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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 2004 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.checkArgument;
import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.base.Preconditions.checkState;
import static com.google.common.base.Strings.nullToEmpty;

import com.google.javascript.jscomp.modules.ModuleMetadataMap;
import com.google.javascript.jscomp.modules.ModuleMetadataMap.ModuleMetadata;
import com.google.javascript.rhino.InputId;
import com.google.javascript.rhino.Node;
import java.util.ArrayDeque;
import java.util.ArrayList;
import java.util.Deque;
import java.util.List;
import javax.annotation.Nullable;

/**
 * NodeTraversal allows an iteration through the nodes in the parse tree,
 * and facilitates the optimizations on the parse tree.
 */
public class NodeTraversal {
  private final AbstractCompiler compiler;
  private final Callback callback;

  /** Contains the current node*/
  private Node curNode;

  /** Contains the enclosing SCRIPT node if there is one, otherwise null. */
  private Node curScript;

  /** The change scope for the current node being visiteds */
  private Node currentChangeScope;

  /**
   * Stack containing the Scopes that have been created. The Scope objects
   * are lazily created; so the {@code scopeRoots} stack contains the
   * Nodes for all Scopes that have not been created yet.
   */
  private final Deque> scopes = new ArrayDeque<>();

  /**
   * A stack of scope roots. See #scopes.
   */
  private final List scopeRoots = new ArrayList<>();

  /**
   * Stack containing the control flow graphs (CFG) that have been created. There are fewer CFGs
   * than scopes, since block-level scopes are not valid CFG roots. The CFG objects are lazily
   * populated: elements are simply the CFG root node until requested by {@link
   * #getControlFlowGraph()}.
   */
  private final Deque cfgs = new ArrayDeque<>();

  /** The current source file name */
  private String sourceName;

  /** The current input */
  private InputId inputId;
  private CompilerInput compilerInput;

  /** The scope creator */
  private final ScopeCreator scopeCreator;

  /** Possible callback for scope entry and exist **/
  private ScopedCallback scopeCallback;

  /** Callback for passes that iterate over a list of change scope roots (FUNCTIONs and SCRIPTs) */
  public interface ChangeScopeRootCallback {
    void enterChangeScopeRoot(AbstractCompiler compiler, Node root);
  }

  /**
   * Callback for tree-based traversals
   */
  public interface Callback {
    /**
     * Visits a node in preorder (before its children) and decides whether its children should be
     * traversed. If the children should be traversed, they will be visited by {@link
     * #shouldTraverse(NodeTraversal, Node, Node)} in preorder and by {@link #visit(NodeTraversal,
     * Node, Node)} in postorder.
     *
     * 
Siblings are always visited left-to-right.
     *
     * 
Implementations can have side-effects (e.g. modify the parse tree). Removing the current
     * node is legal, but removing or reordering nodes above the current node may cause nodes to be
     * visited twice or not at all.
     *
     * @param t The current traversal.
     * @param n The current node.
     * @param parent The parent of the current node.
     * @return whether the children of this node should be visited
     */
    boolean shouldTraverse(NodeTraversal t, Node n, Node parent);

    /**
     * Visits a node in postorder (after its children). A node is visited in postorder iff {@link
     * #shouldTraverse(NodeTraversal, Node, Node)} returned true for its parent. In particular, the
     * root node is never visited in postorder.
     *
     * 
Siblings are always visited left-to-right.
     *
     * 
Implementations can have side-effects (e.g. modify the parse tree). Removing the current
     * node is legal, but removing or reordering nodes above the current node may cause nodes to be
     * visited twice or not at all.
     *
     * @param t The current traversal.
     * @param n The current node.
     * @param parent The parent of the current node.
     */
    void visit(NodeTraversal t, Node n, Node parent);
  }

  /**
   * Callback that also knows about scope changes
   */
  public interface ScopedCallback extends Callback {

    /**
     * Called immediately after entering a new scope. The new scope can
     * be accessed through t.getScope()
     */
    void enterScope(NodeTraversal t);

    /**
     * Called immediately before exiting a scope. The ending scope can
     * be accessed through t.getScope()
     */
    void exitScope(NodeTraversal t);
  }

