2 Why We Need Model-Based Testing

Systems with Complex State 189
11.2.5 State grouping
State grouping is a pruning technique that limits exploration to unique representatives
of user-provided state properties. If during exploration a state is encountered
that does not produce a new value of one of the desired properties, it is pruned away.
In other words, state exploration with grouping seeks to produce distinct examples
of the abstracted state values, rather than distinct states. This can reduce the number
of states greatly while still uncovering “interesting” traces.
An attribute in the form [StateProperty("name")] can be placed on a method,
property or field to define a state property named name. The value of the property in
a given state is the return value of the method, or the value of the field or property in
that state. State properties are sometimes called abstraction functions because they
remove detail.
Here is an example.
namespace Stack
{
static class Contract
{
static Sequence contents = Sequence.EmptySequence;
}
}
static readonly Set elements = new Set(0,1,2,3,4,5);
[Action]
static void Push([Domain("elements")] int x)
{ contents = contents.AddFirst(x); }
static void PopEnabled() { return !contents.IsEmpty; }
[Action]
static void Pop() { contents = contents.Tail; }
[StateFilter]
static bool SizeLimit() { return contents.Count < 6; }
[StateProperty("Count")]
static int Count() { return contents.Count; }
If we explore this model program using the mpv tool without state grouping, we
notice that it branches quickly. However, if we use the Count property as a state
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