Package org.geotools.graph.traverse.standard

Class DijkstraIterator

  • All Implemented Interfaces:
    GraphIterator
    Direct Known Subclasses:
    DirectedDijkstraIterator
    public class DijkstraIterator
extends SourceGraphIterator
    Iterates over the nodes of a graph in pattern using Dijkstra's Shortest Path Algorithm. A Dijkstra iteration returns nodes in an order of increasing cost relative to a specified node (the source node of the iteration).

    In a Dijkstra iteration, a weight is associated with each edge and a cost with each node. The iteration operates by maintaining two sets of nodes. The first the set of nodes whose final cost is known, and the second is the set of nodes whose final cost is unknown. Initially, every node except for the source node has a cost of infinity, and resides in the unknown set. The source node has a cost of zero, and is is a member of the known set.

    The iteration operates as follows:
       sn = source node of iteration
   N = set of all nodes
   K = set of nodes with known cost = {sn}
   U = set of nodes with unknown cost = N - K

   cost(sn) = 0
   for each node $un in U
      cost(un) = infinity

   while(|U| > 0)
      for each node n in K
        find a node un in U that relates to n
        if cost(n) + weight(n,un) < cost(un)
          cost(un) = cost(n) + weight(n,un)

      ln = node with least cost in U
      remove ln from U
      add ln to K

      return ln as next node in iteration
    The following is an illustration of the algorithm. Edge weights are labelled in blue and the final node costs are labelled in red.

    The nodes are returned in order of increasing cost which yields the sequence A,C,B,D,E,F,G,H,I.
    Author:
    Justin Deoliveira, Refractions Research Inc, jdeolive@refractions.net

    • Constructor Detail

      • DijkstraIterator

        public DijkstraIterator​(DijkstraIterator.EdgeWeighter weighter)
        Constructs a new Dijkstra iterator which uses the specified EdgeWeighter.
        Parameters:
        weighter - Calculates weights for edges in the graph being iterated over.

      • DijkstraIterator

        public DijkstraIterator​(DijkstraIterator.EdgeWeighter weighter,
                        DijkstraIterator.NodeWeighter nweighter)
        Constructs a new Dijkstra iterator which uses the specified EdgeWeighter and NodeWeighter
        Parameters:
        weighter - Calculates weights for edges in the graph being iterated over.
        nweighter - Calculates weights for nodes in the graph being iterated over.

    • Method Detail

      • init

        public void init​(Graph graph,
                 GraphTraversal traversal)
        Builds internal priority queue to manage node costs.
        Parameters:
        graph - The graph being whose components are being iterated over.
        See Also:
        org.geotools.graph.traverse.GraphIterator#init(Graph)

      • next

        public Graphable next​(GraphTraversal traversal)
        Returns the next node in the priority queue. If the next node coming out of the queue has infinite cost, then the node is not adjacent to any nodes in the set of nodes with known costs. This situation will end the traversal every other node will also have infinite cost. This usually is the result of a disconnected graph.
        Returns:
        The next component in the iteration, or null if iteration is complete.
        See Also:
        org.geotools.graph.traverse.GraphIterator#next()

      • cont

        public void cont​(Graphable current,
                 GraphTraversal traversal)
        Looks for adjacent nodes to the current node which are in the adjacent node and updates costs.
        Parameters:
        current - The current component of the traversal.
        See Also:
        org.geotools.graph.traverse.GraphIterator#cont(Graphable)

      • killBranch

        public void killBranch​(Graphable current,
                       GraphTraversal traversal)
        Kills the branch of the traversal by not updating the cost of any adjacent nodes.
        Parameters:
        current - The current component of the traversal.
        See Also:
        org.geotools.graph.traverse.GraphIterator#killBranch(Graphable)

      • getCost

        public double getCost​(Graphable component)
        Returns the internal cost of a node which has been calculated by the iterator.
        Parameters:
        component - The component whose cost to return.
        Returns:
        The cost associated with the component.

      • getParent

        public Graphable getParent​(Graphable component)
        Returns the last node in the known set to update the node. The iteration operates by nodes in the known set updating the cost of nodes in the unknown set. Each time an update occurs, the known node is set as the parent of the unkown node.
        Parameters:
        component - The node whose parent to return (child)
        Returns:
        The parent, or null if the method is supplied the source of the iteration.