  Software undergoes constant changes to support new requirements, address
bugs, enhance performance, and ensure maintainability. Thus, developers spend a
great portion of their workday trying to understand and review the code changes
of their teammates. Abstract Syntax Tree (AST) diff tools were developed to
overcome the limitations of line-based diff tools, which are used by the
majority of developers. Despite the notable improvements brought by AST diff
tools in understanding complex changes, they still suffer from serious
limitations, such as (1) lacking multi-mapping support, (2) matching
semantically incompatible AST nodes, (3) ignoring language clues to guide the
matching process, (4) lacking refactoring awareness, and (5) lacking
commit-level diff support. We propose a novel AST diff tool based on
RefactoringMiner that resolves all aforementioned limitations. First, we
improved RefactoringMiner to increase its statement mapping accuracy, and then
we developed an algorithm that generates AST diff for a given commit or pull
request based on the refactoring instances and pairs of matched program element
declarations provided by RefactoringMiner. To evaluate the accuracy of our tool
and compare it with the state-of-the-art tools, we created the first benchmark
of AST node mappings, including 800 bug-fixing commits and 188 refactoring
commits. Our evaluation showed that our tool achieved a considerably higher
precision and recall, especially for refactoring commits, with an execution
time that is comparable with that of the faster tools.
