mirror of
https://github.com/webmproject/libwebp.git
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107 lines
2.9 KiB
C
107 lines
2.9 KiB
C
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// Copyright 2010 Google Inc.
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//
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// This code is licensed under the same terms as WebM:
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// Software License Agreement: http://www.webmproject.org/license/software/
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// Additional IP Rights Grant: http://www.webmproject.org/license/additional/
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// -----------------------------------------------------------------------------
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//
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// Boolean decoder
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//
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// Author: Skal (pascal.massimino@gmail.com)
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#ifndef WEBP_DECODE_BITS_H_
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#define WEBP_DECODE_BITS_H_
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#include <assert.h>
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#include "webp/decode_vp8.h"
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#if defined(__cplusplus) || defined(c_plusplus)
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extern "C" {
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#endif
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//-----------------------------------------------------------------------------
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// Bitreader and code-tree reader
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typedef struct {
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const uint8_t* buf_; // next byte to be read
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const uint8_t* buf_end_; // end of read buffer
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int eof_; // true if input is exhausted
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// boolean decoder
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uint32_t range_; // current range minus 1. In [127, 254] interval.
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uint32_t value_; // current value
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int left_; // how many unused bits (negated)
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} VP8BitReader;
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// Initialize the bit reader and the boolean decoder. Return true if ok.
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int VP8Init(VP8BitReader* const br, const uint8_t* buf, uint32_t size);
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// return the next value made of 'num_bits' bits
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uint32_t VP8GetValue(VP8BitReader* const br, int num_bits);
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static inline uint32_t VP8Get(VP8BitReader* const br) {
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return VP8GetValue(br, 1);
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}
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// return the next value with sign-extension.
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int32_t VP8GetSignedValue(VP8BitReader* const br, int num_bits);
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// Read a bit with proba 'prob'. Speed-critical function!
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extern const uint8_t kVP8Log2Range[128];
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extern const uint8_t kVP8NewRange[128];
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static inline uint32_t VP8GetByte(VP8BitReader* const br) {
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assert(br);
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if (br->buf_ < br->buf_end_) {
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assert(br->buf_);
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return *br->buf_++;
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}
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br->eof_ = 1;
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return 0x80;
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}
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static inline void VP8Shift(VP8BitReader* const br) {
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// range_ is in [0..127] interval here.
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const int shift = kVP8Log2Range[br->range_];
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br->range_ = kVP8NewRange[br->range_];
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br->value_ <<= shift;
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br->left_ += shift;
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if (br->left_ > 0) {
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br->value_ |= VP8GetByte(br) << br->left_;
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br->left_ -= 8;
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}
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}
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static inline uint32_t VP8GetBit(VP8BitReader* const br, int prob) {
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const uint32_t split = (br->range_ * prob) >> 8;
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const uint32_t bit = ((br->value_ >> 8) > split);
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if (bit) {
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br->range_ -= split + 1;
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br->value_ -= (split + 1) << 8;
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} else {
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br->range_ = split;
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}
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if (br->range_ < 0x7f) {
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VP8Shift(br);
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}
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return bit;
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}
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static inline int VP8GetSigned(VP8BitReader* const br, int v) {
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const uint32_t split = br->range_ >> 1;
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const uint32_t bit = ((br->value_ >> 8) > split);
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if (bit) {
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br->range_ -= split + 1;
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br->value_ -= (split + 1) << 8;
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v = -v;
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} else {
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br->range_ = split;
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}
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VP8Shift(br);
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return v;
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}
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#if defined(__cplusplus) || defined(c_plusplus)
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} // extern "C"
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#endif
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#endif // WEBP_DECODE_BITS_H_
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