2 Why We Need Model-Based Testing

Systems with Complex State 167
10.3.5 Bags or multisets
A bag, or multiset, is an unordered collection of possibly repeating elements. Unlike
sets, an element may appear in a bag more than once. Bags are immutable values,
but you can construct new bags by operations such as union and intersection.
Appendix A.2.6 provides a list of bag operations provided by the NModel library.
Like sets, bags are unordered. Two bags are equal if they contain the number of
occurrences of the same elements. The number of occurrences of a given element is
called its multiplicity.
Bags are useful as a way to abstract the order of a sequence while still allowing
for duplicate values.
For example, you might use a bag to model reference counting. In reference
counting, operations track whether an element may be deleted based on the number
of references.
static class ReferenceCounting
{
static Bag references = Bag.EmptyBag;
static Set allocated = Set.EmptySet;
[Action]
static void AddReference(string obj)
{
references = references.Add(obj);
allocated = allocated.Add(obj);
}
static bool RemoveReferenceEnabled(string obj)
{
return references.Contains(obj);
}
[Action]
static void RemoveReference(string obj)
{
references = references.Remove(obj);
}
static bool DeleteObjectEnabled(string obj)
{
return allocated.Contains(obj) && !references.Contains(obj);
}
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