Identifying the Within-Statement Changes to Facilitate Change Understanding

As current tree-differencing approaches ignore changes that occur within a statement or do not present them in an abstract way, it is difficult to automatically understand revisions involving statement updates. We propose a tree-differencing approach to identifying the within-statement changes. It calculates edit operations based on an element-sensitive strategy and the longest common sequence algorithm. Then, it generates the metadata for each edit operation. Meta-data include the type of operation, the type of entity and the name of the element part, the content, the content pattern and all references involved. We have implemented the approach as a free accessible tool. It is built upon ChangeDistiller and refines its statement-update type. Finally, to demonstrate how to use the proposed approach for change understanding, we studied the condition-expression changes in four open projects. We analyzed the non-essential condition changes, the effective changes that definitely affect the condition, and other changes. The results show that for revisions with condition-expression changes, nearly 20% contain non-essential changes, while more than 60% have effective changes. Furthermore, we found many common patterns. For example, we found that half of the revisions with effective changes were caused by adding or removing expressions in logical expressions. And, in these revisions, 47% enhanced the condition, while 49% weakened it.