A question came up a last fall on the OR-Tools forum about how to enforce a chained pickup and delivery problem. The standard pickup and delivery problem is to pick something up from one place, and then deliver that thing to some other place. This question asked about an interesting and fairly common variation of the standard PDP: a chained sequence of pickups and deliveries.

A series of questions on the Google OR Tools mailing list recently led me to question what was really going on with the use of disjunctions. The original poster was kind enough to share his original code with me, but it was in C#. So I took the time to convert the code to python and test out various things. The result surprised me a bit, but I think I understand better how disjunctions work. This post will first present the original problem, then my solution, and then finally explore the interesting issues that came up while using disjunctions.

For many applications of Google’s OR Tools routing library, I find a counting dimension to be quite useful. I’ve talked about it before, but this post focused just on the counting dimension.