A (5/3 + ε)-approximation for unsplittable flow on a path: placing small tasks into boxes

In the unsplittable flow on a path problem (UFP) we are given a path with edge capacities and a collection of tasks. Each task is characterized by a subpath, a profit, and a demand. Our goal is to compute a maximum profit subset of tasks such that, for each edge e, the total demand of selected tasks that use e does not exceed the capacity of e. The current best polynomial-time approximation factor for this problem is 2+є for any constant є>0 [Anagostopoulos et al.-SODA 2014]. This is the best known factor even in the case of uniform edge capacities [Călinescu et al.-IPCO 2002, TALG 2011]. These results, likewise most prior work, are based on a partition of tasks into large and small depending on their ratio of demand to capacity over their respective edges: these algorithms invoke (1+є)-approximations for large and small tasks separately. The known techniques do not seem to be able to combine a big fraction of large and small tasks together (apart from some special cases and quasi-polynomial-time algorithms). The main contribution of this paper is to overcome this critical barrier. Namely, we present a polynomial-time algorithm that obtains roughly all profit from the optimal large tasks plus one third of the profit from the optimal small tasks. In combination with known results, this implies a polynomial-time (5/3+є)-approximation algorithm for UFP. Our algorithm is based on two main ingredients. First, we prove that there exist certain sub-optimal solutions where, roughly speaking, small tasks are packed into boxes. To prove that such solutions can yield high profit we introduce a horizontal slicing lemma which yields a novel geometric interpretation of certain solutions. The resulting boxed structure has polynomial complexity, hence cannot be guessed directly. Therefore, our second contribution is a dynamic program that guesses this structure (plus a packing of large and small tasks) on the fly, while losing at most one third of the profit of the remaining small tasks.