The code seems to me unclear ,unelegant and slow. Code of the book on the web is unnecessarily complex and hard to understand.This is a book of simplistic recipes without formal explanation. So is not usable to learn or understand. Obviously because it was not written in C originally and translation to C was made in a rush.If you are learning buy Statistical Mechanics by Rowley , Understanding Molecular Simulation by Frenkel or Computer Simulation of liquids by Allen.