Watermark Extraction Fails If Watermark Fits Exactly One Time

[mhellmeier]

🐞 Bug Report

Describe the Bug

If a watermark fits exactly one time into the cover text (without a seperator character at the end), the watermarker library is unable to extract the watermark. In such a case, the Result type is still successful (result.isSuccess is true), but the extracted watermark itself is empty (result.value is empty).

To Reproduce

Steps to reproduce the behavior:

1. Use "a" as watermark and "a a a a a a a a a a" as cover text
2. Watermark the cover text with default configuration (RawTrendmark)
3. Try to extract
4. --> The extraction will succeed with an empty watermark inside it

Expected Behavior

The value of the result should not be empty. It should contain the watermark.

System Information

• Component: watermarker & webinterface
• Runtime: Ubuntu
• Version: latest version on main

Additional Context

The initial problem was identified by @eschrewe in a review of another PR (see #139 (comment))

Further, I created a Unit test to demonstrate the problem: 78e6691