Using Earley Parser for Recognizing Totally Ordered Hierarchical Plans

Abstract

Earley Parser is a top-down parser proposed for context-free grammars and used, for example, in the grammar constraint. Parsing trees of context-free grammars are very close to task decomposition trees used in hierarchical planning, specifically when the actions are totally ordered. This paper suggests using the Earley Parser to recognize totally ordered hierarchical plans. Given a sequence of actions – a prefix of the plan – and a task decomposition model, the plan recognition problem asks which task decomposes to a plan containing the given plan prefix. We will show that the Earley parser significantly increases the speed of plan recognition compared to the existing bottom-up parsing-based plan recognizer. The Earley parser’s performance is also on a par with the planning-based plan recognizer despite not using any planning heuristics.