Perfect hashing as an almost perfect subtype test

Abstract

Subtype tests are an important issue in the implementation of object-oriented programming languages. Many techniques have been proposed, but none of them perfectly fulfills the five requirements that we have identified: constant-time, linear-space, multiple inheritance, dynamic loading and inlining. In this article, we propose a subtyping test implementation that involves a combination of usual hashtables and Cohen's display, which is a well-known technique for single inheritance hierarchies. This novel approach is based on perfect hashing, that is, an optimized and truly constant-time variant of hashing that applies to immutable hashtables. We show that the resulting technique closely meets all five requirements. Furthermore, in the framework of JAVA-like languagescc-characterized by single inheritance of classes and multiple subtyping of interfaces-perfect hashing also applies to method invocation when the receiver is typed by an interface. The proposed technique is compared to some alternatives, including the proposal by Palacz and Vitek [2003]. Time-efficiency is assessed at the cycle level in the framework of Driesen's pseudo-code and the linear-space criterion is validated by statistical simulation on benchmarks consisting of large-scale class hierarchies. © 2008 ACM.