CLK/OSBindings/Qt/audiobuffer.h

#pragma once

#include <cstdint>
#include <mutex>
#include <vector>

#include <QIODevice>

/*!
 * \brief An intermediate recepticle for audio data.
 *
 * Provides a QIODevice that will attempt to buffer the minimum amount
 * of data before handing it off to a polling QAudioOutput.
 *
 * Adding an extra buffer increases worst-case latency but resolves two
 * issues uncovered with use of a QAudioOutput by push:
 *
 * 1. knowing how much data is currently buffered, to avoid being at a
 *    permanent disadvantage after startup; and
 * 2. that such QAudioOutputs empirically seem to introduce a minimum
 *    16384-byte latency.
 */
struct AudioBuffer: public QIODevice {
	AudioBuffer() {
		open(QIODevice::ReadOnly | QIODevice::Unbuffered);
	}

	void setDepth(size_t depth) {
		std::lock_guard lock(mutex);
		buffer.resize(depth);
	}

	// AudioBuffer-specific behaviour: always provide the latest data,
	// even if that means skipping some.
	qint64 readData(char *data, const qint64 maxlen) override {
		if(!maxlen) {
			return 0;
		}

		std::lock_guard lock(mutex);
		if(readPointer == writePointer || buffer.empty()) return 0;

		const size_t dataAvailable = std::min(writePointer - readPointer, size_t(maxlen));
		size_t bytesToCopy = dataAvailable;
		while(bytesToCopy) {
			const size_t nextLength = std::min(buffer.size() - (readPointer % buffer.size()), bytesToCopy);
			memcpy(data, &buffer[readPointer % buffer.size()], nextLength);

			bytesToCopy -= nextLength;
			data += nextLength;
			readPointer += nextLength;
		}

		return qint64(dataAvailable);
	}

	qint64 bytesAvailable() const override {
		std::lock_guard lock(mutex);
		return qint64(writePointer - readPointer);
	}

	// Required to make QIODevice concrete; not used.
	qint64 writeData(const char *, qint64) override {
		return 0;
	}

	// Posts a new set of source data. This buffer permits only the amount of data
	// specified by @c setDepth to be enqueued into the future. Additional writes
	// after the buffer is full will overwrite the newest data.
	void write(const std::vector<int16_t> &source) {
		std::lock_guard lock(mutex);
		if(buffer.empty()) return;
		const size_t sourceSize = source.size() * sizeof(int16_t);

		size_t bytesToCopy = sourceSize;
		auto data = reinterpret_cast<const uint8_t *>(source.data());
		while(bytesToCopy) {
			size_t nextLength = std::min(buffer.size() - (writePointer % buffer.size()), bytesToCopy);
			memcpy(&buffer[writePointer % buffer.size()], data, nextLength);

			bytesToCopy -= nextLength;
			data += nextLength;
			writePointer += nextLength;
		}

		readPointer = std::max(readPointer, writePointer - buffer.size());
	}

	private:
		mutable std::mutex mutex;
		std::vector<uint8_t> buffer;
		mutable size_t readPointer = 0;
		size_t writePointer = 0;
};