Two similar code segments, or clones, form a clone pair within a software system. The changes to the clones over time create a clone evolution history. In this work we study late propagation, a specific pattern of clone evolution. In late propagation, one clone in the clone pair is modified, causing the clone pair to become inconsistent. The code segments are then re-synchronized in a later revision.Existing work has established late propagation as a clone evolution pattern, and suggested that the pattern is related to a high number of faults. In this study we examine the characteristics of late propagation in two long-lived software systems using the Simian and CCFinder clone detection tools.We define 8 types of late propagation and compare them to other forms of clone evolution. Our results not only verify that late propagation is more harmful to software systems, but also establish that some specific cases of late propagations are more harmful than others. Specifically, two cases are most risky: (1) when a clone experiences inconsistent changes and then a re synchronizing change without any modification to the other clone in a clone pair; and (2) when two clones undergo an inconsistent modification followed by a re-synchronizing change that modifies both the clones in a clone pair.

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SUMMARYTwo similar code segments, or clones, form a clone pair within a software system. The changes to the clones over time create a clone evolution history. In this work, we study late propagation, a specific pattern of clone evolution. In late propagation, one clone in a clone pair is modified, causing the clone pair to diverge. The code segments are then reconciled in a later commit. Existing work has established late propagation as a clone evolution pattern and suggested that the pattern is related to a high number of faults. In this study, we examine the characteristics of late propagation in three long-lived software systems using the SIMIAN ( Simon Harris, Victoria, Australia, http://www.harukizaemon.com/simian), CCFINDER, and NICAD (Software Technology Laboratory, Queen's University, Kingston, ON, Canada) clone detection tools. We define eight types of late propagation and compare them to other forms of clone evolution. Our results not only verify that late propagation is more harmful to software systems but also establish that some specific types of late propagations are more harmful than others. Specifically, two types are most risky: (1) when a clone experiences diverging changes and then a reconciling change without any modification to the other clone in a clone pair; and (2) when two clones undergo a diverging modification followed by a reconciling change that modifies both the clones in a clone pair. We also observe that the reconciliation in the former case is more prone to faults than in the latter case. We determine that the size of the clones experiencing late propagation has an effect on the fault proneness of specific types of late propagation genealogies. Lastly, we cannot report a correlation between the delay of the propagation of changes and its faults, as the fault proneness of each delay period is system dependent.

Existing clone tracking tools have limited support for sharing clone information between developers in a large scale system. Developers are not notified when new clones are introduced by other developers or when existing clones are modified. We propose a client-server architecture that centrally detects and maintains clone information for an entire software system stored in a version control system. Clients retrieve a list of clones relevant to the code they are working on from the server. Whenever an update is committed to the version control system, the server detects and incrementally updates clone information. We propose techniques to improve the speed of the incremental clone detection. In order to reduce the number of comparisons required for clone detection, we select representative clones from the existing clone list. We build a string-based technique to compare the newly committed code with the representative clones and to update the clone list. In a case study, we show that our approach significantly reduces the clone detection time, while supporting clone detection across the entire software system.

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