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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.javascript.jscomp.NodeTraversal.AbstractPostOrderCallback;
import com.google.javascript.rhino.Node;
import java.util.Arrays;
import java.util.List;

/**
 * A compiler pass to run various peephole optimizations (e.g. constant folding,
 * some useless code removal, some minimizations).
 */
class PeepholeOptimizationsPass implements CompilerPass {

  private final AbstractCompiler compiler;
  private final String passName;
  // NOTE: Use a native array rather than a List to avoid creating iterators for every node in the
  // AST.
  private final AbstractPeepholeOptimization[] peepholeOptimizations;
  private boolean retraverseOnChange;

  /** Creates a peephole optimization pass that runs the given optimizations. */
  PeepholeOptimizationsPass(
      AbstractCompiler compiler, String passName, AbstractPeepholeOptimization... optimizations) {
    this(compiler, passName, Arrays.asList(optimizations));
  }

  PeepholeOptimizationsPass(
      AbstractCompiler compiler,
      String passName,
      List optimizations) {
    this.compiler = compiler;
    this.passName = passName;
    this.peepholeOptimizations = optimizations.toArray(new AbstractPeepholeOptimization[0]);
    this.retraverseOnChange = true;
  }

  @VisibleForTesting
  void setRetraverseOnChange(boolean retraverse) {
    this.retraverseOnChange = retraverse;
  }

  @Override
  public void process(Node externs, Node root) {
    beginTraversal();

    // Repeat to an internal fixed point.
    for (List changedScopeNodes = compiler.getChangedScopeNodesForPass(passName);
        changedScopeNodes == null || !changedScopeNodes.isEmpty();
        changedScopeNodes = compiler.getChangedScopeNodesForPass(passName)) {
      NodeTraversal.traverseScopeRoots(
          compiler, root, changedScopeNodes, new PeepCallback(), false);

      // Cancel the fixed point if requested.
      if (!retraverseOnChange) {
        break;
      }
    }
  }

  private class PeepCallback extends AbstractPostOrderCallback {
    @Override
    public void visit(NodeTraversal t, Node n, Node parent) {
      Node currentNode = n;
      for (AbstractPeepholeOptimization optim : peepholeOptimizations) {
        currentNode = optim.optimizeSubtree(currentNode);
        if (currentNode == null) {
          return;
        }
      }
    }
  }

  /** Make sure that all the optimizations have the current compiler so they can report errors. */
  private void beginTraversal() {
    for (AbstractPeepholeOptimization optimization : peepholeOptimizations) {
      optimization.beginTraversal(compiler);
    }
  }
}