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Wecode integrates with Moss (Measure of Software Similarity) to detect code plagiarism and measure similarity between submissions.

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What is Moss?

Moss is a service developed by Stanford University that compares program source code files to identify similar code segments. It’s language-independent and can detect similarities even when code has been modified.

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Configuration

Before using Moss, you need to configure your Moss User ID:
  1. Navigate to the Moss page for any assignment: /moss/{assignment_id}
  2. Enter your Moss User ID in the configuration form
  3. Click “Update” to save the configuration
You need to register for a free Moss account to obtain a User ID. Visit Stanford Moss to request an account.
The configuration updates the moss script with your user ID: 
$moss_original = trim( file_get_contents(rtrim(Setting::get('tester_path'), '/').'/moss_original') );
$moss_path = rtrim(Setting::get('tester_path'), '/').'/moss';
file_put_contents($moss_path, str_replace('MOSS_USER_ID', $userid, $moss_original));

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Running Detection

To run plagiarism detection on an assignment:
  1. Go to /moss/{assignment_id}
  2. Click the “Detect” button
  3. Wait for Moss to process the submissions

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Detection Process

The detection process:
moss_controller.php:76
private function _detect($assignment_id = FALSE)
{
    if ($assignment_id === FALSE)
        abort(404);

    $lang = Language::all();

    $assignments_path = rtrim(Setting::get('assignments_root'), '/');
    $tester_path = rtrim(Setting::get('tester_path'), '/');
    shell_exec("chmod +x {$tester_path}/moss");
    $items = Submission::get_final_submissions($assignment_id);
    $groups = array();
    foreach ($items as $item) {
        if (!isset($groups[$item->problem_id]))
            $groups[$item->problem_id] = array($item);
        else
            array_push($groups[$item->problem_id], $item);
    }
    foreach ($groups as $problem_id => $group) {
        $list = '';
        $assignment_path = $assignments_path."/assignment_{$assignment_id}";
        foreach ($group as $item){
            $list .= "problem_{$problem_id}/{$item->username}/{$item->file_name}." .(string)Language::find($item->language_id)->extension . " ";
        }

        shell_exec("list='$list'; cd $assignment_path; $tester_path/moss \$list > problem_{$problem_id}/moss_link.txt  2>&1 &");
        shell_exec("cd $assignment_path/problem_{$problem_id}; touch moss_running");

    }
    
    Assignment::where('id', $assignment_id)->update(['moss_update' => date('Y-m-d H:i:s')]);
}

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What Happens During Detection

  1. Grouping: Submissions are grouped by problem
  2. File Collection: Final submissions for each user are collected
  3. Moss Execution: The Moss script is run on all submissions for each problem
  4. Background Processing: Detection runs in the background using shell execution
  5. Status Tracking: A moss_running file is created to indicate processing

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Viewing Results

After detection completes:

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Status Indicators

  • Moss Link: If detection is complete, a URL to the Moss results page is displayed
  • Processing: “submission submitted to moss, awaiting response, please be patience”
  • Not Run: No link displayed if detection hasn’t been run

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Result Display Logic

moss_controller.php:37
foreach ($assignment->problems as $key => $problem){
    $moss_problems[$problem->id]['problem'] = $problem;
    $path = Submission::get_path('', $assignment_id, $problem->id) .'/' ;
    if (file_exists($path . "moss_link.txt") && file_get_contents($path . "moss_link.txt") != ''){
        $moss_problems[$problem->id]['moss'] = shell_exec("tail -n1 $path/moss_link.txt");
        shell_exec("rm $path/moss_running");
    } else if (file_exists($path . "moss_running")){
        $moss_problems[$problem->id]['moss'] = "submission submitted to moss, awaiting respone, please be patience";
    } else {
        $moss_problems[$problem->id]['moss'] = NULL;
    }
}

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Routes

Available Moss routes:
  • GET /moss/{id} - View Moss page for an assignment
  • POST /moss/update/{id} - Update Moss configuration
  • POST /moss/detect/{id} - Run plagiarism detection

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Permissions

Only admins and head instructors can access Moss features. Other users will receive a 403 Forbidden error.

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Best Practices

  1. Run After Deadline: Run Moss detection after the assignment deadline when all submissions are in
  2. Per-Problem Analysis: Results are generated separately for each problem in an assignment
  3. Final Submissions Only: Only final (best-scoring) submissions are analyzed
  4. Review Results: Always manually review Moss results - similar code doesn’t always mean plagiarism

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Troubleshooting

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Moss Not Responding

If the status shows “awaiting response” for a long time:
  1. Check if the moss_running file exists in the problem directory
  2. Check if the Moss service is available
  3. Verify your Moss User ID is correct
  4. Check server logs for errors

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Invalid Moss User ID

If detection fails:
  1. Verify you’ve registered for a Moss account
  2. Check that your User ID is correctly entered
  3. Ensure the moss script has execute permissions

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