1) Yes the source code is available. User:Eran has posted it here
https://github.com/valhallasw/plagiabot
2) This bot ONLY works on new edits within a couple of hours of them
occurring. This reducing the number of false positives. It DOES NOT look at
old edits.
3) This requires human follow up and common sense. One needs to make sure
that a) the source is not PD/CCBYSA b) that it is not wiki text that has
been moved around c) that the authors of both are not the same, etc
4) True positive rate is around 50% which is from my perspective good /
useful. This bot has flagged a lot of copyright issues would have been
missed otherwise.
--
James Heilman
MD, CCFP-EM, Wikipedian
The Wikipedia Open Textbook of Medicine
www.opentextbookofmedicine.com