LARGE-SCALE PARALLEL GRAPH-BASED SIMULATIONS - MATSim

the street network is explained in Section 3.3.3. The same section promotes an alternative data
structure, namely, a self-implemented singly linked list, and gives the test results for these two
implementations. Section 3.3.4 discusses different implementations for the link queues, i.e.,
the spatial queue and the link buffer. The alternatives are using STL-deque, STL-list and
a self-implemented data structure Ring.
3.3.1 The Standard Template Library
The Standard Template Library (STL) is a C++ library composed of the following components:
Collections of standard container types. Containers are implemented as templates, a
special feature of C++ and contain objects of any type. Examples are map, deque
(double-ended queue), vector, list, etc.
Algorithms defined on containers. Examples are: accessing an element, sorting the elements
in a container, searching for an element, etc.
Iterators used for traversing the elements of a container.
The STL not only hides the implementation details of its components but also provides
elegant data structures and algorithms for users. Encapsulation and abstraction properties of
STL enables programmers to focus on application specific issues.
The STL provides two types of containers: Sequence containers store data in a linear sequence.
The “sequence” depends on time and position of insertion. The position of an element
in the container is independent of the element’s value. vector, deque and list are of this
type.
Associative containers, on the other hand, are sorted data structures. They associate the
domain of one type (key) with the domain of another type (value). The position of an element
in such a container depends on its key. Examples are map, multimap, set, etc.
Operations defined on containers, such as insert, delete, or retrieve, differ in performance.
Container selection is dependent on characteristics of the applications and on call frequency.
Some examples are given in next sub-sections.
Each iterator represents a certain position in a container. Regardless of the container for
which it is defined, an iterator comes with a set of basic operators. The basic operators are ++
(stepping forward to the next element), == (equal), != (not equal), = (assignment operator)
and * (dereference operator). Since an iterator is an object, the user must create instances of
the iterator class prior to using them.
3.3.2 Containers: Map vs Vector for Graph Data
Accessing the graph data, i.e. the street network, is one of the key issues in the simulation.
This is because every single item (a link or a node) of the graph is visited several times in each
time step during the simulation run. Moreover, the searching algorithm for the elements of a
container, which represents the graph data, is crucial since searching for a single element in the
graph data is done more than once: (1) when plans are read, the start and the end locations have
to be searched in the graph data, and (2) every time a vehicle on a link at the border needs to
enter the next link, the next link is searched in the graph data to find out, to which computing
node it belongs in the parallel implementation.
The overall approach of using STL containers for the graph data looks as Figure 3.1 (using
graph nodes as the example 1 .)
1 For non-C-experts: typedef aa bb means that from now on, bb will be translated to aa before the
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