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// Copyright Yahoo. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
#pragma once
#include "arrayqueue.hpp"
namespace vespalib {
/**
* Reference implementation of the 'Incremental Minimal Event Barrier'
* algorithm. An event in this context is defined to be something that
* happens during a time interval. An event barrier is a time interval
* for which events may start before or end after, but not both. The
* problem solved by the algorithm is to determine the minimal event
* barrier starting at a given time. In other words; wait for the
* currently active events to complete. The most natural use of this
* algorithm would be to make a thread wait for events happening in
* other threads to complete. The template parameter T defines how the
* detection of a minimal event barrier should be handled. T must
* implement the method 'void completeBarrier()' which does not throw.
**/
template <typename T>
class EventBarrier
{
private:
uint32_t _token;
uint32_t _count;
ArrayQueue<std::pair<uint32_t, T*> > _queue;
EventBarrier(const EventBarrier &);
EventBarrier &operator=(const EventBarrier &);
public:
/**
* At creation there are no active events and no pending barriers.
**/
EventBarrier() : _token(0), _count(0), _queue() {}
/**
* Obtain the current number of active events. This method is
* intended for testing and debugging.
*
* @return number of active events
**/
uint32_t countEvents() const {
uint32_t cnt = _count;
for (uint32_t i = 0; i < _queue.size(); ++i) {
cnt += _queue.peek(i).first;
}
return cnt;
}
/**
* Obtain the current number of pending barriers. This method is
* intended for testing and debugging.
*
* @return number of pending barriers
**/
uint32_t countBarriers() const {
return _queue.size();
}
/**
* Signal the start of an event. The value returned from this
* method must later be passed to the completeEvent method when
* signaling the completion of the event.
*
* @return opaque token identifying the started event
**/
uint32_t startEvent() {
++_count;
return _token;
}
/**
* Signal the completion of an event. The value passed to this
* method must be the same as the return value previously obtained
* from the startEvent method. This method will signal the
* completion of all pending barriers that were completed by the
* completion of this event.
*
* @param token opaque token identifying the completed event
**/
void completeEvent(uint32_t token) {
if (token == _token) {
--_count;
return;
}
--_queue.access(_queue.size() - (_token - token)).first;
while (!_queue.empty() && _queue.front().first == 0) {
_queue.front().second->completeBarrier();
_queue.pop();
}
}
/**
* Initiate the detection of the minimal event barrier starting
* now. If this method returns false it means that no events were
* currently active and the minimal event barrier was infinitely
* small. If this method returns false the handler will not be
* notified of the completion of the barrier. If this method
* returns true it means that the started barrier is pending and
* that the handler passed to this method will be notified of its
* completion at a later time.
*
* @return true if a barrier was started, false if no events were active
* @param handler handler notified of the completion of the barrier
**/
bool startBarrier(T &handler) {
if (_count == 0 && _queue.empty()) {
return false;
}
_queue.push(std::make_pair(_count, &handler));
++_token;
_count = 0;
return true;
}
};
} // namespace vespalib
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