GPC stands for General Polygon Clipping , a library written in C to solve polygon operation (difference, intersection, union , and xor). This library is powerfull enough and has been ported to many programming languages. Visit : http://www.cs.man.ac.uk/~toby/gpc/
GPCJS (this library) This is the javascript version, It based on GPCAS (ActionScript3 version of GPC : https://code.google.com/archive/p/gpcas/).
License : License of this javascrip library retain the original sotware:
http://www.cs.manchester.ac.uk/~toby/alan/software/#Licensing