src/utils/bit_reader.c

// 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 non-inlined methods
//
// Author: Skal (pascal.massimino@gmail.com)

#ifdef HAVE_CONFIG_H
#include "webp/config.h"
#endif

#include "./bit_reader_inl.h"

//------------------------------------------------------------------------------
// VP8BitReader

void VP8InitBitReader(VP8BitReader* const br,
                      const uint8_t* const start, const uint8_t* const end) {
  assert(br != NULL);
  assert(start != NULL);
  assert(start <= end);
  br->range_   = 255 - 1;
  br->buf_     = start;
  br->buf_end_ = end;
  br->value_   = 0;
  br->bits_    = -8;   // to load the very first 8bits
  br->eof_     = 0;
  VP8LoadNewBytes(br);
}

void VP8RemapBitReader(VP8BitReader* const br, ptrdiff_t offset) {
  if (br->buf_ != NULL) {
    br->buf_ += offset;
    br->buf_end_ += offset;
  }
}

const uint8_t kVP8Log2Range[128] = {
     7, 6, 6, 5, 5, 5, 5, 4, 4, 4, 4, 4, 4, 4, 4,
  3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3,
  2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
  2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
  1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
  1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
  1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
  1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
  0
};

// range = ((range - 1) << kVP8Log2Range[range]) + 1
const range_t kVP8NewRange[128] = {
  127, 127, 191, 127, 159, 191, 223, 127,
  143, 159, 175, 191, 207, 223, 239, 127,
  135, 143, 151, 159, 167, 175, 183, 191,
  199, 207, 215, 223, 231, 239, 247, 127,
  131, 135, 139, 143, 147, 151, 155, 159,
  163, 167, 171, 175, 179, 183, 187, 191,
  195, 199, 203, 207, 211, 215, 219, 223,
  227, 231, 235, 239, 243, 247, 251, 127,
  129, 131, 133, 135, 137, 139, 141, 143,
  145, 147, 149, 151, 153, 155, 157, 159,
  161, 163, 165, 167, 169, 171, 173, 175,
  177, 179, 181, 183, 185, 187, 189, 191,
  193, 195, 197, 199, 201, 203, 205, 207,
  209, 211, 213, 215, 217, 219, 221, 223,
  225, 227, 229, 231, 233, 235, 237, 239,
  241, 243, 245, 247, 249, 251, 253, 127
};

void VP8LoadFinalBytes(VP8BitReader* const br) {
  assert(br != NULL && br->buf_ != NULL);
  // Only read 8bits at a time
  if (br->buf_ < br->buf_end_) {
    br->bits_ += 8;
    br->value_ = (bit_t)(*br->buf_++) | (br->value_ << 8);
  } else if (!br->eof_) {
    br->value_ <<= 8;
    br->bits_ += 8;
    br->eof_ = 1;
  }
}

//------------------------------------------------------------------------------
// Higher-level calls

uint32_t VP8GetValue(VP8BitReader* const br, int bits) {
  uint32_t v = 0;
  while (bits-- > 0) {
    v |= VP8GetBit(br, 0x80) << bits;
  }
  return v;
}

int32_t VP8GetSignedValue(VP8BitReader* const br, int bits) {
  const int value = VP8GetValue(br, bits);
  return VP8Get(br) ? -value : value;
}

//------------------------------------------------------------------------------
// VP8LBitReader

#define LBITS 64      // Number of bits prefetched.
#define WBITS 32      // Minimum number of bytes needed after VP8LFillBitWindow.
#define LOG8_WBITS 4  // Number of bytes needed to store WBITS bits.

