如何实现非时间顺序回溯

Question

I'm working on a CDCL SAT-Solver. I don't know how to implement non-chronological backtracking. Is this even possible with recursion or is it only possible in a iterative approach.

Actually what i have done jet is implemented a DPLL Solver which works with recursion. The great differnece from DPLL and CDCL ist that the backracking in the tree is not chronological. Is it even possible to implement something like this with recursion. In my opionion i have two choices in the node of the binary-decision-tree if one of to path leads ia conlict:

1. I try the other path -> but then it would be the same like the DPLL, means a chronological backtracking
2. I return: But then i will never come back to this node.

So am i missing here something. Could it be that the only option is to implement it iterativly?

Answer 1

Non-chronological backtracking (or backjumping as it is usually called) can be implemented in solvers that use recursion for the variable assignments. In languages that support non-local gotos, you would typically use that method. For example in the C language you would use setjmp() to record a point in the stack and longjmp() to backjump to that point. C# has try-catch blocks, Lispy languages might have catch-throw, and so on.

If the language doesn't support non-local goto, then you can implement a substitute in your code. Instead of dpll() returning FALSE, have it return a tuple containing FALSE and the number of levels that need to be backtracked. Upstream callers decrement the counter in the tuple and return it until zero is returned.

Answer 2

You can modify this to get backjumping.

private Assignment recursiveBackJumpingSearch(CSP csp, Assignment assignment) {
    Assignment result = null;
    if (assignment.isComplete(csp.getVariables())) {
        result = assignment;
    }
    else {
        Variable var= selectUnassignedVariable(assignment, csp);

        for (Object value : orderDomainValues(var, assignment, csp)) {
            assignment.setAssignment(var, value);
            fireStateChanged(assignment, csp);
            if (assignment.isConsistent(csp.getConstraints(var))) {

                    result=recursiveBackJumpingSearch(csp, assignment);
                    if (result != null) {
                        break;
                    }
                    if (result == null)
                        numberOfBacktrack++;

            }
            assignment.removeAssignment(var);
        }
    }
    return result;
}