Symmetry reduction in CSP model checking

We present an extension of FDR, the model checker for the process algebra CSP, that exploits symmetry to reduce the size of the state space searched. We define what it means for a process to be symmetric with respect to a group of permutations on the transition labels. We factor the state space of the search by symmetry equivalence, mapping each state to a representative of its equivalence class, thereby considering all symmetric states together. We prove a powerful syntactic result, identifying conditions under which a process will be symmetric in a particular type. We show how to implement such a search using the powerful technique of supercombinators used in the implementation of FDR: we identify conditions on a supercombinator for it to be symmetric and explain how to apply a permutation to a state. Finally, we present a novel efficient technique for calculating representatives of equivalence classes, which normally finds unique representatives; our experiments suggest that this technique typically works faster than other techniques and in particular scales better.