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// Copyright 2014 The Bazel Authors. All rights reserved.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
package com.google.devtools.build.lib.graph;
import com.google.common.collect.Ordering;
import java.util.Collection;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashSet;
import java.util.List;
import java.util.Set;
/**
* <p> The DFS class encapsulates a depth-first search visitation, including
* the order in which nodes are to be visited relative to their successors
* (PREORDER/POSTORDER), whether the forward or transposed graph is to be
* used, and which nodes have been seen already. </p>
*
* <p> A variety of common uses of DFS are offered through methods of
* Digraph; however clients can use this class directly for maximum
* flexibility. See the implementation of
* Digraph.getStronglyConnectedComponents() for an example. </p>
*
* <p> Clients should not modify the enclosing Digraph instance of a DFS
* while a traversal is in progress. </p>
*/
public class DFS<T> {
// (Preferred over a boolean to avoid parameter confusion.)
public enum Order {
PREORDER,
POSTORDER
}
private final Order order; // = (PREORDER|POSTORDER)
private final Comparator<Node<T>> edgeOrder;
private final boolean transpose;
private final Set<Node<T>> marked = new HashSet<>();
/**
* Constructs a DFS instance for searching over the enclosing Digraph
* instance, using the specified visitation parameters.
*
* @param order PREORDER or POSTORDER, determines node visitation order
* @param edgeOrder an ordering in which the edges originating from the same
* node should be visited (if null, the order is unspecified)
* @param transpose iff true, the graph is implicitly transposed during
* visitation.
*/
public DFS(Order order, final Comparator<? super T> edgeOrder, boolean transpose) {
this.order = order;
this.transpose = transpose;
if (edgeOrder == null) {
this.edgeOrder = null;
} else {
this.edgeOrder = new Comparator<Node<T>>() {
@Override
public int compare(Node<T> o1, Node<T> o2) {
return edgeOrder.compare(o1.getLabel(), o2.getLabel());
}
};
}
}
public DFS(Order order, boolean transpose) {
this(order, null, transpose);
}
/**
* Returns the (immutable) set of nodes visited so far.
*/
public Set<Node<T>> getMarked() {
return Collections.unmodifiableSet(marked);
}
public void visit(Node<T> node, GraphVisitor<T> visitor) {
if (!marked.add(node)) {
return;
}
if (order == Order.PREORDER) {
visitor.visitNode(node);
}
Collection<Node<T>> edgeTargets = transpose
? node.getPredecessors() : node.getSuccessors();
if (edgeOrder != null) {
List<Node<T>> mutableNodeList = Ordering.from(edgeOrder).sortedCopy(edgeTargets);
edgeTargets = mutableNodeList;
}
for (Node<T> v: edgeTargets) {
visit(v, visitor);
}
if (order == Order.POSTORDER) {
visitor.visitNode(node);
}
}
}
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