A constant approximation algorithm for sorting buffers

We consider an algorithmic problem that arises in manufacturing applications. The input is a sequence of objects of various types. The scheduler is fed the objects in the sequence one by one, and is equipped with a finite buffer. The goal of the scheduler/sorter is to maximally reduce the number of type transitions. We give the first polynomial-time constant approximation algorithm for this problem. We prove several lemmas about the combinatorial structure of optimal solutions that may be useful in future research, and we show that the unified algorithm based on the local ratio lemma performs well for a slightly larger class of problems than was apparently previously known.