Function documentWaveformDecode

Telegram/SourceFiles/data/data_document.cpp:1362–1393

}

VoiceWaveform documentWaveformDecode(const QByteArray &encoded5bit) {
	auto bitsCount = static_cast<int>(encoded5bit.size() * 8);
	auto valuesCount = bitsCount / 5;
	if (!valuesCount) {
		return VoiceWaveform();
	}

	// Read each 5 bit of encoded5bit as 0-31 unsigned char.
	// We count the index of the byte in which the desired 5-bit sequence starts.
	// And then we read a uint16 starting from that byte to guarantee to get all of those 5 bits.
	//
	// BUT! if it is the last byte we have, we're not allowed to read a uint16 starting with it.
	// Because it will be an overflow (we'll access one byte after the available memory).
	// We see, that only the last 5 bits could start in the last available byte and be problematic.
	// So we read in a general way all the entries in a general way except the last one.
	auto result = VoiceWaveform(valuesCount, 0);
	auto bitsData = encoded5bit.constData();
	for (auto i = 0, l = valuesCount - 1; i != l; ++i) {
		auto byteIndex = (i * 5) / 8;
		auto bitShift = (i * 5) % 8;
		auto value = *reinterpret_cast<const uint16*>(bitsData + byteIndex);
		result[i] = static_cast<char>((value >> bitShift) & 0x1F);
	}
	auto lastByteIndex = ((valuesCount - 1) * 5) / 8;
	auto lastBitShift = ((valuesCount - 1) * 5) % 8;
	auto lastValue = (lastByteIndex == encoded5bit.size() - 1)
		? static_cast<uint16>(*reinterpret_cast<const uchar*>(bitsData + lastByteIndex))
		: *reinterpret_cast<const uint16*>(bitsData + lastByteIndex);
	result[valuesCount - 1] = static_cast<char>((lastValue >> lastBitShift) & 0x1F);

	return result;
}

QByteArray documentWaveformEncode5bit(const VoiceWaveform &waveform) {
	auto bitsCount = waveform.size() * 5;