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#include "q3semaphore.h"

#include "qmutex.h"

#include "qwaitcondition.h"

QT_BEGIN_NAMESPACE

/*!

    \class Q3Semaphore

    \threadsafe

    \brief The Q3Semaphore class provides a robust integer semaphore.

    \compat

    A Q3Semaphore can be used to serialize thread execution, in a

    similar way to a QMutex. A semaphore differs from a mutex, in

    that a semaphore can be accessed by more than one thread at a

    time.

    For example, suppose we have an application that stores data in a

    large tree structure. The application creates 10 threads

    (commonly called a thread pool) to perform searches on the tree.

    When the application searches the tree for some piece of data, it

    uses one thread per base node to do the searching. A semaphore

    could be used to make sure that two threads don't try to search

    the same branch of the tree at the same time.

    A non-computing example of a semaphore would be dining at a

    restaurant. A semaphore is initialized to have a maximum count

    equal to the number of chairs in the restaurant. As people

    arrive, they want a seat. As seats are filled, the semaphore is

    accessed, once per person. As people leave, the access is

    released, allowing more people to enter. If a party of 10 people

    want to be seated, but there are only 9 seats, those 10 people

    will wait, but a party of 4 people would be seated (taking the

    available seats to 5, making the party of 10 people wait longer).

    When a semaphore is created it is given a number which is the

    maximum number of concurrent accesses it will permit. This amount

    may be changed using operator++(), operator--(), operator+=() and

    operator-=(). The number of accesses allowed is retrieved with

    available(), and the total number with total(). Note that the

    incrementing functions will block if there aren't enough available

    accesses. Use tryAccess() if you want to acquire accesses without

    blocking.

*/

#ifdef max

#undef max

#endif

class Q3SemaphorePrivate {

public:

    Q3SemaphorePrivate(int);

    QMutex mutex;

    QWaitCondition cond;

    int value, max;

};

Q3SemaphorePrivate::Q3SemaphorePrivate(int m)

    : value(0), max(m)

{

}

/*!

    Creates a new semaphore. The semaphore can be concurrently

    accessed at most \a maxcount times.

*/

Q3Semaphore::Q3Semaphore(int maxcount)

{

    d = new Q3SemaphorePrivate(maxcount);

}

/*!

    Destroys the semaphore.

    \warning If you destroy a semaphore that has accesses in use the

    resultant behavior is undefined.

*/

Q3Semaphore::~Q3Semaphore()

{

    delete d;

}

/*!

    Postfix ++ operator.

    Try to get access to the semaphore. If \l available() == 0, this

    call will block until it can get access, i.e. until available() \>

    0.

*/

int Q3Semaphore::operator++(int)

{

    QMutexLocker locker(&d->mutex);

    while (d->value >= d->max)

        d->cond.wait(locker.mutex());

    ++d->value;

    if (d->value > d->max)

        d->value = d->max;

    return d->value;

}

/*!

    Postfix -- operator.

    Release access of the semaphore. This wakes all threads waiting

    for access to the semaphore.

*/

int Q3Semaphore::operator--(int)

{

    QMutexLocker locker(&d->mutex);

    --d->value;

    if (d->value < 0)

        d->value = 0;

    d->cond.wakeAll();

    return d->value;

}

/*!

    Try to get access to the semaphore. If \l available() \< \a n, this

    call will block until it can get all the accesses it wants, i.e.

    until available() \>= \a n.

*/

int Q3Semaphore::operator+=(int n)

{

    QMutexLocker locker(&d->mutex);

    if (n < 0 || n > d->max) {

        qWarning("Q3Semaphore::operator+=: parameter %d out of range", n);

        n = n < 0 ? 0 : d->max;

    }

    while (d->value + n > d->max)

        d->cond.wait(locker.mutex());

    d->value += n;

    return d->value;

}

/*!

    Release \a n accesses to the semaphore.

*/

int Q3Semaphore::operator-=(int n)

{

    QMutexLocker locker(&d->mutex);

    if (n < 0 || n > d->value) {

        qWarning("Q3Semaphore::operator-=: parameter %d out of range", n);

        n = n < 0 ? 0 : d->value;

    }

    d->value -= n;

    d->cond.wakeAll();

    return d->value;

}

/*!

    Returns the number of accesses currently available to the

    semaphore.

*/

int Q3Semaphore::available() const

{

    QMutexLocker locker(&d->mutex);

    return d->max - d->value;

}

/*!

    Returns the total number of accesses to the semaphore.

*/

int Q3Semaphore::total() const

{

    QMutexLocker locker(&d->mutex);

    return d->max;

}

/*!

    Try to get access to the semaphore. If \l available() \< \a n, this

    function will return false immediately. If \l available() \>= \a n,

    this function will take \a n accesses and return true. This

    function does \e not block.

*/

bool Q3Semaphore::tryAccess(int n)

{

    QMutexLocker locker(&d->mutex);

    if (d->value + n > d->max)

        return false;

    d->value += n;

    return true;

}

QT_END_NAMESPACE