1// Copyright 2010 Google Inc. 2// 3// This code is licensed under the same terms as WebM: 4// Software License Agreement: http://www.webmproject.org/license/software/ 5// Additional IP Rights Grant: http://www.webmproject.org/license/additional/ 6// ----------------------------------------------------------------------------- 7// 8// Boolean decoder 9// 10// Author: Skal (pascal.massimino@gmail.com) 11 12#ifndef WEBP_DEC_BITS_H_ 13#define WEBP_DEC_BITS_H_ 14 15#include <assert.h> 16#include "webp/decode_vp8.h" 17 18#if defined(__cplusplus) || defined(c_plusplus) 19extern "C" { 20#endif 21 22//----------------------------------------------------------------------------- 23// Bitreader and code-tree reader 24 25typedef struct { 26 const uint8_t* buf_; // next byte to be read 27 const uint8_t* buf_end_; // end of read buffer 28 int eof_; // true if input is exhausted 29 30 // boolean decoder 31 uint32_t range_; // current range minus 1. In [127, 254] interval. 32 uint32_t value_; // current value 33 int missing_; // number of missing bits in value_ (8bit) 34} VP8BitReader; 35 36// Initialize the bit reader and the boolean decoder. 37void VP8InitBitReader(VP8BitReader* const br, 38 const uint8_t* const start, const uint8_t* const end); 39 40// return the next value made of 'num_bits' bits 41uint32_t VP8GetValue(VP8BitReader* const br, int num_bits); 42static inline uint32_t VP8Get(VP8BitReader* const br) { 43 return VP8GetValue(br, 1); 44} 45 46// return the next value with sign-extension. 47int32_t VP8GetSignedValue(VP8BitReader* const br, int num_bits); 48 49// Read a bit with proba 'prob'. Speed-critical function! 50extern const uint8_t kVP8Log2Range[128]; 51extern const uint8_t kVP8NewRange[128]; 52static inline uint32_t VP8GetByte(VP8BitReader* const br) { 53 assert(br); 54 if (br->buf_ < br->buf_end_) { 55 assert(br->buf_); 56 return *br->buf_++; 57 } 58 br->eof_ = 1; 59 return 0xff; 60} 61 62static inline uint32_t VP8BitUpdate(VP8BitReader* const br, uint32_t split) { 63 uint32_t bit; 64 const uint32_t value_split = (split + 1) << 8; 65 // Make sure we have a least 8 bits in 'value_' 66 if (br->missing_ > 0) { 67 br->value_ |= VP8GetByte(br) << br->missing_; 68 br->missing_ -= 8; 69 } 70 bit = (br->value_ >= value_split); 71 if (bit) { 72 br->range_ -= split + 1; 73 br->value_ -= value_split; 74 } else { 75 br->range_ = split; 76 } 77 return bit; 78} 79 80static inline void VP8Shift(VP8BitReader* const br) { 81 // range_ is in [0..127] interval here. 82 const int shift = kVP8Log2Range[br->range_]; 83 br->range_ = kVP8NewRange[br->range_]; 84 br->value_ <<= shift; 85 br->missing_ += shift; 86} 87 88static inline uint32_t VP8GetBit(VP8BitReader* const br, int prob) { 89 const uint32_t split = (br->range_ * prob) >> 8; 90 const uint32_t bit = VP8BitUpdate(br, split); 91 if (br->range_ < 0x7f) { 92 VP8Shift(br); 93 } 94 return bit; 95} 96 97static inline int VP8GetSigned(VP8BitReader* const br, int v) { 98 const uint32_t split = br->range_ >> 1; 99 const uint32_t bit = VP8BitUpdate(br, split); 100 VP8Shift(br); 101 return bit ? -v : v; 102} 103 104#if defined(__cplusplus) || defined(c_plusplus) 105} // extern "C" 106#endif 107 108#endif // WEBP_DEC_BITS_H_ 109