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/*
Copyright (c) 2007-2013 Contributors as noted in the AUTHORS file
This file is part of 0MQ.
0MQ is free software; you can redistribute it and/or modify it under
the terms of the GNU Lesser General Public License as published by
the Free Software Foundation; either version 3 of the License, or
(at your option) any later version.
0MQ 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 Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public License
along with this program. If not, see <http://www.gnu.org/licenses/>.
*/
#ifndef __ZMQ_YQUEUE_HPP_INCLUDED__
#define __ZMQ_YQUEUE_HPP_INCLUDED__
#include <stdlib.h>
#include <stddef.h>
#include "err.hpp"
#include "atomic_ptr.hpp"
namespace zmq
{
// yqueue is an efficient queue implementation. The main goal is
// to minimise number of allocations/deallocations needed. Thus yqueue
// allocates/deallocates elements in batches of N.
//
// yqueue allows one thread to use push/back function and another one
// to use pop/front functions. However, user must ensure that there's no
// pop on the empty queue and that both threads don't access the same
// element in unsynchronised manner.
//
// T is the type of the object in the queue.
// N is granularity of the queue (how many pushes have to be done till
// actual memory allocation is required).
template <typename T, int N> class yqueue_t
{
public:
// Create the queue.
inline yqueue_t ()
{
begin_chunk = (chunk_t*) malloc (sizeof (chunk_t));
alloc_assert (begin_chunk);
begin_pos = 0;
back_chunk = NULL;
back_pos = 0;
end_chunk = begin_chunk;
end_pos = 0;
}
// Destroy the queue.
inline ~yqueue_t ()
{
while (true) {
if (begin_chunk == end_chunk) {
free (begin_chunk);
break;
}
chunk_t *o = begin_chunk;
begin_chunk = begin_chunk->next;
free (o);
}
chunk_t *sc = spare_chunk.xchg (NULL);
free (sc);
}
// Returns reference to the front element of the queue.
// If the queue is empty, behaviour is undefined.
inline T &front ()
{
return begin_chunk->values [begin_pos];
}
// Returns reference to the back element of the queue.
// If the queue is empty, behaviour is undefined.
inline T &back ()
{
return back_chunk->values [back_pos];
}
// Adds an element to the back end of the queue.
inline void push ()
{
back_chunk = end_chunk;
back_pos = end_pos;
if (++end_pos != N)
return;
chunk_t *sc = spare_chunk.xchg (NULL);
if (sc) {
end_chunk->next = sc;
sc->prev = end_chunk;
} else {
end_chunk->next = (chunk_t*) malloc (sizeof (chunk_t));
alloc_assert (end_chunk->next);
end_chunk->next->prev = end_chunk;
}
end_chunk = end_chunk->next;
end_pos = 0;
}
// Removes element from the back end of the queue. In other words
// it rollbacks last push to the queue. Take care: Caller is
// responsible for destroying the object being unpushed.
// The caller must also guarantee that the queue isn't empty when
// unpush is called. It cannot be done automatically as the read
// side of the queue can be managed by different, completely
// unsynchronised thread.
inline void unpush ()
{
// First, move 'back' one position backwards.
if (back_pos)
--back_pos;
else {
back_pos = N - 1;
back_chunk = back_chunk->prev;
}
// Now, move 'end' position backwards. Note that obsolete end chunk
// is not used as a spare chunk. The analysis shows that doing so
// would require free and atomic operation per chunk deallocated
// instead of a simple free.
if (end_pos)
--end_pos;
else {
end_pos = N - 1;
end_chunk = end_chunk->prev;
free (end_chunk->next);
end_chunk->next = NULL;
}
}
// Removes an element from the front end of the queue.
inline void pop ()
{
if (++ begin_pos == N) {
chunk_t *o = begin_chunk;
begin_chunk = begin_chunk->next;
begin_chunk->prev = NULL;
begin_pos = 0;
// 'o' has been more recently used than spare_chunk,
// so for cache reasons we'll get rid of the spare and
// use 'o' as the spare.
chunk_t *cs = spare_chunk.xchg (o);
free (cs);
}
}
private:
// Individual memory chunk to hold N elements.
struct chunk_t
{
T values [N];
chunk_t *prev;
chunk_t *next;
};
// Back position may point to invalid memory if the queue is empty,
// while begin & end positions are always valid. Begin position is
// accessed exclusively be queue reader (front/pop), while back and
// end positions are accessed exclusively by queue writer (back/push).
chunk_t *begin_chunk;
int begin_pos;
chunk_t *back_chunk;
int back_pos;
chunk_t *end_chunk;
int end_pos;
// People are likely to produce and consume at similar rates. In
// this scenario holding onto the most recently freed chunk saves
// us from having to call malloc/free.
atomic_ptr_t<chunk_t> spare_chunk;
// Disable copying of yqueue.
yqueue_t (const yqueue_t&);
const yqueue_t &operator = (const yqueue_t&);
};
}
#endif
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