2 Why We Need Model-Based Testing

186 Analyzing Systems with Complex State
an understandable separation between scenario and contract model program. It also
makes it possible for there to be several scenarios, not just one. Also, even though it
is not explorable, the full contract model program is useful for other purposes, such
as determining whether a trace taken from a log file is valid.
11.2 Pruning techniques
Once you have made your model program explorable by using the technique described
in the previous section, you may still notice that it has an unbounded number
of possible states. In this section we introduce some techniques for managing this
proliferation of states.
11.2.1 State and transition limits
A simple way of limiting the number of transitions is to impose a limit and then
halt exploration after the maximum number has been reached. You can do this by
providing a maxTransitions command-line argument to the mpv tool.
Although this technique is simple, it isn’t very systematic. Still, it is a useful stop
gap when other pruning techniques haven’t been applied yet.
11.2.2 State filters
Sometimes it’s useful to consider only a subset of the possible states. For example,
consider the following explorable, but infinite, model program:
namespace Counter2
{
static class Contract
{
public static int counter = 0;
}
}
[Action]
static void Increment() { counter = counter + 1; }
static bool DecrementEnabled() { return counter > 0; }
[Action]
static void Decrement() { counter = counter - 1; }
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