CheatHit: plagiarism detection tool for programming assignments

Installation

Install Python 3.9 or higher, then run:

pip install cheathit

Usage

Running

Run CheatHit as follows:

cheathit /submission/directory

Or with parameters:

cheathit /submission/directory --path=group/student/problem/attempt --min-ngram=3 --max-ngram=10 --min-ratio=0.5 --max-clique=4

Or, if you want to save results to a file:

cheathit /submission/directory > /path/to/file

Parameters

--path

Specify the structure of the submission directory with this parameter. Use the student, group, problem, and attempt sections separated with slashes, e.g., group/group/student/problem/problem/attempt. Each subsequent section gets CheatHit one level down the directory tree; the last level must be a file containing the submission.

• student corresponds to the set of programs submitted by an individual student;
• group corresponds to a group of students such that cheating is likely to take place within such a group (e.g., a school class);
• problem corresponds to a separate task shared by the students;
• attempt corresponds to a separate submission of a student.

The student section is required (i.e., there should be at least one of these in --path); the other three sections are optional. If the same section appears in --path multiple times, CheatHit will simply concatenate its values to obtain the “true” representation of the section.

The default value of --path is student, which is suitable for cases when there is a single directory with many files, one file per student.

--min-ngram

Minimum ngram size (number of consecutive tokens) to analyze across the submissions.

The default value of --min-ngram is 1.

--max-ngram

Maximum ngram size (number of consecutive tokens) to analyze across the submissions.

The default value of --max-ngram is 20.

--min-ratio

Minimum ratio of the number of tokens shared by two submissions to the number of tokens in the longer of the submissions required so that the pair is included in the report.

The default value of --min-ratio is 0.2.

--max-clique

If an ngram (a sequence of tokens) occurs in submissions of more than --max-clique students (or in submissions of students from more than --max-clique groups), it is not considered distinctive.

The default value of --max-clique is:

• 2 if students are assigned groups (--path includes group),
• 5 otherwise.

Tokenization

CheatHit tokenizes source code into alphanumeric words (which can also contain underscores) and non-alphanumeric characters. Whitespace, semicolons, and commas are ignored. Two special markers, <START> and <END>, are added to the beginning and end of a token sequence. Hence,

CheatHit supports C++, Python v2 & v3, and _even_ VB.NET; awesome!

would be tokenized as

['<START>', 'CheatHit', 'supports', 'C', '+', '+', 'Python', 'v2', '&', 'v3', 'and', '_even_', 'VB', '.', 'NET', 'awesome', '!', '<END>']

Results

For each pair of students CheatHit will report how much code is shared between the students while adjusting for how distinctive the shared code is. See the Parameters and Tokenization sections above for an insight into what CheatHit considers distinctive.