/****************************************************************************
** $Id:  qt/qsemaphore_unix.cpp   3.0.3   edited Oct 12 12:18 $
**
** QSemaphore class for Unix
**
** Created : 20010725
**
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**
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** LICENSE.QPL included in the packaging of this file.
**
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** Foundation and appearing in the file LICENSE.GPL included in the
** packaging of this file.
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** licenses may use this file in accordance with the Qt Commercial License
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#if defined(QT_THREAD_SUPPORT)

#include "qsemaphore.h"
#include "qmutex.h"
#include "qwaitcondition.h"


/*!
  \class QSemaphore qthread.h
  \brief The QSemaphore class provides a robust integer semaphore.

  \ingroup thread
  \ingroup environment

  A QSemaphore 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 restuarant.
  A semaphore is initialized to have a maximum count equal to the number
  of chairs in the restuarant.  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.
*/


class QSemaphorePrivate {
public:
    QSemaphorePrivate(int);

    QMutex mutex;
    QWaitCondition cond;

    int value, max;
};


QSemaphorePrivate::QSemaphorePrivate(int m)
    : mutex(FALSE), value(0), max(m)
{
}


/*!
  Creates a new semaphore.  The semaphore can be concurrently accessed at
  most \a maxcount times.
*/
QSemaphore::QSemaphore(int maxcount)
{
    d = new QSemaphorePrivate(maxcount);
}


/*!
  Destroys the semaphore.
*/
QSemaphore::~QSemaphore()
{
    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 QSemaphore::operator++(int)
{
    int ret;

    d->mutex.lock();

    while (d->value >= d->max)
	d->cond.wait(&(d->mutex));

    ++(d->value);
    if (d->value > d->max) d->value = d->max;
    ret = d->value;

    d->mutex.unlock();

    return ret;
}


/*!
  Postfix -- operator.

  Release access of the semaphore.  This wakes all threads waiting for
  access to the semaphore.
 */
int QSemaphore::operator--(int)
{
    int ret;

    d->mutex.lock();

    --(d->value);
    if (d->value < 0) d->value = 0;
    ret = d->value;

    d->cond.wakeAll();
    d->mutex.unlock();

    return ret;
}


/*!
  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 QSemaphore::operator+=(int n)
{
    int ret;

    d->mutex.lock();

    while (d->value + n > d->max)
	d->cond.wait(&(d->mutex));

    d->value += n;

#ifdef QT_CHECK_RANGE
    if (d->value > d->max) {
	qWarning("QSemaphore::operator+=: attempt to allocate more resources than available");
	d->value = d->max;
    }
#endif

    ret = d->value;

    d->mutex.unlock();

    return ret;
}


/*!
  Release \a n accesses to the semaphore.
 */
int QSemaphore::operator-=(int n)
{
    int ret;

    d->mutex.lock();

    d->value -= n;

#ifdef QT_CHECK_RANGE
    if (d->value < 0) {
	qWarning("QSemaphore::operator-=: attempt to deallocate more resources than taken");
	d->value = 0;
    }
#endif

    ret = d->value;

    d->cond.wakeOne();
    d->mutex.unlock();

    return ret;
}


/*!
  This function returns the number of accesses currently available to
  the semaphore.
 */
int QSemaphore::available() const {
    int ret;

    d->mutex.lock();
    ret = d->max - d->value;
    d->mutex.unlock();

    return ret;
}


/*!
  This function returns the total number of accesses to the semaphore.
 */
int QSemaphore::total() const {
    int ret;

    d->mutex.lock();
    ret = d->max;
    d->mutex.unlock();

    return ret;
}


/*!
  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 QSemaphore::tryAccess(int n)
{
    if (! d->mutex.tryLock())
	return FALSE;

    if (d->value + n > d->max) {
	d->mutex.unlock();
	return FALSE;
    }

    d->value += n;

#ifdef QT_CHECK_RANGE
    if (d->value > d->max) {
	qWarning("QSemaphore::operator+=: attempt to allocate more resources than available");
	d->value = d->max;
    }
#endif

    d->mutex.unlock();

    return TRUE;
}

#endif // QT_THREAD_SUPPORT
