segments intersection

Try Google before asking next time okay?

Here is one that came up on Google with source:

http://www.lems.brown.edu/~wq/projects/cs252.html

Anyway the algorithm is pretty simple so the time to integrate the code into your own code will proably take longer that just writing it yourself.

If you need more of this kind of things I recommend reading De Berg: Computational Geometry. Springer-Verlag. It is a really good book.