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// Copyright 2010 Google Inc. All Rights Reserved.
// Use of this source code is governed by a BSD-style license
// that can be found in the COPYING file in the root of the source
// tree. An additional intellectual property rights grant can be found
// in the file PATENTS. All contributing project authors may
// be found in the AUTHORS file in the root of the source tree.
// -----------------------------------------------------------------------------
// Boolean decoder
// Author: Skal (
// Vikas Arora (
#include <assert.h>
#ifdef _MSC_VER
#include <stdlib.h> // _byteswap_ulong
#include "../webp/types.h"
#ifdef __cplusplus
extern "C" {
// The Boolean decoder needs to maintain infinite precision on the value_ field.
// However, since range_ is only 8bit, we only need an active window of 8 bits
// for value_. Left bits (MSB) gets zeroed and shifted away when value_ falls
// below 128, range_ is updated, and fresh bits read from the bitstream are
// brought in as LSB. To avoid reading the fresh bits one by one (slow), we
// cache BITS of them ahead. The total of (BITS + 8) bits must fit into a
// natural register (with type bit_t). To fetch BITS bits from bitstream we
// use a type lbit_t.
// BITS can be any multiple of 8 from 8 to 56 (inclusive).
// Pick values that fit natural register size.
#if defined(__i386__) || defined(_M_IX86) // x86 32bit
#define BITS 24
#elif defined(__x86_64__) || defined(_M_X64) // x86 64bit
#define BITS 56
#elif defined(__arm__) || defined(_M_ARM) // ARM
#define BITS 24
#elif defined(__aarch64__) // ARM 64bit
#define BITS 56
#elif defined(__mips__) // MIPS
#define BITS 24
#else // reasonable default
#define BITS 24
// Derived types and constants:
// bit_t = natural register type for storing 'value_' (which is BITS+8 bits)
// range_t = register for 'range_' (which is 8bits only)
#if (BITS > 24)
typedef uint64_t bit_t;
typedef uint32_t bit_t;
typedef uint32_t range_t;
// Bitreader
typedef struct VP8BitReader VP8BitReader;
struct VP8BitReader {
// boolean decoder (keep the field ordering as is!)
bit_t value_; // current value
range_t range_; // current range minus 1. In [127, 254] interval.
int bits_; // number of valid bits left
// read buffer
const uint8_t* buf_; // next byte to be read
const uint8_t* buf_end_; // end of read buffer
const uint8_t* buf_max_; // max packed-read position on buffer
int eof_; // true if input is exhausted
// Initialize the bit reader and the boolean decoder.
void VP8InitBitReader(VP8BitReader* const br,
const uint8_t* const start, size_t size);
// Sets the working read buffer.
void VP8BitReaderSetBuffer(VP8BitReader* const br,
const uint8_t* const start, size_t size);
// Update internal pointers to displace the byte buffer by the
// relative offset 'offset'.
void VP8RemapBitReader(VP8BitReader* const br, ptrdiff_t offset);
// return the next value made of 'num_bits' bits
uint32_t VP8GetValue(VP8BitReader* const br, int num_bits);
static WEBP_INLINE uint32_t VP8Get(VP8BitReader* const br) {
return VP8GetValue(br, 1);
// return the next value with sign-extension.
int32_t VP8GetSignedValue(VP8BitReader* const br, int num_bits);
// bit_reader_inl.h will implement the following methods:
// static WEBP_INLINE int VP8GetBit(VP8BitReader* const br, int prob)
// static WEBP_INLINE int VP8GetSigned(VP8BitReader* const br, int v)
// and should be included by the .c files that actually need them.
// This is to avoid recompiling the whole library whenever this file is touched,
// and also allowing platform-specific ad-hoc hacks.
// -----------------------------------------------------------------------------
// Bitreader for lossless format
// maximum number of bits (inclusive) the bit-reader can handle:
#define VP8L_MAX_NUM_BIT_READ 24
#define VP8L_LBITS 64 // Number of bits prefetched (= bit-size of vp8l_val_t).
#define VP8L_WBITS 32 // Minimum number of bytes ready after VP8LFillBitWindow.
typedef uint64_t vp8l_val_t; // right now, this bit-reader can only use 64bit.
typedef struct {
vp8l_val_t val_; // pre-fetched bits
const uint8_t* buf_; // input byte buffer
size_t len_; // buffer length
size_t pos_; // byte position in buf_
int bit_pos_; // current bit-reading position in val_
int eos_; // true if a bit was read past the end of buffer
} VP8LBitReader;
void VP8LInitBitReader(VP8LBitReader* const br,
const uint8_t* const start,
size_t length);
// Sets a new data buffer.
void VP8LBitReaderSetBuffer(VP8LBitReader* const br,
const uint8_t* const buffer, size_t length);
// Reads the specified number of bits from read buffer.
// Flags an error in case end_of_stream or n_bits is more than the allowed limit
// of VP8L_MAX_NUM_BIT_READ (inclusive).
// Flags eos_ if this read attempt is going to cross the read buffer.
uint32_t VP8LReadBits(VP8LBitReader* const br, int n_bits);
// Return the prefetched bits, so they can be looked up.
static WEBP_INLINE uint32_t VP8LPrefetchBits(VP8LBitReader* const br) {
return (uint32_t)(br->val_ >> (br->bit_pos_ & (VP8L_LBITS - 1)));
// Returns true if there was an attempt at reading bit past the end of
// the buffer. Doesn't set br->eos_ flag.
static WEBP_INLINE int VP8LIsEndOfStream(const VP8LBitReader* const br) {
assert(br->pos_ <= br->len_);
return br->eos_ || ((br->pos_ == br->len_) && (br->bit_pos_ > VP8L_LBITS));
// For jumping over a number of bits in the bit stream when accessed with
// VP8LPrefetchBits and VP8LFillBitWindow.
static WEBP_INLINE void VP8LSetBitPos(VP8LBitReader* const br, int val) {
br->bit_pos_ = val;
br->eos_ = VP8LIsEndOfStream(br);
// Advances the read buffer by 4 bytes to make room for reading next 32 bits.
// Speed critical, but infrequent part of the code can be non-inlined.
extern void VP8LDoFillBitWindow(VP8LBitReader* const br);
static WEBP_INLINE void VP8LFillBitWindow(VP8LBitReader* const br) {
if (br->bit_pos_ >= VP8L_WBITS) VP8LDoFillBitWindow(br);
#ifdef __cplusplus
} // extern "C"