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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.

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/*
 * Copyright 2011 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.Predicate;
import com.google.javascript.jscomp.graph.DiGraph.DiGraphEdge;
import com.google.javascript.jscomp.graph.DiGraph.DiGraphNode;

/**
 * Prunes a graph, creating a new graph with nodes removed.
 *
 * If a node is removed from the graph, any paths through that node
 * will be replaced with edges. In other words, if A and B are nodes
 * in the original graph and the pruned graph, then there exists a path
 * from A -> B in the original graph iff there's a path from A -> B
 * in the pruned graph.
 *
 * We do not make any guarantees about what edges are in the pruned graph.
 *
 * @author [email protected] (Nick Santos)
 */
public class GraphPruner {
  private final DiGraph graph;

  public GraphPruner(DiGraph graph) {
    this.graph = graph;
  }

  public LinkedDirectedGraph prune(Predicate keep) {
    LinkedDirectedGraph workGraph = cloneGraph(graph);

    // Create a work graph where all nodes with a path between them have
    // an edge.
    for (DiGraphNode node : workGraph.getDirectedGraphNodes()) {
      for (DiGraphEdge inEdge : node.getInEdges()) {
        for (DiGraphEdge outEdge : node.getOutEdges()) {
          N source = inEdge.getSource().getValue();
          N dest = outEdge.getDestination().getValue();
          if (!workGraph.isConnectedInDirection(source, dest)) {
            workGraph.connect(source, outEdge.getValue(), dest);
          }
        }
      }
    }


    // Build a complete subgraph of workGraph.
    LinkedDirectedGraph resultGraph =
        LinkedDirectedGraph.create();
    for (DiGraphNode node : workGraph.getDirectedGraphNodes()) {
      if (keep.apply(node.getValue())) {
        resultGraph.createNode(node.getValue());

        for (DiGraphEdge outEdge : node.getOutEdges()) {
          N source = node.getValue();
          N dest = outEdge.getDestination().getValue();
          if (keep.apply(dest)) {
            resultGraph.createNode(dest);
            if (source != dest &&
                !resultGraph.isConnectedInDirection(source, dest)) {
              resultGraph.connect(source, outEdge.getValue(), dest);
            }
          }
        }
      }
    }

    return resultGraph;
  }

  private static  LinkedDirectedGraph cloneGraph(
      DiGraph graph) {
    LinkedDirectedGraph newGraph = LinkedDirectedGraph.create();
    for (DiGraphNode node : graph.getDirectedGraphNodes()) {
      newGraph.createNode(node.getValue());

      for (DiGraphEdge outEdge : node.getOutEdges()) {
        N dest = outEdge.getDestination().getValue();
        newGraph.createNode(dest);
        newGraph.connect(node.getValue(), outEdge.getValue(), dest);
      }
    }

    return newGraph;
  }
}