#if !defined(WEBP_FORCE_ALIGNED) && \
    (defined(__arm__) || defined(_M_ARM) || defined(__aarch64__) || \
     defined(__i386__) || defined(_M_IX86) || \
     defined(__x86_64__) || defined(_M_X64))
#define VP8L_USE_UNALIGNED_LOAD
#endif

static const uint32_t kBitMask[VP8L_MAX_NUM_BIT_READ + 1] = {
  0,
  0x000001, 0x000003, 0x000007, 0x00000f,
  0x00001f, 0x00003f, 0x00007f, 0x0000ff,
  0x0001ff, 0x0003ff, 0x0007ff, 0x000fff,
  0x001fff, 0x003fff, 0x007fff, 0x00ffff,
  0x01ffff, 0x03ffff, 0x07ffff, 0x0fffff,
  0x1fffff, 0x3fffff, 0x7fffff, 0xffffff
};

void VP8LInitBitReader(VP8LBitReader* const br, const uint8_t* const start,
                       size_t length) {
  size_t i;
  vp8l_val_t value = 0;
  assert(br != NULL);
  assert(start != NULL);
  assert(length < 0xfffffff8u);   // can't happen with a RIFF chunk.

  br->len_ = length;
  br->val_ = 0;
  br->bit_pos_ = 0;
  br->eos_ = 0;
  br->error_ = 0;

  if (length > sizeof(br->val_)) {
    length = sizeof(br->val_);
  }
  for (i = 0; i < length; ++i) {
    value |= (vp8l_val_t)start[i] << (8 * i);
  }
  br->val_ = value;
  br->pos_ = length;
  br->buf_ = start;
}

// Special version that assumes br->pos_ <= br_len_.
static int IsEndOfStreamSpecial(const VP8LBitReader* const br) {
  assert(br->pos_ <= br->len_);
  return br->pos_ == br->len_ && br->bit_pos_ >= LBITS;
}

static int IsEndOfStream(const VP8LBitReader* const br) {
  return (br->pos_ > br->len_) || IsEndOfStreamSpecial(br);
}

void VP8LBitReaderSetBuffer(VP8LBitReader* const br,
                            const uint8_t* const buf, size_t len) {
  assert(br != NULL);
  assert(buf != NULL);
  assert(len < 0xfffffff8u);   // can't happen with a RIFF chunk.
  br->buf_ = buf;
  br->len_ = len;
  br->eos_ = IsEndOfStream(br);
}

// If not at EOS, reload up to LBITS byte-by-byte
static void ShiftBytes(VP8LBitReader* const br) {
  while (br->bit_pos_ >= 8 && br->pos_ < br->len_) {
    br->val_ >>= 8;
    br->val_ |= ((vp8l_val_t)br->buf_[br->pos_]) << (LBITS - 8);
    ++br->pos_;
    br->bit_pos_ -= 8;
  }
}

void VP8LFillBitWindow(VP8LBitReader* const br) {
  if (br->bit_pos_ >= WBITS) {
    // TODO(jzern): given the fixed read size it may be possible to force
    //              alignment in this block.
#if defined(VP8L_USE_UNALIGNED_LOAD)
    if (br->pos_ + sizeof(br->val_) < br->len_) {
      br->val_ >>= WBITS;
      br->bit_pos_ -= WBITS;
      // The expression below needs a little-endian arch to work correctly.
      // This gives a large speedup for decoding speed.
      br->val_ |= (vp8l_val_t)*(const uint32_t*)(br->buf_ + br->pos_) <<
                  (LBITS - WBITS);
      br->pos_ += LOG8_WBITS;
      return;
    }
#endif
    ShiftBytes(br);       // Slow path.
    br->eos_ = IsEndOfStreamSpecial(br);
  }
}

uint32_t VP8LReadBits(VP8LBitReader* const br, int n_bits) {
  assert(n_bits >= 0);
  // Flag an error if end_of_stream or n_bits is more than allowed limit.
  if (!br->eos_ && n_bits <= VP8L_MAX_NUM_BIT_READ) {
    const uint32_t val =
        (uint32_t)(br->val_ >> br->bit_pos_) & kBitMask[n_bits];
    const int new_bits = br->bit_pos_ + n_bits;
    br->bit_pos_ = new_bits;
    // If this read is going to cross the read buffer, set the eos flag.
    br->eos_ = IsEndOfStreamSpecial(br);
    ShiftBytes(br);
    return val;
  } else {
    br->error_ = 1;
    return 0;
  }
}

//------------------------------------------------------------------------------