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/*!

    \namespace QtConcurrent

    \inmodule QtCore

    \since 4.4

    \brief The QtConcurrent namespace provides high-level APIs that make it

    possible to write multi-threaded programs without using low-level

    threading primitives.

    See the \l {Concurrent Programming}{Qt Concurrent} chapter in

    the \l{threads.html}{threading} documentation.

    \inheaderfile QtCore

    \ingroup thread

*/

/*!

    \namespace QtConcurrent::internal

    \internal

    \brief The QtConcurrent::internal namespace contains QtConcurrent

    implementation details.

*/

/*!

    \enum QtConcurrent::ReduceOption

    This enum specifies the order of which results from the map or filter 

    function are passed to the reduce function.

    \value UnorderedReduce Reduction is done in an arbitrary order.

    \value OrderedReduce Reduction is done in the order of the

    original sequence.

    \value SequentialReduce Reduction is done sequentally: only one

    thread will enter the reduce function at a time. (Parallel reduction

    might be supported in a future version of Qt Concurrent.)

*/

/*!

    \headerfile <QtConcurrentMap>

    \title Concurrent Map and Map-Reduce

    \ingroup thread

    \brief The <QtConcurrentMap> header provides concurrent Map and MapReduce.

    These functions are a part of the \l {Concurrent Programming}{Qt Concurrent} framework.

    The QtConcurrent::map(), QtConcurrent::mapped() and

    QtConcurrent::mappedReduced() functions run computations in parallel on

    the items in a sequence such as a QList or a QVector. QtConcurrent::map()

    modifies a sequence in-place, QtConcurrent::mapped() returns a new

    sequence containing the modified content, and QtConcurrent::mappedReduced()

    returns a single result.

    Each of the above functions has a blocking variant that returns

    the final result instead of a QFuture. You use them in the same

    way as the asynchronous variants.

    \snippet doc/src/snippets/code/src_corelib_concurrent_qtconcurrentmap.cpp 7

    Note that the result types above are not QFuture objects, but real result

    types (in this case, QList<QImage> and QImage).

    \section1 Concurrent Map

    QtConcurrent::mapped() takes an input sequence and a map function. This map

    function is then called for each item in the sequence, and a new sequence

    containing the return values from the map function is returned.

    The map function must be of the form:

    \snippet doc/src/snippets/code/src_corelib_concurrent_qtconcurrentmap.cpp 0

    T and U can be any type (and they can even be the same type), but T must

    match the type stored in the sequence. The function returns the modified

    or \e mapped content.

    This example shows how to apply a scale function to all the items

    in a sequence:

    \snippet doc/src/snippets/code/src_corelib_concurrent_qtconcurrentmap.cpp 1

    The results of the map are made available through QFuture.  See the

    QFuture and QFutureWatcher documentation for more information on how to

    use QFuture in your applications.

    If you want to modify a sequence in-place, use QtConcurrent::map(). The

    map function must then be of the form:

    \snippet doc/src/snippets/code/src_corelib_concurrent_qtconcurrentmap.cpp 2

    Note that the return value and return type of the map function are not

    used.

    Using QtConcurrent::map() is similar to using QtConcurrent::mapped():

    \snippet doc/src/snippets/code/src_corelib_concurrent_qtconcurrentmap.cpp 3

    Since the sequence is modified in place, QtConcurrent::map() does not

    return any results via QFuture. However, you can still use QFuture and

    QFutureWatcher to monitor the status of the map.

    \section1 Concurrent Map-Reduce

    QtConcurrent::mappedReduced() is similar to QtConcurrent::mapped(), but

    instead of returning a sequence with the new results, the results are

    combined into a single value using a reduce function.

    The reduce function must be of the form:

    \snippet doc/src/snippets/code/src_corelib_concurrent_qtconcurrentmap.cpp 4

    T is the type of the final result, U is the return type of the map

    function. Note that the return value and return type of the reduce

    function are not used.

    Call QtConcurrent::mappedReduced() like this:

    \snippet doc/src/snippets/code/src_corelib_concurrent_qtconcurrentmap.cpp 5

    The reduce function will be called once for each result returned by the map

    function, and should merge the \e{intermediate} into the \e{result}

    variable.  QtConcurrent::mappedReduced() guarantees that only one thread

    will call reduce at a time, so using a mutex to lock the result variable

    is not neccesary. The QtConcurrent::ReduceOptions enum provides a way to

    control the order in which the reduction is done. If

    QtConcurrent::UnorderedReduce is used (the default), the order is

    undefined, while QtConcurrent::OrderedReduce ensures that the reduction

    is done in the order of the original sequence.

