DStarRepeater/Common/RingBuffer.h

/*
 *   Copyright (C) 2006-2009,2012,2013,2015 by Jonathan Naylor G4KLX
 *
 *   This program is free software; you can redistribute it and/or modify
 *   it under the terms of the GNU General Public License as published by
 *   the Free Software Foundation; either version 2 of the License, or
 *   (at your option) any later version.
 *
 *   This program is distributed in the hope that it will be useful,
 *   but WITHOUT ANY WARRANTY; without even the implied warranty of
 *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *   GNU General Public License for more details.
 *
 *   You should have received a copy of the GNU General Public License
 *   along with this program; if not, write to the Free Software
 *   Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
 */

/*
 * Single-producer / single-consumer (SPSC) lock-free ring buffer.
 *
 * Used primarily for modem-to-repeater data transfer: the modem driver thread
 * writes received frames via addData(), while the repeater thread reads them
 * via getData() or peek().  The atomic m_iPtr (write index) and m_oPtr (read
 * index) make the empty/full checks safe across exactly two threads without a
 * mutex, provided each pointer is only advanced by its respective thread.
 *
 * Important: this is NOT safe for multiple concurrent producers or consumers.
 * CModem wraps accesses with a mutex when both threads need stronger ordering
 * guarantees (e.g. when writing the 2-byte framing header atomically with the
 * payload).
 *
 * The buffer wastes one slot so that iPtr == oPtr always means empty rather
 * than ambiguously full.  Capacity is therefore (length - 1) elements.
 */

#ifndef RingBuffer_H
#define RingBuffer_H

#include <atomic>
#include <cassert>
#include <cstring>

template<class T> class CRingBuffer {
public:
	CRingBuffer(unsigned int length) :
	m_length(length),
	m_buffer(nullptr),
	m_iPtr(0U),
	m_oPtr(0U)
	{
		assert(length > 0U);

		m_buffer = new T[length];

		::memset(m_buffer, 0x00, m_length * sizeof(T));
	}

	~CRingBuffer()
	{
		delete[] m_buffer;
	}

	// Write nSamples elements from buffer.  Returns 0 if there is not enough
	// free space; the caller must retry or discard.
	unsigned int addData(const T* buffer, unsigned int nSamples)
	{
		if (nSamples > freeSpace())
			return 0U;

		for (unsigned int i = 0U; i < nSamples; i++) {
			m_buffer[m_iPtr++] = buffer[i];

			if (m_iPtr == m_length)
				m_iPtr = 0U;
		}

		return nSamples;
	}

	// Read up to nSamples elements into buffer.  Returns the actual count read
	// (may be less than requested if the buffer does not have enough data).
	unsigned int getData(T* buffer, unsigned int nSamples)
	{
		unsigned int data = dataSize();

		if (data < nSamples)
			nSamples = data;

		for (unsigned int i = 0U; i < nSamples; i++) {
			buffer[i] = m_buffer[m_oPtr++];

			if (m_oPtr == m_length)
				m_oPtr = 0U;
		}

		return nSamples;
	}

	// Like getData() but does not advance the read pointer (non-destructive).
	unsigned int peek(T* buffer, unsigned int nSamples)
	{
		unsigned int data = dataSize();

		if (data < nSamples)
			nSamples = data;

		unsigned int ptr = m_oPtr;
		for (unsigned int i = 0U; i < nSamples; i++) {
			buffer[i] = m_buffer[ptr++];

			if (ptr == m_length)
				ptr = 0U;
		}

		return nSamples;
	}

	void clear()
	{
		m_iPtr  = 0U;
		m_oPtr  = 0U;

		::memset(m_buffer, 0x00, m_length * sizeof(T));
	}

	unsigned int freeSpace() const
	{
		if (m_oPtr == m_iPtr)
			return m_length - 1U;

		if (m_oPtr > m_iPtr)
			return m_oPtr - m_iPtr - 1U;

		return m_length - (m_iPtr - m_oPtr) - 1U;
	}

	bool hasSpace(unsigned int length) const
	{
		return freeSpace() >= length;
	}

	bool hasData() const
	{
		return m_oPtr != m_iPtr;
	}

	bool isEmpty() const
	{
		return m_oPtr == m_iPtr;
	}

private:
	unsigned int              m_length;   // Allocated slot count (capacity + 1)
	T*                        m_buffer;
	std::atomic<unsigned int> m_iPtr;     // Write index; advanced only by the producer
	std::atomic<unsigned int> m_oPtr;     // Read index; advanced only by the consumer

	unsigned int dataSize() const
	{
		if (m_iPtr >= m_oPtr)
			return m_iPtr - m_oPtr;

		return m_length - (m_oPtr - m_iPtr);
	}
};

#endif