/*
 * Copyright (C) 2006-2009 Andre Noll <maan@systemlinux.org>
 *
 * Licensed under the GPL v2. For licencing details see COPYING.
 */

/** \file ringbuffer.c Simple ringbuffer implementation */

#include "para.h"
#include "ringbuffer.h"
#include "string.h"

/**
 * Holds all information about one ring buffer
 *
 * It is intentionally not exported via ringbuffer.h. Think abstract.
 */
struct ringbuffer
{
	/** The size of this ring buffer. */
	unsigned size;
	/** The actual entries of the ringbuffer. */
	void **entries;
	/** The next entry will be added at this position. */
	int head;
	/** How many entries the ring buffer contains. */
	unsigned filled;
};

/**
 * Initialize a new ringbuffer.
 *
 * \param size The number of entries the ringbuffer holds.
 *
 * This function initializes a circular ring buffer which can hold up to \a
 * size entries of arbitrary type. If performance is an issue, \a size should
 * be a power of two to make the underlying modulo operations cheap. Arbitrary
 * many ringbuffers may be initialized via this function. Each ringbuffer is
 * identified by a 'cookie'.
 *
 * \return  A 'cookie' which identifies the ringbuffer just created and
 * which must be passed to ringbuffer_add() and ringbuffer_get().
 */
struct ringbuffer *ringbuffer_new(unsigned size)
{
	struct ringbuffer *rb = para_calloc(sizeof(struct ringbuffer));
	rb->entries = para_calloc(size * sizeof(void *));
	rb->size = size;
	return rb;
}

/**
 * Add one entry to a ringbuffer.
 *
 * \param rb The ringbuffer identifier.
 * \param data The data to be inserted.
 *
 * Insert \a data into the ringbuffer associated with \a cookie.  As soon as
 * the ringbuffer fills up, its oldest entry is disregarded and replaced by \a
 * data.
 *
 * \return The old \a data pointer which is going to be disregarded, or
 * NULL if the ringbuffer is not yet full.
 */
void *ringbuffer_add(struct ringbuffer *rb, void *data)
{
	void *ret = rb->entries[rb->head];
	rb->entries[rb->head] = data;
	rb->head = (rb->head + 1) % rb->size;
	if (rb->filled < rb->size)
		rb->filled++;
	return ret;
}

/**
 * Get one entry from a ringbuffer.
 *
 * \param rb The ringbuffer identifier.
 * \param num The number of the entry.
 *
 * \return A pointer to data previously added, or NULL if entry number
 * \a num is not available. \a num counts backwards from zero, i.e.
 * ringbuffer_get_entry(0) gets the entry which was added most recently.
 */
void *ringbuffer_get(struct ringbuffer *rb, int num)
{
	int pos = (rb->head + rb->size - 1 - num) % rb->size;
//	fprintf(stderr, "pos = %d\n", pos);
	return rb->entries[pos];
}

/**
 * Get the number of entries in the ring buffer.
 *
 * \param rb The ringbuffer identifier
 *
 * This function always succeeds and never returns a number greater than the
 * size of the ring buffer.
 */
unsigned ringbuffer_filled(struct ringbuffer *rb)
{
	return rb->filled;
}