Compressed Tree Canonization

Abstract: Straight-line (linear) context-free tree (SLT) grammars have been used to compactly represent ordered trees. It is well known that equivalence of SLT grammars is decidable in polynomial time. Here we extend this result and show that isomorphism of unordered trees given as SLT grammars is decidable in polynomial time. The proof constructs a compressed version of the canonical form of the tree represented by the input SLT grammar. The result is generalized to unrooted trees by "re-rooting" the compressed trees in polynomial time. We further show that bisimulation equivalence of unrooted unordered trees represented by SLT grammars is decidable in polynomial time. For non-linear SLT grammars which can have double-exponential compression ratios, we prove that unordered isomorphism is PSPACE-hard and in EXPTIME. The same complexity bounds are shown for bisimulation equivalence.