    \section1 Additional API Features

    \section2 Using Iterators instead of Sequence

    Each of the above functions has a variant that takes an iterator range

    instead of a sequence. You use them in the same way as the sequence

    variants:

    \snippet doc/src/snippets/code/src_corelib_concurrent_qtconcurrentmap.cpp 6

    \section2 Blocking Variants

    Each of the above functions has a blocking variant that returns

    the final result instead of a QFuture. You use them in the same

    way as the asynchronous variants.

    \snippet doc/src/snippets/code/src_corelib_concurrent_qtconcurrentmap.cpp 7

    Note that the result types above are not QFuture objects, but real result

    types (in this case, QList<QImage> and QImage).

    \section2 Using Member Functions

    QtConcurrent::map(), QtConcurrent::mapped(), and

    QtConcurrent::mappedReduced() accept pointers to member functions.

    The member function class type must match the type stored in the sequence:

    \snippet doc/src/snippets/code/src_corelib_concurrent_qtconcurrentmap.cpp 8

    Note that when using QtConcurrent::mappedReduced(), you can mix the use of

    normal and member functions freely:

    \snippet doc/src/snippets/code/src_corelib_concurrent_qtconcurrentmap.cpp 9

    \section2 Using Function Objects

    QtConcurrent::map(), QtConcurrent::mapped(), and

    QtConcurrent::mappedReduced() accept function objects, which can be used to

    add state to a function call. The result_type typedef must define the 

    result type of the function call operator:

    \snippet doc/src/snippets/code/src_corelib_concurrent_qtconcurrentmap.cpp 14

    \section2 Using Bound Function Arguments

    Note that Qt does not provide support for bound functions. This is

    provided by 3rd party libraries like

    \l{http://www.boost.org/libs/bind/bind.html}{Boost} or

    \l{http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1836.pdf}{C++

    TR1 Library Extensions}.

    If you want to use a map function that takes more than one argument you can

    use boost::bind() or std::tr1::bind() to transform it onto a function that

    takes one argument.

    As an example, we'll use QImage::scaledToWidth():

    \snippet doc/src/snippets/code/src_corelib_concurrent_qtconcurrentmap.cpp 10

    scaledToWidth takes three arguments (including the "this" pointer) and

    can't be used with QtConcurrent::mapped() directly, because

    QtConcurrent::mapped() expects a function that takes one argument. To use

    QImage::scaledToWidth() with QtConcurrent::mapped() we have to provide a

    value for the \e{width} and the \e{transformation mode}:

    \snippet doc/src/snippets/code/src_corelib_concurrent_qtconcurrentmap.cpp 11

    The return value from boost::bind() is a function object (functor) with

    the following signature:

    \snippet doc/src/snippets/code/src_corelib_concurrent_qtconcurrentmap.cpp 12

    This matches what QtConcurrent::mapped() expects, and the complete example

    becomes:

    \snippet doc/src/snippets/code/src_corelib_concurrent_qtconcurrentmap.cpp 13

*/

/*!

    \fn QFuture<void> QtConcurrent::map(Sequence &sequence, MapFunction function)

    \relates <QtConcurrentMap>

    Calls \a function once for each item in \a sequence. The \a function is

    passed a reference to the item, so that any modifications done to the item

    will appear in \a sequence.

*/

/*!

    \fn QFuture<void> QtConcurrent::map(Iterator begin, Iterator end, MapFunction function)

    \relates <QtConcurrentMap>

    Calls \a function once for each item from \a begin to \a end. The

    \a function is passed a reference to the item, so that any modifications

    done to the item will appear in the sequence which the iterators belong to.

*/

/*!

    \fn QFuture<T> QtConcurrent::mapped(const Sequence &sequence, MapFunction function)

    \relates <QtConcurrentMap>

    Calls \a function once for each item in \a sequence and returns a future

    with each mapped item as a result. You can use QFuture::const_iterator or

    QFutureIterator to iterate through the results.

*/

/*!

