tom | 41c3fcc | 2014-08-30 17:57:15 -0700 | [diff] [blame] | 1 | package org.onlab.graph; |
| 2 | |
| 3 | import java.util.HashMap; |
| 4 | import java.util.HashSet; |
| 5 | import java.util.Map; |
| 6 | import java.util.Set; |
| 7 | import java.util.Stack; |
| 8 | |
| 9 | /** |
| 10 | * DFS graph search algorithm implemented via iteration rather than recursion. |
| 11 | */ |
| 12 | public class DepthFirstSearch<V extends Vertex, E extends Edge<V>> |
| 13 | extends AbstractGraphPathSearch<V, E> { |
| 14 | |
| 15 | /** |
| 16 | * Graph edge types as classified by the DFS algorithm. |
| 17 | */ |
| 18 | public static enum EdgeType { |
| 19 | TREE_EDGE, FORWARD_EDGE, BACK_EDGE, CROSS_EDGE |
| 20 | } |
| 21 | |
| 22 | @Override |
| 23 | public SpanningTreeResult search(Graph<V, E> graph, V src, V dst, |
| 24 | EdgeWeight<V, E> weight) { |
| 25 | checkArguments(graph, src, dst); |
| 26 | |
| 27 | // Prepare the search result. |
| 28 | SpanningTreeResult result = new SpanningTreeResult(src, dst); |
| 29 | |
| 30 | // The source vertex has cost 0, of course. |
| 31 | result.updateVertex(src, null, 0.0, true); |
| 32 | |
| 33 | // Track finished vertexes and keep a stack of vertexes that have been |
| 34 | // started; start this stack with the source on it. |
| 35 | Set<V> finished = new HashSet<>(); |
| 36 | Stack<V> stack = new Stack<>(); |
| 37 | stack.push(src); |
| 38 | |
| 39 | while (!stack.isEmpty()) { |
| 40 | V vertex = stack.peek(); |
| 41 | if (vertex.equals(dst)) { |
| 42 | // If we have reached our destination, bail. |
| 43 | break; |
| 44 | } |
| 45 | |
| 46 | double cost = result.cost(vertex); |
| 47 | boolean tangent = false; |
| 48 | |
| 49 | // Visit all egress edges of the current vertex. |
| 50 | for (E edge : graph.getEdgesFrom(vertex)) { |
| 51 | // If we have seen the edge already, skip it. |
| 52 | if (result.isEdgeMarked(edge)) { |
| 53 | continue; |
| 54 | } |
| 55 | |
| 56 | // Examine the destination of the current edge. |
| 57 | V nextVertex = edge.dst(); |
| 58 | if (!result.hasCost(nextVertex)) { |
| 59 | // If this vertex have not finished this vertex yet, |
| 60 | // not started it, then start it as a tree-edge. |
| 61 | result.markEdge(edge, EdgeType.TREE_EDGE); |
| 62 | double newCost = cost + (weight == null ? 1.0 : weight.weight(edge)); |
| 63 | result.updateVertex(nextVertex, edge, newCost, true); |
| 64 | stack.push(nextVertex); |
| 65 | tangent = true; |
| 66 | break; |
| 67 | |
| 68 | } else if (!finished.contains(nextVertex)) { |
| 69 | // We started the vertex, but did not yet finish it, so |
| 70 | // it must be a back-edge. |
| 71 | result.markEdge(edge, EdgeType.BACK_EDGE); |
| 72 | } else { |
| 73 | // The target has been finished already, so what we have |
| 74 | // here is either a forward-edge or a cross-edge. |
| 75 | result.markEdge(edge, isForwardEdge(result, edge) ? |
| 76 | EdgeType.FORWARD_EDGE : EdgeType.CROSS_EDGE); |
| 77 | } |
| 78 | } |
| 79 | |
| 80 | // If we have not been sent on a tangent search and reached the |
| 81 | // end of the current scan normally, mark the node as finished |
| 82 | // and pop it off the vertex stack. |
| 83 | if (!tangent) { |
| 84 | finished.add(vertex); |
| 85 | stack.pop(); |
| 86 | } |
| 87 | } |
| 88 | |
| 89 | // Finally, but the paths on the search result and return. |
| 90 | result.buildPaths(); |
| 91 | return result; |
| 92 | } |
| 93 | |
| 94 | /** |
| 95 | * Determines whether the specified edge is a forward edge using the |
| 96 | * accumulated set of parent edges for each vertex. |
| 97 | * |
| 98 | * @param result search result |
| 99 | * @param edge edge to be classified |
| 100 | * @return true if the edge is a forward edge |
| 101 | */ |
| 102 | protected boolean isForwardEdge(DefaultResult result, E edge) { |
| 103 | // Follow the parent edges until we hit the edge source vertex |
| 104 | V target = edge.src(); |
| 105 | V vertex = edge.dst(); |
| 106 | Set<E> parentEdges; |
| 107 | while ((parentEdges = result.parents.get(vertex)) != null) { |
| 108 | for (E parentEdge : parentEdges) { |
| 109 | vertex = parentEdge.src(); |
| 110 | if (vertex.equals(target)) { |
| 111 | return true; |
| 112 | } |
| 113 | } |
| 114 | } |
| 115 | return false; |
| 116 | } |
| 117 | |
| 118 | /** |
| 119 | * Graph search result which includes edge classification for building |
| 120 | * a spanning tree. |
| 121 | */ |
| 122 | public class SpanningTreeResult extends DefaultResult { |
| 123 | |
| 124 | protected final Map<E, EdgeType> edges = new HashMap<>(); |
| 125 | |
| 126 | /** |
| 127 | * Creates a new spanning tree result. |
| 128 | * |
| 129 | * @param src search source |
| 130 | * @param dst optional search destination |
| 131 | */ |
| 132 | public SpanningTreeResult(V src, V dst) { |
| 133 | super(src, dst); |
| 134 | } |
| 135 | |
| 136 | /** |
| 137 | * Returns the map of edge type. |
| 138 | * |
| 139 | * @return edge to edge type bindings |
| 140 | */ |
| 141 | public Map<E, EdgeType> edges() { |
| 142 | return edges; |
| 143 | } |
| 144 | |
| 145 | /** |
| 146 | * Indicates whether or not the edge has been marked with type. |
| 147 | * |
| 148 | * @param edge edge to test |
| 149 | * @return true if the edge has been marked already |
| 150 | */ |
| 151 | boolean isEdgeMarked(E edge) { |
| 152 | return edges.containsKey(edge); |
| 153 | } |
| 154 | |
| 155 | /** |
| 156 | * Marks the edge with the specified type. |
| 157 | * |
| 158 | * @param edge edge to mark |
| 159 | * @param type edge type |
| 160 | */ |
| 161 | void markEdge(E edge, EdgeType type) { |
| 162 | edges.put(edge, type); |
| 163 | } |
| 164 | |
| 165 | } |
| 166 | |
| 167 | } |