From the github repo it looks like it is using directly "google custom search engine", to index a limited set of websites https://cse.google.com https://cse.goo

From the github repo it looks like it is using directly "google custom search engine", to index a limited set of websites https://cse.google.com https://cse.google.com (recognizing the css classes here https://github.com/txthinking/soso/blob/master/main.js#L41 https://github.com/txthinking/soso/blob/master/main.js#L41)