com.facebook.buck.graph.AbstractBottomUpTraversal.java Source code

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Here is the source code for com.facebook.buck.graph.AbstractBottomUpTraversal.java

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/*
 * Copyright 2012-present Facebook, Inc.
 *
 * 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.facebook.buck.graph;

import com.google.common.base.Preconditions;
import com.google.common.collect.Iterables;
import com.google.common.collect.Lists;
import com.google.common.collect.Maps;
import com.google.common.collect.Sets;

import java.util.Map;
import java.util.Queue;
import java.util.Set;
import java.util.concurrent.atomic.AtomicInteger;

/**
 * Class that performs a "bottom-up" traversal of a DAG. For any given node, every node to which it
 * has an outgoing edge will be visited before the given node.
 */
public abstract class AbstractBottomUpTraversal<T, V> {

    private final TraversableGraph<T> graph;

    private final Set<T> visitedNodes;

    private final Queue<T> nodesToExplore;

    // AtomicInteger is used to decrement the integer value in-place.
    private final Map<T, AtomicInteger> effectiveOutDegreesOfExplorableNodes;

    public AbstractBottomUpTraversal(TraversableGraph<T> graph) {
        this.graph = Preconditions.checkNotNull(graph);
        this.visitedNodes = Sets.newHashSet();
        this.nodesToExplore = Lists.newLinkedList();
        this.effectiveOutDegreesOfExplorableNodes = Maps.newHashMap();
    }

    public final void traverse() {
        Iterables.addAll(nodesToExplore, graph.getNodesWithNoOutgoingEdges());
        while (!nodesToExplore.isEmpty()) {
            T node = nodesToExplore.remove();
            if (visitedNodes.contains(node)) {
                Preconditions.checkState(false,
                        "The queue of nodes to explore should not contain a node that has already been"
                                + " visited.");
            }

            visit(node);
            visitedNodes.add(node);

            // Only add a node to the set of nodes to be explored if all the nodes it depends on have
            // been visited already. We achieve the same by keeping track of the out degrees of explorable
            // nodes. After visiting a node, decrement the out degree of each of its parent node. When the
            // out degree reaches zero, it is safe to add that node to the list of nodes to explore next.
            for (T exploreCandidate : graph.getIncomingNodesFor(node)) {
                if (!effectiveOutDegreesOfExplorableNodes.containsKey(exploreCandidate)) {
                    effectiveOutDegreesOfExplorableNodes.put(exploreCandidate,
                            new AtomicInteger(Iterables.size(graph.getOutgoingNodesFor(exploreCandidate))));
                }
                if (effectiveOutDegreesOfExplorableNodes.get(exploreCandidate).decrementAndGet() == 0) {
                    nodesToExplore.add(exploreCandidate);
                }
            }
        }
    }

    public abstract void visit(T node);

    public abstract V getResult();

    protected TraversableGraph<T> getGraph() {
        return graph;
    }
}