LARGE-SCALE PARALLEL GRAPH-BASED SIMULATIONS - MATSim

t y p e d e f multimap- Time , Veh WaitQueue ;
WaitQueue waitQueue ;
t y p e d e f multimap- Time , Veh ParkQueue ;
ParkQueue parkQueue ;
Figure 3.8: Declarations for waiting and parking queues with the STL-multimap
t y p e d e f Linked- Time , Veh WaitQueue ;
WaitQueue waitQueue ;
t y p e d e f Linked- Time , Veh ParkQueue ;
ParkQueue parkQueue ;
Figure 3.9: Declarations for waiting and parking queues with linked lists
queue is not full, the vehicle is moved from the waiting queue into the spatial queue so that it
can start its trip.
Hence, in each time step, waiting and parking queues are checked for the eligible vehicles.
In realistic scenarios, most of the vehicles wait in these queues because their drivers are actually
performing an activity. Checking for all eligible vehicles and accessing their information and
moving them to other queues when necessary comes with computational cost. Therefore, an
appropriate data structure should be used for these queues. That data structure needs to make
available the vehicle with the next scheduled departure at low performance cost. For this, a
partial ordering would in fact be sufficient. However, there is no data structure in the STL
which supplies a fully efficient partial ordering. Therefore, two fully ordered data structures
were tested: the STL-multimap, and a self-implemented singly linked list.
Note that this section only discusses waiting and parking queues. Data structures for link
cells will be explained in the next subsection.
Multimap
An easy-to-use fully sorted data structure is the STL-multimap. One just inserts key-item
pairs, with the keys equal to the start time of vehicles and the items being pointers to vehicles,
and the resulting data structure is automatically sorted. The difference between the STL-map,
as mentioned above, and the STL-multimap is that the latter accepts multiple elements for
the same key. This is necessary since it is possible that several vehicles want to depart at the
same time step. The declarations by using STL-multimap are given in Figure 3.8.
User-defined singly linked list
There are three operations defined on these queues: Insertion of an element into a queue, retrieving
the first element of the queue, and deleting the first element of the queue. Unfortunately
these operations are rather costly with the STL-multimap. From the experiences in the C
version of the simulation where a linked list was been used for these queues, a linked list is
implemented also in the C++ version to handle waiting and parking queues.
The Linked class in Figure 3.9 represents a singly linked list where each item in the list
has a pointer to the next item. Insertion at the end of the list and insertion according to a key
value into the sorted list are available. The latter is important so that vehicles can be sorted
according to their start times.
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