Building Large Compressed PDBs for the Sliding Tile Puzzle

The performance of heuristic search algorithms depends crucially on the effectiveness of the heuristic. A pattern database (PDB) is a powerful heuristic in the form of a pre-computed lookup table. Larger PDBs provide better bounds and thus allow more cut-offs in the search process. We computed 9-9-6, 9-8-7, and 8-8-8 PDBs for the 24-puzzle that are three orders of magnitude larger (up to 1.4 TB) than the 6-6-6-6 PDB. This was possible by performing a parallel breadth-first search in the compressed pattern space. Our experiments indicate an average 8-fold improvement of the 9-9-6 PDB over the 6-6-6-6 PDB on the 24-puzzle. Combining several large PDBs yields a 13-fold improvement.