You find a reference to the algorithm in the comments:
% The algorithm is similar to that described in the following reference.
%
% @article{514556,
% author = {Kai Hormann and Alexander Agathos},
% title = {The point in polygon problem for arbitrary polygons},
% journal = {Comput. Geom. Theory Appl.},
% volume = {20},
% number = {3},
% year = {2001},
% issn = {0925-7721},
% pages = {131--144},
% doi = {http://dx.doi.org/10.1016/S0925-7721(01)00012-8},
% publisher = {Elsevier Science Publishers B. V.},
% address = {Amsterdam, The Netherlands, The Netherlands},
