How to Directed Graph of 2p2 Hand Evaluator

I'm referring to the famous 2+2 hand evaluator : https://github.com/tangentforks/TwoPlusTwoHandEvaluator/blob/master/generate_table.cpp

I wanna know what method/algorithm is used to generate the lookup table aka the giant directed graph. Can someone give me a small example and illustrate how the lookup table is generated?