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

import java.util.LinkedHashMap;
import java.util.Map;

/**
 * This code implements the instrumentation pass over the AST
 * (returned by JSCompiler).
 * @author [email protected] (Praveen Kumashi)
 *
 */
@GwtIncompatible("FileInstrumentationData")
class CoverageInstrumentationPass implements CompilerPass {

  final AbstractCompiler compiler;
  private Map instrumentationData;
  private CoverageReach reach;

  public static final String JS_INSTRUMENTATION_OBJECT_NAME = "__jscov";

  public enum CoverageReach {
    ALL,
    CONDITIONAL
  }

  /**
   *
   * @param compiler the compiler which generates the AST.
   */
  public CoverageInstrumentationPass(AbstractCompiler compiler,
      CoverageReach reach) {
    this.compiler = compiler;
    this.reach = reach;
    instrumentationData = new LinkedHashMap<>();
  }

  /**
   * Creates the js code to be added to source. This code declares and
   * initializes the variables required for collection of coverage data.
   */
  private void addHeaderCode(Node script) {
    script.addChildToFront(createConditionalObjectDecl(JS_INSTRUMENTATION_OBJECT_NAME, script));
  }

  @Override
  public void process(Node externsNode, Node rootNode) {
    if (rootNode.hasChildren()) {
      NodeTraversal.traverseEs6(compiler, rootNode,
          new CoverageInstrumentationCallback(
              compiler, instrumentationData, reach));

      Node firstScript = rootNode.getFirstChild();
      Preconditions.checkState(firstScript.isScript());
      addHeaderCode(firstScript);
    }
  }

  private Node createConditionalObjectDecl(String name, Node srcref) {
    String jscovDecl =
        " var "
            + name
            + " = window.top.__jscov || "
            + "(window.top.__jscov = {fileNames:[], instrumentedLines: [], executedLines: []});";
    Node script = compiler.parseSyntheticCode(jscovDecl);
    Node var = script.removeFirstChild();
    return var.useSourceInfoIfMissingFromForTree(srcref);
  }
}