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cregit: Token-level blame information in git version control repositories

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Abstract

The blame feature of version control systems is widely used—both by practitioners and researchers—to determine who has last modified a given line of code, and the commit where this contribution was made. The main disadvantage of blame is that, when a line is modified several times, it only shows the last commit that modified it—occluding previous changes to other areas of the same line. In this paper, we developed a method to increase the granularity of blame in git: instead of tracking lines of code, this method is capable of tracking tokens in source code. We evaluate its effectiveness with an empirical study in which we compare the accuracy of blame in git (per line) with our proposed blame-per-token method. We demonstrate that, in 5 large open source systems, blame-per-token is capable of properly identifying the commit that introduced a token with an accuracy between 94.5% and 99.2%, while blame-per-line can only achieve an accuracy between 75% and 91% (with a margin of error of +/-5% and a confidence interval of 95%). We also classify the reasons why either blame method fails, highlighting each method’s weaknesses. The blame-per-token method has been implemented in an open source tool called cregit, which is currently in use by the Linux Foundation to identify the persons who have contributed to the source code of the Linux kernel.

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Notes

  1. https://rtyley.github.io/bfg-repo-cleaner/

  2. cloc is an OSS tool to count lines of code and comments https://github.com/AlDanial/cloc

  3. For this reason, other “diff” algorithms have been proposed, such as “patient diff” (originally implemented in the version control system Bazaar, and also implemented in git). Patient-diff tries to maximize the number of unique unchanged lines by repeatedly running Myers’ diff on sections of the input). For a discussion of its benefits, we refer elsewhere (Schindelin 2009).

  4. https://github.com/git/git/commit/c9018b0305a56436c85b292edbeacff04b0ebb5d

  5. http://turingmachine.org/2018/cregit

  6. http://github.com/cregit/evaluation

  7. https://github.com/GumTreeDiff/gumtree

  8. https://blogs.s-osg.org/made-change-using-cregit-debugging/

  9. http://github.com/cregit

  10. http://cregit.linuxsources.org

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Authors and Affiliations

  1. University of Victoria, Victoria, BC, V8P 5C2, Canada

    Daniel M. German

  2. Polytechnique Montréal, Montreal, QC, H3T 1J4, Canada

    Bram Adams

  3. Linux Foundation, San Francisco, CA, 94129, USA

    Kate Stewart

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  1. Daniel M. German
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Correspondence to Daniel M. German.

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Communicated by: Romain Robbes

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German, D.M., Adams, B. & Stewart, K. cregit: Token-level blame information in git version control repositories. Empir Software Eng 24, 2725–2763 (2019). https://doi.org/10.1007/s10664-019-09704-x

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