2 Why We Need Model-Based Testing

172 Modeling Systems with Structured State
In the code snippets that follow, we will omit for brevity the RevisionControl
namespace declaration that defines the model’s scope, but this can be assumed.
Revision data type
In addition to the int and string data types, we will use an additional data record
to define the concept of a “revision.”
enum Op { Add, Delete, Change }
class Revision : CompoundValue
{
public readonly Op op;
public readonly int revisionNumber;
}
public Revision(Op op, int revisionNumber)
{
this.op = op;
this.revisionNumber = revisionNumber;
}
The enumeration Op defines the kind of edit operations provided by the revision
control system. Edits may consist of adding a file, changing the contents of an existing
file, or deleting a file.
We define a new compound value type Revision that is a pair of values: an
operation and a revision number that says in which version of the repository the
change appears. This lets the repository keep accurate information about all previous
versions.
Repository file state
The repository maintains a per-file record of each revision that been made. We can
model the change list as a sequence of revisions. The database of changes is a map
associating file names to change lists.
partial static class Repository
{
static Map db =
Map.EmptyMap;
}
The first element of a change log is the most recent revision.
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