// Copyright (C) 2023 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only

/*!
    \example primecounter
    \meta tags {widgets, threads}
    \title Prime Counter
    \ingroup qtconcurrentexamples
    \brief Demonstrates how to monitor the progress of concurrent operations.

    The following example demonstrates how to create an interactive and
    non-blocking QtWidgets application using the QFutureWatcher class and the
    \l {Concurrent Filter-Reduce} {filteredReduced} functions from
    \l {Qt Concurrent}. With this example, the user can create a QList of
    integers that can be resized. The list will be automatically filled with
    natural numbers starting from 1 up to n. The program will then check for
    prime numbers within the list and display the total count of prime numbers
    found.

    \image primecounter.png

    \include examples-run.qdocinc

    \section1 Setting up the connections

    The \l {Qt Concurrent} library provides the
    \l {Concurrent Filter-Reduce} {filteredReduced} functions, which can operate
    in two modes:
    \l {QtConcurrent::ReduceOption} {OrderedReduce and UnorderedReduce}. In
    \c OrderedReduce mode, the reducing function is called in the order of the
    original sequence, whereas in \c UnorderedReduce mode, the elements are
    accessed randomly.

    After configuring the UI with the desired elements, it is necessary to
    connect them to the signals of the concurrent operations using the Qt
    \l {Signals & Slots} mechanism. In this example, we use the QFutureWatcher
    class to monitor the progress of the concurrent operations and provide the
    signals required to implement the interactive GUI.

    \dots
    \snippet primecounter/primecounter.cpp 1
    \dots

    The QFutureWatcher class plays a vital role in this example as it provides
    the signals required to update the UI in response to changes in the
    concurrent operations.

    \section1 Starting the concurrent operation

    After connecting all the \l {Signals & Slots}, and when the user presses
    the QPushButton, the \c {start()} function is called.

    In the \c {start()} function, we call the
    \l {Concurrent Filter-Reduce} {filteredReduced} function from Qt Concurrent
    and set the future on the QFutureWatcher member. To ensure that this
    operation runs truly concurrently, we specify a separate QThreadPool as the
    first parameter. This approach also avoids any possible blocking in the
    global thread pool. We pass the QList of integers as the container, a
    static filter and reduce function, and finally the
    \l {QtConcurrent::} {ReduceOption} flag.

    \dots
    \snippet primecounter/primecounter.cpp 2
    \dots

    Let's examine the filter and reduce functions. These functions are declared
    static in this example since they do not depend on any member variable.
    However, they could easily be specified as lambdas or member functions.

    The filter function marks elements for subsequent reduction with the reduce
    function. This implementation is a simple prime filter. As this function
    takes a const reference as an argument, it allows thread-safe operation on
    the container it operates on.

    \dots
    \snippet primecounter/primecounter.cpp 3
    \dots

    The reduce function takes a modifiable reference of the same type as the
    container it operates on as its first parameter. The second parameter is the
    previously filtered element from the filter function. In this example, we
    count the number of primes.

    \dots
    \snippet primecounter/primecounter.cpp 4
    \dots

*/
