bit_reader.cc

// Copyright (c) 2012 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
 
#include <packager/media/base/bit_reader.h>
 
#include <algorithm>
#include <cstddef>
#include <cstdint>
 
#include <absl/log/check.h>
 
namespace shaka {
namespace media {
 
BitReader::BitReader(const uint8_t* data, size_t size)
    : data_(data),
      initial_size_(size),
      bytes_left_(size),
      num_remaining_bits_in_curr_byte_(0) {
  DCHECK(data_ != NULL && bytes_left_ > 0);
 
  UpdateCurrByte();
}
 
BitReader::~BitReader() {}
 
bool BitReader::SkipBits(size_t num_bits) {
  // Skip any bits in the current byte waiting to be processed, then
  // process full bytes until less than 8 bits remaining.
  if (num_bits > num_remaining_bits_in_curr_byte_) {
    num_bits -= num_remaining_bits_in_curr_byte_;
    num_remaining_bits_in_curr_byte_ = 0;
 
    size_t num_bytes = num_bits / 8;
    num_bits %= 8;
    if (bytes_left_ < num_bytes) {
      bytes_left_ = 0;
      return false;
    }
    bytes_left_ -= num_bytes;
    data_ += num_bytes;
    UpdateCurrByte();
 
    // If there is no more data remaining, only return true if we
    // skipped all that were requested.
    if (num_remaining_bits_in_curr_byte_ == 0)
      return (num_bits == 0);
  }
 
  // Less than 8 bits remaining to skip. Use ReadBitsInternal to verify
  // that the remaining bits we need exist, and adjust them as necessary
  // for subsequent operations.
  uint64_t not_needed;
  return ReadBitsInternal(num_bits, &not_needed);
}
 
void BitReader::SkipToNextByte() {
  // Already aligned.
  if (num_remaining_bits_in_curr_byte_ == 8)
    return;
 
  num_remaining_bits_in_curr_byte_ = 0;
  UpdateCurrByte();
}
 
bool BitReader::SkipBytes(size_t num_bytes) {
  if (num_bytes == 0)
    return true;
  if (num_remaining_bits_in_curr_byte_ != 8)
    return false;
 
  data_ += num_bytes - 1;  // One additional byte in curr_byte_.
  if (num_bytes > bytes_left_ + 1)
    return false;
  bytes_left_ -= num_bytes - 1;
  num_remaining_bits_in_curr_byte_ = 0;
  UpdateCurrByte();
  return true;
}
 
bool BitReader::ReadBitsInternal(size_t num_bits, uint64_t* out) {
  DCHECK_LE(num_bits, 64u);
 
  *out = 0;
 
  while (num_remaining_bits_in_curr_byte_ != 0 && num_bits != 0) {
    size_t bits_to_take = std::min(num_remaining_bits_in_curr_byte_, num_bits);
 
    *out <<= bits_to_take;
    *out += curr_byte_ >> (num_remaining_bits_in_curr_byte_ - bits_to_take);
    num_bits -= bits_to_take;
    num_remaining_bits_in_curr_byte_ -= bits_to_take;
    curr_byte_ &= (1 << num_remaining_bits_in_curr_byte_) - 1;
 
    if (num_remaining_bits_in_curr_byte_ == 0)
      UpdateCurrByte();
  }
 
  return num_bits == 0;
}
 
void BitReader::UpdateCurrByte() {
  DCHECK_EQ(num_remaining_bits_in_curr_byte_, 0u);
 
  if (bytes_left_ == 0)
    return;
 
  // Load a new byte and advance pointers.
  curr_byte_ = *data_;
  ++data_;
  --bytes_left_;
  num_remaining_bits_in_curr_byte_ = 8;
}
 
}  // namespace media
}  // namespace shaka