UT_ThreadQueue.h

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/*
 * PROPRIETARY INFORMATION.  This software is proprietary to
 * Side Effects Software Inc., and is not to be reproduced,
 * transmitted, or disclosed in any way without written permission.
 *
 * NAME:        UT_ThreadQueue.h ( UT Library, C++)
 *
 * COMMENTS:
 *      Modelled after Python's Queue, this is a simple queue
 *      which you can use to communicate to another thread with.
 */

#ifndef __UT_ThreadQueue__
#define __UT_ThreadQueue__

#include "UT_API.h"
#include "UT_Condition.h"
#include "UT_NonCopyable.h"
#include "UT_RingBuffer.h"
#include "UT_Thread.h"
#include <utility>

///
/// UT_ThreadQueue
///
/// Designed to be templated on a POD.
///
template <typename T>
class UT_ThreadQueue
{
public:
    /// A max cost of -1 will never block on adding items to the queue.
    UT_ThreadQueue(exint maxcost = -1) 
    { 
        myMaxCost = maxcost; 
        myCurrentCost = 0; 
    }
    ~UT_ThreadQueue() {}

    /// Prohibit copying as I am not sure if lock semantics will work.
    UT_NON_COPYABLE(UT_ThreadQueue)

    /// Synchronized test for number of entries
    int                 entries() const
    {
        UT_AutoLock     l(myLock);

        return myList.entries();
    }

    /// Removes an item, returns false if failed to remove because
    /// the queue was empty.
    bool                remove(T &item)
    {
        UT_AutoLock     l(myLock);

        if (isEmpty())
            return false;
        item = std::move(myList.popFirst());
        exint cost = myCost.popFirst();
        myCurrentCost -= cost;

        // Let people know it may now be empty.
        myCond.triggerAll();
        return true;
    }

    /// Blocks until the item is ready.
    T                   waitAndRemove()
    {
        UT_AutoLock     l(myLock);
        T               result;

        while (!remove(result))
        {
            myCond.waitForTrigger(myLock);
        }
        return result;
    }

    /// Blocks until the queue is empty.
    /// May not still be empty, since someone could add after
    /// you reach empty!
    void                waitForEmpty()
    {
        UT_AutoLock     l(myLock);

        while (!isEmpty())
        {
            myCond.waitForTrigger(myLock);
        }
    }

    /// Blocks until a queue change occurs, returns the new queue size.
    /// Returns zero immediately if queue is empty.
    int                 waitForQueueChange()
    {
        UT_AutoLock     l(myLock);

        if (isEmpty())
            return 0;

        myCond.waitForTrigger(myLock);
        return myList.entries();
    }

    /// Adds the item to the queue.
    /// Will block if it the total cost would be exceeded by
    /// adding this to the queue.
    /// Zero cost never block, nor does a max cost of -1
    /// @{
    void                append(const T &item, exint cost = 0)
                            { appendImpl(item, cost); }
    void                append(T &&item, exint cost = 0)
                            { appendImpl(std::move(item), cost); }
    /// @}

private:
    /// Non-synchronized, do not call outside of the lock.
    bool                isEmpty() const
    {
        return !myList.entries();
    }

    template <typename S>
    void                appendImpl(S &&item, exint cost)
    {
        UT_AutoLock     l(myLock);

        // Check if we'll exceed the cost.
        while (1)
        {
            // IF we are empty, add even if too expensive!
            if (cost && myMaxCost >= 0 && (myCurrentCost > 0) && (myCurrentCost + cost > myMaxCost))
            {
                // Too expensive to add right now, so await a trigger.
                myCond.waitForTrigger(myLock);
            }
            else
            {
                // Enough room for the new item.
                break;
            }
        }

        // Append
        myList.push(std::forward<S>(item));
        myCost.push(cost);
        myCurrentCost += cost;

        // Alert anyone waiting to wake up.
        myCond.triggerAll();
    }

private:
    UT_RingBuffer<T>    myList;
    UT_RingBuffer<exint>        myCost;
    exint               myMaxCost;
    exint               myCurrentCost;

    mutable UT_Lock     myLock;
    UT_Condition        myCond;
};

#endif