  /**
   * Abstract callback to visit all nodes in postorder. Note: Do not create anonymous subclasses of
   * this. Instead, write a lambda expression which will be interpreted as an
   * AbstractPostOrderCallbackInterface.
   *
   */
  public abstract static class AbstractPostOrderCallback implements Callback {
    @Override
    public final boolean shouldTraverse(NodeTraversal nodeTraversal, Node n, Node parent) {
      return true;
    }
  }

  /**
   * Abstract callback to visit all non-extern nodes in postorder. Note: Even though type-summary
   * nodes are included under the externs roots, they are traversed by this callback.
   */
  public abstract static class ExternsSkippingCallback implements Callback {
    @Override
    public final boolean shouldTraverse(NodeTraversal t, Node n, Node parent) {
      return !n.isScript() || !n.isFromExterns() || NodeUtil.isFromTypeSummary(n);
    }
  }

  /** Abstract callback to visit all nodes in postorder. */
  @FunctionalInterface
  public static interface AbstractPostOrderCallbackInterface {
    void visit(NodeTraversal t, Node n, Node parent);
  }

  private static Callback makePostOrderCallback(AbstractPostOrderCallbackInterface lambda) {
    return new Callback() {
      @Override
      public final boolean shouldTraverse(NodeTraversal nodeTraversal, Node n, Node parent) {
        return true;
      }

      @Override
      public final void visit(NodeTraversal t, Node n, Node parent) {
        lambda.visit(t, n, parent);
      }
    };
  }

  /** Abstract callback to visit all nodes in preorder. */
  public abstract static class AbstractPreOrderCallback implements Callback {
    @Override
    public final void visit(NodeTraversal t, Node n, Node parent) {}
  }

  /** Abstract scoped callback to visit all nodes in postorder. */
  public abstract static class AbstractScopedCallback implements ScopedCallback {
    @Override
    public final boolean shouldTraverse(NodeTraversal nodeTraversal, Node n, Node parent) {
      return true;
    }

    @Override
    public void enterScope(NodeTraversal t) {}

    @Override
    public void exitScope(NodeTraversal t) {}
  }

  /**
   * Abstract callback to visit all nodes but not traverse into function
   * bodies.
   */
  public abstract static class AbstractShallowCallback implements Callback {
    @Override
    public final boolean shouldTraverse(NodeTraversal nodeTraversal, Node n, Node parent) {
      // We do want to traverse the name of a named function, but we don't
      // want to traverse the arguments or body.
      return parent == null || !parent.isFunction() || n == parent.getFirstChild();
    }
  }

  /**
   * Abstract callback to visit all structure and statement nodes but doesn't traverse into
   * functions or expressions.
   */
  public abstract static class AbstractShallowStatementCallback implements Callback {
    @Override
    public final boolean shouldTraverse(NodeTraversal nodeTraversal, Node n, Node parent) {
      return parent == null
          || NodeUtil.isControlStructure(parent)
          || NodeUtil.isStatementBlock(parent);
    }
  }

  /**
   * Abstract callback that knows when a global script, goog.provide file, goog.module,
   * goog.loadModule, ES module or CommonJS module is entered or exited. This includes both whole
   * file modules and bundled modules, as well as files in the global scope.
   */
  public abstract static class AbstractModuleCallback implements Callback {
    protected final AbstractCompiler compiler;
    private final ModuleMetadataMap moduleMetadataMap;

    @Nullable private ModuleMetadata currentModule;
    @Nullable private Node scopeRoot;
    private boolean inLoadModule;

    AbstractModuleCallback(AbstractCompiler compiler, ModuleMetadataMap moduleMetadataMap) {
      this.compiler = compiler;
      this.moduleMetadataMap = moduleMetadataMap;
    }

    /**
     * Called when the traversal enters a global file or module.
     *
     * @param currentModule The entered global file or module.
     * @param moduleScopeRoot The root scope for the entered module or SCRIPT for global files.
     */
    protected void enterModule(ModuleMetadata currentModule, Node moduleScopeRoot) {}

    /**
     * Called when the traversal exits a global file or module.
     *
     * @param oldModule The exited global file or module.
     * @param moduleScopeRoot The root scope for the exited module or SCRIPT for global files.
     */
    protected void exitModule(ModuleMetadata oldModule, Node moduleScopeRoot) {}