    \fn QFuture<T> QtConcurrent::mapped(ConstIterator begin, ConstIterator end, MapFunction function)

    \relates <QtConcurrentMap>

    Calls \a function once for each item from \a begin to \a end and returns a

    future with each mapped item as a result. You can use

    QFuture::const_iterator or QFutureIterator to iterate through the results.

*/

/*!

    \fn QFuture<T> QtConcurrent::mappedReduced(const Sequence &sequence,

    MapFunction mapFunction, ReduceFunction reduceFunction,

    QtConcurrent::ReduceOptions reduceOptions)

    \relates <QtConcurrentMap>

    Calls \a mapFunction once for each item in \a sequence. The return value of

    each \a mapFunction is passed to \a reduceFunction.

    Note that while \a mapFunction is called concurrently, only one thread at a

    time will call \a reduceFunction. The order in which \a reduceFunction is

    called is determined by \a reduceOptions.

*/

/*!

    \fn QFuture<T> QtConcurrent::mappedReduced(ConstIterator begin,

    ConstIterator end, MapFunction mapFunction, ReduceFunction reduceFunction,

    QtConcurrent::ReduceOptions reduceOptions)

    \relates <QtConcurrentMap>

    Calls \a mapFunction once for each item from \a begin to \a end. The return

    value of each \a mapFunction is passed to \a reduceFunction.

    Note that while \a mapFunction is called concurrently, only one thread at a

    time will call \a reduceFunction. By default, the order in which

    \a reduceFunction is called is undefined.

    \note QtConcurrent::OrderedReduce results in the ordered reduction.

*/

/*!

  \fn void QtConcurrent::blockingMap(Sequence &sequence, MapFunction function)

  Calls \a function once for each item in \a sequence. The \a function is

  passed a reference to the item, so that any modifications done to the item

  will appear in \a sequence.

  \note This function will block until all items in the sequence have been processed.

  \sa map()

*/

/*!

  \fn void QtConcurrent::blockingMap(Iterator begin, Iterator end, MapFunction function)

  Calls \a function once for each item from \a begin to \a end. The

  \a function is passed a reference to the item, so that any modifications

  done to the item will appear in the sequence which the iterators belong to.

  \note This function will block until the iterator reaches the end of the

  sequence being processed.

  \sa map()

*/

/*!

  \fn T QtConcurrent::blockingMapped(const Sequence &sequence, MapFunction function)

  Calls \a function once for each item in \a sequence and returns a Sequence containing

  the results. The type of the results will match the type returned my the MapFunction.

  \note This function will block until all items in the sequence have been processed.

  \sa mapped()

*/

/*!

  \fn T QtConcurrent::blockingMapped(ConstIterator begin, ConstIterator end, MapFunction function)

  Calls \a function once for each item from \a begin to \a end and returns a

  container with the results. Specify the type of container as the a template

  argument, like this:

  \code

     QList<int> ints = QtConcurrent::blockingMapped<QList<int> >(beginIterator, endIterator, fn);

  \endcode

  \note This function will block until the iterator reaches the end of the

  sequence being processed.

  \sa mapped()

*/

/*!

  \fn T QtConcurrent::blockingMappedReduced(const Sequence &sequence, MapFunction mapFunction, ReduceFunction reduceFunction, QtConcurrent::ReduceOptions reduceOptions)

  \relates <QtConcurrentMap>

  Calls \a mapFunction once for each item in \a sequence. The return value of

  each \a mapFunction is passed to \a reduceFunction.

  Note that while \a mapFunction is called concurrently, only one thread at a

  time will call \a reduceFunction. The order in which \a reduceFunction is

  called is determined by \a reduceOptions.

  \note This function will block until all items in the sequence have been processed.

  \sa mapped()

*/

/*!

  \fn T QtConcurrent::blockingMappedReduced(ConstIterator begin, ConstIterator end, MapFunction mapFunction, ReduceFunction reduceFunction, QtConcurrent::ReduceOptions reduceOptions)

  \relates <QtConcurrentMap>

  Calls \a mapFunction once for each item from \a begin to \a end. The return

  value of each \a mapFunction is passed to \a reduceFunction.

  Note that while \a mapFunction is called concurrently, only one thread at a

  time will call \a reduceFunction. The order in which \a reduceFunction is

  called is undefined.

  \note This function will block until the iterator reaches the end of the

  sequence being processed.

  \sa blockingMappedReduced()

*/