2 Why We Need Model-Based Testing

Systems with Finite Models 103
more automatic way that uses composition to check whether a run is allowed, which
is still simpler than writing a unit test.
6.2.4 An exploration algorithm
In this subsection we explain how exploration works by presenting an exploration
algorithm. You do not need to understand the algorithm in order to use the mpv
tool to analyze finite model programs. But to analyze infinite model programs,
you must provide additional information to guide exploration (Chapter 11). Then it
becomes necessary to understand how exploration works. We prepare for that now
by considering exhaustive exploration, which is the simplest case.
Exploration generates an FSM from a model program. It executes the model
program, automatically selecting actions to execute, monitoring, and recording each
state transition as it occurs, building a data structure that contains all the information
shown in state transition tables or diagrams (Table 6.2, Figure 6.2).
Recall that we prepare for exploration by providing a factory method that creates
a LibraryModelProgram object from the compiled model program assembly
(Section 6.2.1, Figure 6.3). Exploration executes the model program indirectly, by
invoking the methods of this object.
The LibraryModelProgram class is derived from the ModelProgram base class. We
now provide a simplified description of this class, which omits many details. The
names and members (etc.) of the types and methods in this simplified description
are not the same as in the actual library. To avoid confusion with the actual modeling
library, we call the class ModelProgramSimple here. It uses these types (which are
also not exactly the same as the ones in the actual modeling library):
State The state of a model program, represented by a dictionary that associates
each state variable name with its value.
Action The invocation of an action, including the action method name and all of
its arguments.
Transition A state transition, with constructor Transition(State current,
Action a, State next)
Our ModelProgramSimple class provides these properties and methods:
State InitialState Property that returns the model program’s initial state
Set GetActions(State s) Method that returns the set of actions that
are enabled in state s.
State GetTargetState(State s, Action a) Method that returns the next state
reached by executing action a in the current state s.
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