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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 static com.google.common.base.Preconditions.checkState;

import com.google.common.annotations.GwtIncompatible;
import com.google.javascript.rhino.IR;
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).
 */
@GwtIncompatible("FileInstrumentationData")
class CoverageInstrumentationPass implements CompilerPass {

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

  public enum InstrumentOption {
    LINE_ONLY,  // Collect coverage for every executable statement.
    BRANCH_ONLY  // Collect coverage for control-flow branches.
  }

  public static final String JS_INSTRUMENTATION_OBJECT_NAME = "__jscov";

  public enum CoverageReach {
    ALL,         // Instrument all statements.
    CONDITIONAL  // Do not instrument global statements.
  }

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

  public CoverageInstrumentationPass(AbstractCompiler compiler, CoverageReach reach) {
    this(compiler, reach, InstrumentOption.LINE_ONLY);
  }

  /**
   * 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));

    // Make subsequent usages of "window" and "window.top" work in a Web Worker context.
    script.addChildToFront(
        compiler.parseSyntheticCode(
            "if (!self.window) { self.window = self; self.window.top = self; }")
        .removeFirstChild()
        .useSourceInfoIfMissingFromForTree(script));
  }

  @Override
  public void process(Node externsNode, Node rootNode) {
    if (rootNode.hasChildren()) {
      if (instrumentOption == InstrumentOption.BRANCH_ONLY) {
        NodeTraversal.traverse(
            compiler,
            rootNode,
            new BranchCoverageInstrumentationCallback(compiler, instrumentationData));
      } else {
        NodeTraversal.traverse(
            compiler, rootNode, new CoverageInstrumentationCallback(instrumentationData, reach));
      }
      Node firstScript = rootNode.getFirstChild();
      checkState(firstScript.isScript());
      // If any passes run after we need to preserve the MODULE_BODY structure of scripts - we can't
      // just add to a script if it is a module.
      if (firstScript.hasChildren() && firstScript.getFirstChild().isModuleBody()) {
        firstScript = firstScript.getFirstChild();
      }
      addHeaderCode(firstScript);
    }
  }

  private Node createConditionalObjectDecl(String name, Node srcref) {
    // Make sure to quote properties so they are not renamed.
    Node jscovData;
    switch (instrumentOption) {
      case BRANCH_ONLY:
        jscovData =
            IR.objectlit(
                IR.quotedStringKey("fileNames", IR.arraylit()),
                IR.quotedStringKey("branchPresent", IR.arraylit()),
                IR.quotedStringKey("branchesInLine", IR.arraylit()),
                IR.quotedStringKey("branchesTaken", IR.arraylit()));
        break;
      case LINE_ONLY:
        jscovData =
            IR.objectlit(
                IR.quotedStringKey("fileNames", IR.arraylit()),
                IR.quotedStringKey("instrumentedLines", IR.arraylit()),
                IR.quotedStringKey("executedLines", IR.arraylit()));
        break;
      default:
        throw new AssertionError("Unexpected option: " + instrumentOption);
    }

    // Add the __jscov var to the window as a quoted key so it can be found even if property
    // renaming is enabled.
    Node var =
        IR.var(
            IR.name(name),
            IR.or(
                IR.getelem(IR.getprop(IR.name("window"), "top"), IR.string("__jscov")),
                IR.assign(
                    IR.getelem(IR.getprop(IR.name("window"), "top"), IR.string("__jscov")),
                    jscovData)));
    return var.useSourceInfoIfMissingFromForTree(srcref);
  }
}