Improving pattern tracking with a language-aware tree differencing algorithm

Tracking code fragments of interest is important in monitoring a software project over multiple versions. Various approaches, including our previous work on Herodotos, exploit the notion of Longest Common Subsequence, as computed by readily available tools such as GNU Diff, to map corresponding code fragments. Nevertheless, the efficient code differencing algorithms are typically line-based or word-based, and thus do not report changes at the level of language constructs. Furthermore, they identify only additions and removals, but not the moving of a block of code from one part of a file to another. Code fragments of interest that fall within the added and removed regions of code have to be manually correlated across versions, which is tedious and error-prone. When studying a very large code base over a long time, the number of manual correlations can become an obstacle to the success of a study. In this paper, we investigate the effect of replacing the current line-based algorithm used by Herodotos by tree-matching, as provided by the algorithm of the differencing tool GumTree. In contrast to the line-based approach, the tree-based approach does not generate any manual correlations, but it incurs a high execution time. To address the problem, we propose a hybrid strategy that gives the best of both approaches.