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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 2010 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.annotations.VisibleForTesting;
import com.google.common.base.Preconditions;
import com.google.javascript.rhino.Node;

/**
 * An abstract class whose implementations run peephole optimizations:
 * optimizations that look at a small section of code and either remove
 * that code (if it is not needed) or replaces it with smaller code.
 *
 */
abstract class AbstractPeepholeOptimization {

  private NodeTraversal currentTraversal;

  /**
   * Given a node to optimize and a traversal, optimize the node. Subclasses
   * should override to provide their own peephole optimization.
   *
   * @param subtree The subtree that will be optimized.
   * @return The new version of the subtree (or null if the subtree or one of
   * its parents was removed from the AST). If the subtree has not changed,
   * this method must return {@code subtree}.
   */
  abstract Node optimizeSubtree(Node subtree);

  /**
   * Helper method for reporting an error to the compiler when applying a
   * peephole optimization.
   *
   * @param diagnostic The error type
   * @param n The node for which the error should be reported
   */
  protected void error(DiagnosticType diagnostic, Node n) {
    JSError error = currentTraversal.makeError(n, diagnostic, n.toString());
    currentTraversal.getCompiler().report(error);
  }

  /**
   * Helper method for telling the compiler that something has changed.
   * Subclasses must call these if they have changed the AST.
   */
  protected void reportCodeChange() {
    Preconditions.checkNotNull(currentTraversal);
    currentTraversal.getCompiler().reportCodeChange();
  }

  /**
   * Are the nodes equal for the purpose of inlining?
   * If type aware optimizations are on, type equality is checked.
   */
  protected boolean areNodesEqualForInlining(Node n1, Node n2) {
    /* Our implementation delegates to the compiler. We provide this
     * method because we don't want to expose Compiler to PeepholeOptimizations.
     */
    Preconditions.checkNotNull(currentTraversal);
    return currentTraversal.getCompiler().areNodesEqualForInlining(n1, n2);
  }

  /**
   *  Is the current AST normalized? (e.g. has the Normalize pass been run
   *  and has the Denormalize pass not yet been run?)
   */
  protected boolean isASTNormalized() {
    Preconditions.checkNotNull(currentTraversal);
    Preconditions.checkNotNull(currentTraversal.getCompiler());

    return currentTraversal.getCompiler().getLifeCycleStage().isNormalized();
  }

  /**
   * Informs the optimization that a traversal will begin.
   */
  void beginTraversal(NodeTraversal traversal) {
    currentTraversal = traversal;
  }

  /**
   * Informs the optimization that a traversal has completed.
   */
  void endTraversal(NodeTraversal traversal) {
    currentTraversal = null;
  }

  // NodeUtil's mayEffectMutableState and mayHaveSideEffects need access to the
  // compiler object, route them through here to give them access.

  /**
   * @return Whether the node may create new mutable state, or change existing
   * state.
   */
  boolean mayEffectMutableState(Node n) {
    return NodeUtil.mayEffectMutableState(n, currentTraversal.getCompiler());
  }

  /**
   * @return Whether the node may have side effects when executed.
   */
  boolean mayHaveSideEffects(Node n) {
    return NodeUtil.mayHaveSideEffects(n, currentTraversal.getCompiler());
  }

  /**
   * @return Whether the source code version is EcmaScript 5 or later.
   *     Workarounds for quirks in browsers that do not support ES5 can be
   *     ignored when this is true.
   */
  boolean isEcmaScript5OrGreater() {
    return currentTraversal != null
        && currentTraversal.getCompiler().acceptEcmaScript5();
  }

  /**
   * @return the current coding convention.
   */
  CodingConvention getCodingConvention() {
    // Note: this assumes a thread safe coding convention object.
    return currentTraversal.getCompiler().getCodingConvention();
  }

  /**
   * Check if the specified node is null or is still in the AST.
   */
  @VisibleForTesting
  static Node validateResult(Node n) {
    done: {
      if (n != null && !n.isScript()
          && (!n.isBlock() || !n.isSyntheticBlock())) {
        for (Node parent : n.getAncestors()) {
          if (parent.isScript()) {
            break done;
          }
        }
        Preconditions.checkState(false);
      }
    }
    return n;
  }
}