public List <Action> search(Problem p)
{
assert(p is BidirectionalProblem);
searchOutcome = SearchOutcome.PATH_NOT_FOUND;
clearInstrumentation();
Problem op = ((BidirectionalProblem)p).getOriginalProblem();
Problem rp = ((BidirectionalProblem)p).getReverseProblem();
CachedStateQueue <Node> opFrontier = new CachedStateQueue <Node>();
CachedStateQueue <Node> rpFrontier = new CachedStateQueue <Node>();
GraphSearch ogs = new GraphSearch();
GraphSearch rgs = new GraphSearch();
// Ensure the instrumentation for these
// are cleared down as their values
// are used in calculating the overall
// bidirectional metrics.
ogs.clearInstrumentation();
rgs.clearInstrumentation();
Node opNode = new Node(op.getInitialState());
Node rpNode = new Node(rp.getInitialState());
opFrontier.insert(opNode);
rpFrontier.insert(rpNode);
setQueueSize(opFrontier.Count + rpFrontier.Count);
setNodesExpanded(ogs.getNodesExpanded() + rgs.getNodesExpanded());
while (!(opFrontier.isEmpty() && rpFrontier.isEmpty()))
{
// Determine the nodes to work with and expand their fringes
// in preparation for testing whether or not the two
// searches meet or one or other is at the GOAL.
if (!opFrontier.isEmpty())
{
opNode = opFrontier.pop();
opFrontier.AddRange(ogs.getResultingNodesToAddToFrontier(opNode,
op));
}
else
{
opNode = null;
}
if (!rpFrontier.isEmpty())
{
rpNode = rpFrontier.pop();
rpFrontier.AddRange(rgs.getResultingNodesToAddToFrontier(rpNode,
rp));
}
else
{
rpNode = null;
}
setQueueSize(opFrontier.Count + rpFrontier.Count);
setNodesExpanded(ogs.getNodesExpanded() + rgs.getNodesExpanded());
//
// First Check if either frontier contains the other's state
if (null != opNode && null != rpNode)
{
Node popNode = null;
Node prpNode = null;
if (opFrontier.containsNodeBasedOn(rpNode.getState()))
{
popNode = opFrontier.getNodeBasedOn(rpNode.getState());
prpNode = rpNode;
}
else if (rpFrontier.containsNodeBasedOn(opNode.getState()))
{
popNode = opNode;
prpNode = rpFrontier.getNodeBasedOn(opNode.getState());
// Need to also check whether or not the nodes that
// have been taken off the frontier actually represent the
// same state, otherwise there are instances whereby
// the searches can pass each other by
}
else if (opNode.getState().Equals(rpNode.getState()))
{
popNode = opNode;
prpNode = rpNode;
}
if (null != popNode && null != prpNode)
{
List <Action> actions = retrieveActions(op, rp, popNode,
prpNode);
// It may be the case that it is not in fact possible to
// traverse from the original node to the goal node based on
// the reverse path (i.e. unidirectional links: e.g.
// InitialState(A)<->C<-Goal(B) )
if (null != actions)
{
return(actions);
}
}
}
//
// Check if the original problem is at the GOAL state
if (null != opNode && SearchUtils.isGoalState(op, opNode))
{
// No need to check return value for null here
// as an action path discovered from the goal
// is guaranteed to exist
return(retrieveActions(op, rp, opNode, null));
}
//
// Check if the reverse problem is at the GOAL state
if (null != rpNode && SearchUtils.isGoalState(rp, rpNode))
{
List <Action> actions = retrieveActions(op, rp, null, rpNode);
// It may be the case that it is not in fact possible to
// traverse from the original node to the goal node based on
// the reverse path (i.e. unidirectional links: e.g.
// InitialState(A)<-Goal(B) )
if (null != actions)
{
return(actions);
}
}
}
// Empty List can indicate already at Goal
// or unable to find valid set of actions
return(new List <Action>());
}