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Retrieving and classifying instances of source code plagiarism


Automatic detection of source code plagiarism is an important research field for both the commercial software industry and within the research community. Existing methods of plagiarism detection primarily involve exhaustive pairwise document comparison, which does not scale well for large software collections. To achieve scalability, we approach the problem from an information retrieval (IR) perspective. We retrieve a ranked list of candidate documents in response to a pseudo-query representation constructed from each source code document in the collection. The challenge in source code document retrieval is that the standard bag-of-words (BoW) representation model for such documents is likely to result in many false positives being retrieved, because of the use of identical programming language specific constructs and keywords. To address this problem, we make use of an abstract syntax tree (AST) representation of the source code documents. While the IR approach is efficient, it is essentially unsupervised in nature. To further improve its effectiveness, we apply a supervised classifier (pre-trained with features extracted from sample plagiarized source code pairs) on the top ranked retrieved documents. We report experiments on the SOCO-2014 dataset comprising 12K Java source files with almost 1M lines of code. Our experiments confirm that the AST based approach produces significantly better retrieval effectiveness than a standard BoW representation, i.e., the AST based approach is able to identify a higher number of plagiarized source code documents at top ranks in response to a query source code document. The supervised classifier, trained on features extracted from sample plagiarized source code pairs, is shown to effectively filter and thus further improve the ranked list of retrieved candidate plagiarized documents.

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  1. Refer to the case of Oracle America, Inc. versus Google, Inc. for an example of possible source code plagiarism:,


  3. The code fragments of Fig. 1 are motivating examples only, and do not form a part of our experimental dataset.

  4. According to Java programming conventions, an identifier name starting with an upper case letter denotes a ‘class’ name, whereas one beginning with a lower case denotes the name of a variable or a method, hence, “Add” and “add” have different semantic meanings.

  5. We refer to these simply as “functions” and do not make a distinction between a function and a class method.










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The authors would like to thank to Enrique Flores, Paolo Rosso and Lidia Moreno for providing us with important details regarding the participating systems in the SOCO 2014 shared task. The first two authors are supported by Science Foundation Ireland (SFI) as a part of the ADAPT Centre at DCU (Grant No.: 13/RC/2106). The work of the last three authors was partially funded by CONACyT under the Thematic Networks program (Language Technologies Thematic Network Project No. 260178, 271622). Additionally, they would also like to thank to UAM Cuajimalpa and SNI-CONACyT for their support.

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Correspondence to Debasis Ganguly.

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Ganguly, D., Jones, G.J.F., Ramírez-de-la-Cruz, A. et al. Retrieving and classifying instances of source code plagiarism. Inf Retrieval J 21, 1–23 (2018).

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  • Source code plagiarism detection
  • Field based indexing and retrieval
  • Lexical, Structural and stylistic features
  • Document representation