You should check out QuickCheck for catching edge cases you did not think of. The idea behind QuickCheck is simple--you specify invariants in your code (called "properties") and the framework tests them with random inputs.

This tool is very widely used in Haskell, but it's been ported to a whole bunch of other languages and could make your testing more thorough. In Haskell it's also easy to use and more fun than normal tests, but I don't know what it would be like in a different language.