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Closure Compiler is a JavaScript optimizing compiler. It parses your JavaScript, analyzes it, removes dead code and rewrites and minimizes what's left. It also checks syntax, variable references, and types, and warns about common JavaScript pitfalls. It is used in many of Google's JavaScript apps, including Gmail, Google Web Search, Google Maps, and Google Docs. This binary checks for style issues such as incorrect or missing JSDoc usage, and missing goog.require() statements. It does not do more advanced checks such as typechecking.

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/*
 * Copyright 2008 The Closure Compiler Authors.
 *
 * 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.javascript.jscomp.graph;

import com.google.common.base.Preconditions;
import com.google.javascript.jscomp.graph.DiGraph.DiGraphEdge;
import com.google.javascript.jscomp.graph.DiGraph.DiGraphNode;

import java.util.HashSet;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.Set;

/**
 * A utility class for doing fixed-point computations. We traverse
 * the edges over the given directed graph until the graph reaches
 * a steady state.
 *
 * @author [email protected] (Nick Santos)
 *
 * @param  Value type that the graph node stores.
 * @param  Value type that the graph edge stores.
 */
public final class FixedPointGraphTraversal {
  // TODO(nicksantos): Generalize the algorithm for undirected graphs, if we
  // need it.

  private final EdgeCallback callback;

  public static final String NON_HALTING_ERROR_MSG =
    "Fixed point computation not halting";

  /**
   * Create a new traversal.
   * @param callback A callback for updating the state of the graph each
   *     time an edge is traversed.
   */
  public FixedPointGraphTraversal(EdgeCallback callback) {
    this.callback = callback;
  }

  /**
   * Helper method for creating new traversals.
   */
  public static  FixedPointGraphTraversal newTraversal(
      EdgeCallback callback) {
    return new FixedPointGraphTraversal<>(callback);
  }

  /**
   * Compute a fixed point for the given graph.
   * @param graph The graph to traverse.
   */
  public void computeFixedPoint(DiGraph graph) {
    Set nodes = new HashSet<>();
    for (DiGraphNode node : graph.getDirectedGraphNodes()) {
      nodes.add(node.getValue());
    }
    computeFixedPoint(graph, nodes);
  }

  /**
   * Compute a fixed point for the given graph, entering from the given node.
   * @param graph The graph to traverse.
   * @param entry The node to begin traversing from.
   */
  public void computeFixedPoint(DiGraph graph, N entry) {
    Set entrySet = new HashSet<>();
    entrySet.add(entry);
    computeFixedPoint(graph, entrySet);
  }

  /**
   * Compute a fixed point for the given graph, entering from the given nodes.
   * @param graph The graph to traverse.
   * @param entrySet The nodes to begin traversing from.
   */
  public void computeFixedPoint(DiGraph graph, Set entrySet) {
    int cycleCount = 0;
    long nodeCount = graph.getNodes().size();

    // Choose a bail-out heuristically in case the computation
    // doesn't converge.
    long maxIterations = Math.max(nodeCount * nodeCount * nodeCount, 100);

    // Use a LinkedHashSet, so that the traversal is deterministic.
    LinkedHashSet> workSet =
         new LinkedHashSet<>();
    for (N n : entrySet) {
      workSet.add(graph.getDirectedGraphNode(n));
    }
    for (; !workSet.isEmpty() && cycleCount < maxIterations; cycleCount++) {
      // For every out edge in the workSet, traverse that edge. If that
      // edge updates the state of the graph, then add the destination
      // node to the resultSet, so that we can update all of its out edges
      // on the next iteration.
      DiGraphNode source = workSet.iterator().next();
      N sourceValue = source.getValue();

      workSet.remove(source);

      List> outEdges = source.getOutEdges();
      for (DiGraphEdge edge : outEdges) {
        N destNode = edge.getDestination().getValue();
        if (callback.traverseEdge(sourceValue, edge.getValue(), destNode)) {
          workSet.add(edge.getDestination());
        }
      }
    }

    Preconditions.checkState(cycleCount != maxIterations,
        NON_HALTING_ERROR_MSG);
  }

  /** Edge callback */
  public static interface EdgeCallback {
    /**
     * Update the state of the destination node when the given edge
     * is traversed. For the fixed-point computation to work, only the
     * destination node may be modified. The source node and the edge must
     * not be modified.
     *
     * @param source The start node.
     * @param e The edge.
     * @param destination The end node.
     * @return Whether the state of the destination node changed.
     */
    boolean traverseEdge(Node source, Edge e, Node destination);
  }
}