Caching Search States in Permutation Problems

When the search for a solution to a constraint satisfaction problem backtracks, it is not usually worthwhile to remember the assignment that failed, because the same assignment will not occur again. However, we show that for some problems recording assignments is useful, because other assignments can lead to the same state of the search. We demonstrate this in two classes of permutation problem, a satisfaction problem and an optimization problem. Caching states visited has proved effective in reducing both search effort and run-time for difficult instances of each class, and the space requirements are manageable.