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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 2009 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;

import com.google.common.base.Preconditions;
import com.google.common.collect.ImmutableList;
import com.google.javascript.rhino.IR;
import com.google.javascript.rhino.Node;
import com.google.javascript.rhino.Token;

import java.util.ArrayList;
import java.util.List;

/**
 * Proxy that provides a high level interface that compiler passes can
 * use to replace or remove sections of the AST.
 *
 */
class AstChangeProxy {

  /**
   * Interface used to notify client code about changes done by
   * AstChangeProxy.
   */
  interface ChangeListener {

    /**
     * Notifies clients about node removals.
     */
    void nodeRemoved(Node node);
  }

  private final List listeners;

  AstChangeProxy() {
    listeners = new ArrayList<>();
  }

  /**
   * Registers a change listener.
   */
  final void registerListener(ChangeListener listener) {
    listeners.add(listener);
  }

  /**
   * Unregisters a change listener.
   */
  final void unregisterListener(ChangeListener listener) {
    listeners.remove(listener);
  }

  /**
   * Notifies listeners about a removal.
   */
  private void notifyOfRemoval(Node node) {
    for (ChangeListener listener : listeners) {
      listener.nodeRemoved(node);
    }
  }

  /**
   * Removes a node from the parent's child list.
   */
  final void removeChild(Node parent, Node node) {
    parent.removeChild(node);

    notifyOfRemoval(node);
  }

  /**
   * Replaces a node from the parent's child list.
   */
  final void replaceWith(Node parent, Node node, Node replacement) {
    replaceWith(parent, node, ImmutableList.of(replacement));
  }

  /**
   * Replaces a node with the provided list.
   */
  final void replaceWith(Node parent, Node node, List replacements) {
    Preconditions.checkNotNull(replacements, "\"replacements\" is null.");

    int size = replacements.size();

    if ((size == 1) && node.isEquivalentTo(replacements.get(0))) {
      // trees are equal... don't replace
      return;
    }

    Token parentType = parent.getToken();

    Preconditions.checkState(size == 1 ||
        parentType == Token.BLOCK ||
        parentType == Token.SCRIPT ||
        parentType == Token.LABEL);

    if (parentType == Token.LABEL && size != 1) {
      Node block = IR.block();
      for (Node newChild : replacements) {
        newChild.useSourceInfoIfMissingFrom(node);
        block.addChildToBack(newChild);
      }
      parent.replaceChild(node, block);
    } else {
      for (Node newChild : replacements) {
        newChild.useSourceInfoIfMissingFrom(node);
        parent.addChildBefore(newChild, node);
      }
      parent.removeChild(node);
    }
    notifyOfRemoval(node);
  }
}