On the Optimal Order of Reading Source Code Changes for Review

Change-based code review, e.g., in the form of pull requests, is the dominant style of code review in practice. An important option to improve review's efficiency is cognitive support for the reviewer. Nevertheless, review tools present the change parts under review sorted in alphabetical order of file path, thus leaving the effort of understanding the construction, connections, and logic of the changes on the reviewer. This leads to the question: How should a code review tool order the parts of a code change to best support the reviewer? We answer this question with a middle-range theory, which we generated inductively in a mixed methods study, based on interviews, an online survey, and existing findings from related areas. Our results indicate that an optimal order is mainly an optimal grouping of the change parts by relatedness. We present our findings as a collection of principles and formalize them as a partial order relation among review orders.