add some unit tests, and improve the heuristics

Change-Id: I95aa97a87e178ce8d506bd245710d0ae66ad08a4
Reviewed-on: https://gerrit.libreoffice.org/63647Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>
Tested-by: Noel Grandin <noel.grandin@collabora.co.uk>