Yeah, the underground belts in the first and fourth row look odd, I currently don’t see a reason why the couldn’t be replaced by simple belts, which would be somewhat cheaper. Maybe that’s an additional UPS optimization applied in that design?
I agree that using underground belts makes harder than it should to understand the example. I replaced it with simple belts. I re-calculated the design with the latest version of the code that has an optimization function that favors normal belts to underground ones https://github.com/gianluca-venturini/factorio-tools/blob/ma...
But I can't help to feel that the following paragraph should be removed, since it really seems to be a bit confused about the concepts of both Np-complete and NP-hard.
> Finding a feasible solution is NP-hard because we can't do any better than start placing the components on a 2D grid and check if the properties are respected. Furthermore, finding the optimal solution (i.e., minimum number of components) is NP-complete because it will require enumerating all the possible solutions in order to find the best one.