102 lines
3.9 KiB
Java
102 lines
3.9 KiB
Java
package net.datastructures;
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import java.util.Iterator;
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//begin#fragment DFS
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/** Generic DFS traversal of a graph using the template method pattern.
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* Parameterized types:
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* V, the type for the elements stored at vertices
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* E, the type for the elements stored at edges
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* I, the type for the information object passed to the execute method
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* R, the type for the result object returned by the DFS
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*/
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public class DFS<V, E, I, R> {
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protected Graph<V, E> graph; // The graph being traversed
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protected Vertex<V> start; // The start vertex for the DFS
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protected I info; // Information object passed to DFS
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protected R visitResult; // The result of a recursive traversal call
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protected static Object STATUS = new Object(); // The status attribute
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protected static Object VISITED = new Object(); // Visited value
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protected static Object UNVISITED = new Object(); // Unvisited value
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//end#fragment DFS
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//begin#fragment DFS2
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/** Execute a depth first search traversal on graph g, starting
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* from a start vertex s, passing in an information object (in) */
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public R execute(Graph<V, E> g, Vertex<V> s, I in) {
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graph = g;
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start = s;
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info = in;
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for(Vertex<V> v: graph.vertices()) unVisit(v); // mark vertices as unvisited
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for(Edge<E> e: graph.edges()) unVisit(e); // mark edges as unvisited
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setup(); // perform any necessary setup prior to DFS traversal
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return finalResult(dfsTraversal(start));
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}
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/** Recursive template method for a generic DFS traversal. */
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protected R dfsTraversal(Vertex<V> v) {
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initResult();
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if (!isDone())
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startVisit(v);
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if (!isDone()) {
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visit(v);
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for (Edge<E> e: graph.incidentEdges(v)) {
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if (!isVisited(e)) {
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// found an unexplored edge, explore it
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visit(e);
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Vertex<V> w = graph.opposite(v, e);
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if (!isVisited(w)) {
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// w is unexplored, this is a discovery edge
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traverseDiscovery(e, v);
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if (isDone()) break;
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visitResult = dfsTraversal(w); // get result from DFS-tree child
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if (isDone()) break;
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}
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else {
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// w is explored, this is a back edge
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traverseBack(e, v);
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if (isDone()) break;
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}
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}
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}
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}
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if(!isDone())
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finishVisit(v);
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return result();
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}
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//end#fragment DFS2
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//begin#fragment decorations
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/** Mark a position (vertex or edge) as visited. */
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protected void visit(DecorablePosition<?> p) { p.put(STATUS, VISITED); }
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/** Mark a position (vertex or edge) as unvisited. */
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protected void unVisit(DecorablePosition<?> p) { p.put(STATUS, UNVISITED); }
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/** Test if a position (vertex or edge) has been visited. */
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protected boolean isVisited(DecorablePosition<?> p) {
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return (p.get(STATUS) == VISITED);
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}
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//end#fragment decorations
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// Auxiliary methods (all initially null) for specializing a generic DFS
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//begin#fragment auxiliary
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/** Setup method that is called prior to the DFS execution. */
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protected void setup() {}
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/** Initializes result (called first, once per vertex visited). */
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protected void initResult() {}
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/** Called when we encounter a vertex (v). */
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protected void startVisit(Vertex<V> v) {}
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/** Called after we finish the visit for a vertex (v). */
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protected void finishVisit(Vertex<V> v) {}
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/** Called when we traverse a discovery edge (e) from a vertex (from). */
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protected void traverseDiscovery(Edge<E> e, Vertex<V> from) {}
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/** Called when we traverse a back edge (e) from a vertex (from). */
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protected void traverseBack(Edge<E> e, Vertex<V> from) {}
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/** Determines whether the traversal is done early. */
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protected boolean isDone() { return false; /* default value */ }
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/** Returns a result of a visit (if needed). */
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protected R result() { return null; /* default value */ }
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/** Returns the final result of the DFS execute method. */
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protected R finalResult(R r) { return r; /* default value */ }
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//end#fragment auxiliary
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//begin#fragment Tail
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} // end of DFS class
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//end#fragment Tail
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