Unifying Instance Optimality for Static Problems: A Case Study on Sorting a DAG

We study the DAG sorting problem: a partial order $\mathcal{P}$ on $n$ keys is to be discovered by querying as few edges of an input graph $G=(V=[n],E)$ as possible. The graph $G$ only contains edges between ordered pairs, and $G$ is promised to contain the transitive reduction of the DAG describing $\mathcal{P}$. We present two technical and one conceptual result. We first show that DAG sorting is closely related to the fundamental problem of sorting with priced information. \emph{Our first technical result} shows the existence of an algorithm with a $\widetilde{O}(n^{3/4})$ competitive ratio for the $\{0,1,n,\infty\}$ cost version. Thus the $\Omega(n)$ lower bound for maximum cannot extend to sorting, reopening the question of the existence of a $o(n)$-competitive algorithm for the general version. \emph{As our main conceptual contribution}, we define a notion of instance-optimality for the specific problem of DAG sorting, and also unify the existing landscape of instance-optimal algorithms for other static problems existing in literature. This includes problems like sorting [Estivill-Castro and Woods, ACM Comput. Surv. 1992], convex hull [Afshani, Barbay and Chan, JACM 2017], and adaptive joins [Demaine, L\'{o}pez-Ortiz and Munro, SODA 2000]. Our unified notion of instance-optimality is also related to FPT algorithms and algorithms with predictions. We consider the special case of DAG sorting where the input graph is bipartite. \emph{As our second technical result}, we show that a recent algorithm for bichromatic sorting [Goswami and Jacob, ITCS 2024] gives an algorithm for bipartite DAG sorting which is instance-optimal to a factor $O(\log^{3}n)$. This generalizes the famous nuts-and-bolts problem to the setting where the number of nuts and bolts are different, and there is no promise of a matching between them, and the resulting order might not be total.