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Chapter 6. Change Dynamics
ing it with another syntax tree. In contrast to the String matching approach,
the Abstract Syntax Tree (AST) based matching is programming
language aware and is able to identify the types of changes more precisely.
Though this approach is comparatively more complex, it has
increasingly been adopted by contemporary IDE’s like Eclipse [73] to
perform an intelligent diff allowing developers to compare two versions
of a class. The AST based matching approach has also been enhanced
to better detect structural changes in object oriented programs by combining
it with call graphs analysis [157], and program behaviour analysis
[7].
The metric based finger printing approach involves computing a set of
measures from the abstraction that is being monitored and comparing
this set with a set collected after the change has been made. If there
is a difference between these two sets, the abstraction is considered to
have been changed. This approach, depending on the implementation
can be aware of the underlying programming language semantics, especially
if an AST is used to represent the program under analysis. A
key advantage of the metric based approach is its ability in providing
a consistent qualitative distance measure which can be used to determine
the magnitude of a change. One such distance measure is the
n-dimensional Euclidian distance measure where the two sets of measures
are treated as the input vectors from which a distance measure is
computed [34,67,155]. The resulting distance falls on the ordinal scale
and only provides sufficient information to permit ordering of changes
by magnitude. That is, we can determine that a specific class has had
a greater level of change than another. However, these measures are
unbounded and hence need to be normalised if they are to be used in a
comparative analysis (discussed in additional detail in the next section).
Change detection algorithms have applications beyond an intelligent
diff of the source code [242]. Variations of these algorithms have been
applied to detect different types of refactoring [288], as well as to identify
duplicate blocks of code (clones) in a software system [156, 195]
as these have been shown to reduce maintainability [236]. Clones are
in general caused by developers copying and pasting blocks of code,
which is considered a poor programming practice [236]. These clones
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