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### Party

Example party demonstrates the possibilities of an interval map (`interval_map` or `split_interval_map`). An `interval_map` maps intervals to a given content. In this case the content is a set of party guests represented by their name strings.

As time goes by, groups of people join the party and leave later in the evening. So we add a time interval and a name set to the `interval_map` for the attendance of each group of people, that come together and leave together. On every overlap of intervals, the corresponding name sets are accumulated. At the points of overlap the intervals are split. The accumulation of content is done via an operator += that has to be implemented for the content parameter of the `interval_map`. Finally the interval_map contains the history of attendance and all points in time, where the group of party guests changed.

Party demonstrates a principle that we call aggregate on overlap: On insertion a value associated to the interval is aggregated with those values in the interval_map that overlap with the inserted value. There are two behavioral aspects to aggregate on overlap: a decompositional behavior and an accumulative behavior.

- The decompositional behavior splits up intervals on the time dimension of the interval_map so that the intervals are split whenever associated values change.
- The accumulative behavior accumulates associated values on every overlap of an insertion for the associated values.

The aggregation function is += by default. Different aggregations can be used, if desired.

    // The next line includes <boost/date_time/posix_time/posix_time.hpp>
    // and a few lines of adapter code.
    #include <boost/icl/ptime.hpp>
    #include <iostream>
    #include <boost/icl/interval_map.hpp>

    using namespace std;
    using namespace boost::posix_time;
    using namespace boost::icl;

    // Type set<string> collects the names of party guests. Since std::set is
    // a model of the itl's set concept, the concept provides an operator +=
    // that performs a set union on overlap of intervals.
    typedef std::set<string> GuestSetT;

    void boost_party()
    {
        GuestSetT mary_harry;
        mary_harry.insert("Mary");
        mary_harry.insert("Harry");

        GuestSetT diana_susan;
        diana_susan.insert("Diana");
        diana_susan.insert("Susan");

        GuestSetT peter;
        peter.insert("Peter");

        // A party is an interval map that maps time intervals to sets of guests
        interval_map<ptime, GuestSetT> party;

        party.add( // add and element
          make_pair(
            interval<ptime>::right_open(
              time_from_string("2008-05-20 19:30"),
              time_from_string("2008-05-20 23:00")),
            mary_harry));

        party += // element addition can also be done via operator +=
          make_pair(
            interval<ptime>::right_open(
              time_from_string("2008-05-20 20:10"),
              time_from_string("2008-05-21 00:00")),
            diana_susan);

        party +=
          make_pair(
            interval<ptime>::right_open(
              time_from_string("2008-05-20 22:15"),
              time_from_string("2008-05-21 00:30")),
            peter);

        interval_map<ptime, GuestSetT>::iterator it = party.begin();
        cout << "----- History of party guests -------------------------\n";
        while(it != party.end())
        {
            interval<ptime>::type when = it->first;
            // Who is at the party within the time interval 'when' ?
            GuestSetT who = (*it++).second;
            cout << when << ": " << who << endl;
        }

    }

    int main()
    {
        cout << ">>Interval Container Library: Sample boost_party.cpp <<\n";
        cout << "-------------------------------------------------------\n";
        boost_party();
        return 0;
    }

    // Program output:
    /*-----------------------------------------------------------------------------
    >>Interval Container Library: Sample boost_party.cpp <<
    -------------------------------------------------------
    ----- History of party guests -------------------------
    [2008-May-20 19:30:00, 2008-May-20 20:10:00): Harry Mary
    [2008-May-20 20:10:00, 2008-May-20 22:15:00): Diana Harry Mary Susan
    [2008-May-20 22:15:00, 2008-May-20 23:00:00): Diana Harry Mary Peter Susan
    [2008-May-20 23:00:00, 2008-May-21 00:00:00): Diana Peter Susan
    [2008-May-21 00:00:00, 2008-May-21 00:30:00): Peter
    -----------------------------------------------------------------------------*/

| Caution
We are introducing `interval_maps` using an interval map of sets of strings, because of it's didactic advantages. The party example is used to give an immediate access to the basic ideas of interval maps and aggregate on overlap. For real world applications, an interval_map of sets is not necessarily recommended. It has the same efficiency problems as a `std::map` of `std::sets`. There is a big realm though of using interval_maps with numerical and other efficient data types for the associated values.