM_BIT_READ (inclusive). 133// Flags eos_ if this read attempt is going to cross the read buffer. 134uint32_t VP8LReadBits(VP8LBitReader* const br, int n_bits); 135 136// Return the prefetched bits, so they can be looked up. 137static WEBP_INLINE uint32_t VP8LPrefetchBits(VP8LBitReader* const br) { 138 return (uint32_t)(br->val_ >> br->bit_pos_); 139} 140 141// For jumping over a number of bits in the bit stream when accessed with 142// VP8LPrefetchBits and VP8LFillBitWindow. 143static WEBP_INLINE void VP8LSetBitPos(VP8LBitReader* const br, int val) { 144 br->bit_pos_ = val; 145} 146 147// Advances the read buffer by 4 bytes to make room for reading next 32 bits. 148void VP8LFillBitWindow(VP8LBitReader* const br); 149 150#ifdef __cplusplus 151} // extern "C" 152#endif 153 154#endif /* WEBP_UTILS_BIT_READER_H_ */ 155