Refactoring via program slicing and sliding

Mark Weiser’s observation that “programmers use slices when debugging”, back in 1982, started a new field of research. Program slicing, the study of meaningful subprograms that capture a subset of an existing program’s behaviour, aims at providing programmers with tools to assist in a variety of software development and maintenance activities. Two decades later, the work leading to this thesis was initiated with the observation that “programmers use slices when refactoring”. Hence, the thesis explores ways in which known refactoring techniques can be automated through slicing and related program analyses. Common to all slicing related refactorings, as explored in this thesis, is the goal of improving reusability, comprehensibility and hence maintainability of existing code. A problem of slice extraction is posed and its potential contribution to refactoring research highlighted. Limitations of existing slice-extraction solutions include low applicability and high levels of code duplication. Advanced techniques for the automation of slice extraction are proposed. The key to their success lies in a novel program representation introduced in the thesis. We think of a program as a collection of transparency slides, placed one on top of the other. On each such slide, a subset of the original statement, not necessarily a contiguous one, is printed. Thus, a subset of a statement’s slides can be extracted from the remaining slides in an operation of sideways movement, called sliding. Semantic properties of such sliding operations are extensively studied through known techniques of predicate calculus and program semantics. This thesis makes four significant contributions to the slicing and refactoring fields of research. Firstly, it develops a theoretical framework for slicing-based behaviour-preserving transformations of existing code. Secondly, it provides a provably correct slicing algorithm. This application of our theory acts as evidence for its expressive power whilst enabling constructive descriptions of slicingbased transformations. Thirdly, it applies the above framework and slicing algorithm in solving the problem of slice extraction. The solution, a family of provably correct sliding transformations, provides high levels of accuracy and applicability. Finally, the thesis outlines the application of sliding to known refactorings, making them automatable for the first time. These contributions provide strong evidence that, indeed, slicing and related analyses can assist in building automatic tools for refactoring. To Dana, Amir, Zohara and Ze’ev; and to loved Bärbel.