The contract used in this repository was created based on the ping-pong example contract using the following command: erdpy contract new example-contract --template ping-pong-egld. You can find the original source code here.
The contract was not modified, but only supplemented by exemplary tests in the Given-When-Then (GWT) style. You can run the GWT tests like this: cargo test --test gwt_test
Find out more about the intentions behind this in this blog post.