Method readBits

YUViewLib/src/parser/common/SubByteReader.cpp:46–114

    : byteVector(inArr), posInBufferBytes(inArrOffset), initialPosInBuffer(inArrOffset){};

std::tuple<uint64_t, std::string> SubByteReader::readBits(size_t nrBits)
{
  uint64_t out        = 0;
  auto     nrBitsRead = nrBits;

  // The return unsigned int is of depth 64 bits
  if (nrBits > 64)
    throw std::logic_error("Trying to read more than 64 bits at once from the bitstream.");
  if (nrBits == 0)
    return {0, ""};

  while (nrBits > 0)
  {
    if (this->posInBufferBits == 8 && nrBits != 0)
    {
      // We read all bits we could from the current byte but we need more. Go to
      // the next byte.
      if (!this->gotoNextByte())
        // We are at the end of the buffer but we need to read more. Error.
        throw std::logic_error("Error while reading annexB file. Trying to "
                               "read over buffer boundary.");
    }

    // How many bits can be gotton from the current byte?
    auto curBitsLeft = 8 - this->posInBufferBits;

    size_t readBits; // Nr of bits to read
    size_t offset;   // Offset for reading from the right
    if (nrBits >= curBitsLeft)
    {
      // Read "curBitsLeft" bits
      readBits = curBitsLeft;
      offset   = 0;
    }
    else
    {
      // Read "nrBits" bits
      assert(nrBits < 8 && nrBits < curBitsLeft);
      readBits = nrBits;
      offset   = curBitsLeft - nrBits;
    }

    // Shift output value so that the new bits fit
    out = out << readBits;

    char c   = this->byteVector[this->posInBufferBytes];
    c        = c >> offset;
    int mask = ((1 << readBits) - 1);

    // Write bits to output
    out += (c & mask);

    // Update counters
    nrBits -= readBits;
    this->posInBufferBits += readBits;
  }

  std::string bitsRead;