2 Why We Need Model-Based Testing

204 Testing Systems with Complex State
12.4.1 Coverage points
There are many ways to measure coverage. Coverage points provide a uniform way
to handle different coverage measures. A coverage point is a part or property of
a program used to measure coverage, by counting how many times it is executed,
visited, or otherwise exercised. A state, an action, or a branch in the code could be
a coverage point.
Let us a consider a fixed explorable model program. Suppose that given a state
s and an action a, one can calculate a collection of coverage points that are visited
(covered) in the model program if a is executed from state s. In general, the same
coverage point can be visited multiple times but the order in which the coverage
points are visited is irrelevant; thus, the collection is intuitively a bag. A coverage
point is represented by a term. The type of a function that, given a state and an action,
computes a bag of coverage points is called a CoverageFunction and is defined in
the NModel.Conformance namespace as follows. 1
delegate Bag CoverageFunction(IState s, Action a);
Coverage points can reflect structural coverage as well as behavioral coverage of
the model program. Here are some concrete examples of some common notions of
coverage. The first two are examples of structural coverage of the model program,
whereas the remaining three are examples of behavioral coverage because they are
independent of the structure of the model program.
Vocabulary coverage. The simplest example of a coverage function is one that
returns a bag containing just the action symbol. In other words, it is ignored
from which state the action is explored and what arguments the action takes.
Bag GetVocabularyCoverage(IState s, Action a)
{ return new Bag(a.FunctionSymbol.AsTerm); }
Annotation coverage. Suppose you want to cover different cases or code
branches inside one action method. You can annotate the model program with
explicit coverage points using the Execute.AddCoveragePoint method in order
to record what case has been taken. The following example shows how to annotate
the bag model in Figure 12.2 with two coverage points that distinguish
between the two cases when an element that is to be deleted occurs in the bag
or does not occur in the bag.
[Action]
static void Delete(string element)
{
1 A method type such as CoverageFunction is called a delegate in .NET.
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