Orienting rewrite rules with the Knuth–Bendix order

We consider two decision problems related to the Knuth–Bendix order (KBO). The first problem is orientability: given a system of rewrite rules R, does there exist an instance of KBO which orients every ground instance of every rewrite rule in R. The second problem is whether a given instance of KBO orients every ground instance of a given rewrite rule. This problem can also be reformulated as the problem of solving a single ordering constraint for the KBO. We prove that both problems can be solved in the time polynomial in the size of the input. The polynomial-time algorithm for orientability builds upon an algorithm for solving systems of homogeneous linear inequalities over integers. We show that the orientability problem is P-complete. The polynomial-time algorithm for solving a single ordering constraint does not need to solve systems of linear inequalities and can be run in time O(n²). Also we show that if a system is orientable using a real-valued instance of KBO, then it is also orientable using an integer-valued instance of KBO. Therefore, all our results hold both for the integer-valued and the real-valued KBO.