    @Override
    public final boolean shouldTraverse(NodeTraversal t, Node n, Node parent) {
      switch (n.getToken()) {
        case SCRIPT:
          currentModule =
              moduleMetadataMap.getModulesByPath().get(t.getInput().getPath().toString());
          checkNotNull(currentModule);
          scopeRoot = n.hasChildren() && n.getFirstChild().isModuleBody() ? n.getFirstChild() : n;
          enterModule(currentModule, scopeRoot);
          break;
        case BLOCK:
          if (NodeUtil.isBundledGoogModuleScopeRoot(n)) {
            scopeRoot = n;
            inLoadModule = true;
          }
          break;
        case CALL:
          if (inLoadModule && n.getFirstChild().matchesQualifiedName("goog.module")) {
            ModuleMetadata newModule =
                moduleMetadataMap.getModulesByGoogNamespace().get(n.getLastChild().getString());
            checkNotNull(newModule);
            // In the event of multiple goog.module statements (an error), don't call enterModule
            // more than once.
            if (newModule != currentModule) {
              currentModule = newModule;
              enterModule(currentModule, scopeRoot);
            }
          }
          break;
        default:
          break;
      }
      return shouldTraverse(t, n, currentModule, scopeRoot);
    }

    /**
     * See {@link Callback#shouldTraverse}.
     *
     * @param t The current traversal.
     * @param n The current node.
     * @param currentModule The current module, or null if not inside a module (e.g. AST root).
     * @param moduleScopeRoot The root scope for the current module, or null if not inside a module
     *     (e.g. AST root).
     * @return whether the children of this node should be visited
     */
    protected boolean shouldTraverse(
        NodeTraversal t,
        Node n,
        @Nullable ModuleMetadata currentModule,
        @Nullable Node moduleScopeRoot) {
      return true;
    }

    @Override
    public final void visit(NodeTraversal t, Node n, Node parent) {
      switch (n.getToken()) {
        case SCRIPT:
          checkNotNull(currentModule);
          exitModule(currentModule, scopeRoot);
          currentModule = null;
          scopeRoot = null;
          break;
        case BLOCK:
          if (NodeUtil.isBundledGoogModuleScopeRoot(n)) {
            checkNotNull(currentModule);
            exitModule(currentModule, scopeRoot);
            scopeRoot = n.getGrandparent().getGrandparent();
            inLoadModule = false;
            currentModule =
                moduleMetadataMap.getModulesByPath().get(t.getInput().getPath().toString());
            checkNotNull(currentModule);
          }
          break;
        default:
          break;
      }

      visit(t, n, currentModule, scopeRoot);
    }

    /**
     * See {@link Callback#visit}.
     *
     * @param t The current traversal.
     * @param n The current node.
     * @param currentModule The current module, or null if not inside a module (e.g. AST root).
     * @param moduleScopeRoot The root scope for the current module, or null if not inside a module
     *     (e.g. AST root).
     */
    protected void visit(
        NodeTraversal t,
        Node n,
        @Nullable ModuleMetadata currentModule,
        @Nullable Node moduleScopeRoot) {}
  }

  /**
   * Creates a node traversal using the specified callback interface
   * and the scope creator.
   */
  public NodeTraversal(AbstractCompiler compiler, Callback cb, ScopeCreator scopeCreator) {
    this.callback = cb;
    if (cb instanceof ScopedCallback) {
      this.scopeCallback = (ScopedCallback) cb;
    }
    this.compiler = compiler;
    this.scopeCreator = scopeCreator;
  }

  private void throwUnexpectedException(Throwable unexpectedException) {
    // If there's an unexpected exception, try to get the
    // line number of the code that caused it.
    String message = unexpectedException.getMessage();

    // TODO(user): It is possible to get more information if curNode or
    // its parent is missing. We still have the scope stack in which it is still
    // very useful to find out at least which function caused the exception.
    if (inputId != null) {
      message =
          unexpectedException.getMessage() + "\n"
          + formatNodeContext("Node", curNode)
          + (curNode == null ?  "" : formatNodeContext("Parent", curNode.getParent()));
    }
    compiler.throwInternalError(message, unexpectedException);
  }

  private String formatNodeContext(String label, Node n) {
    if (n == null) {
      return "  " + label + ": NULL";
    }
    return "  " + label + "(" + n.toString(false, false, false) + "): "
        + formatNodePosition(n);
  }

  /**
   * Traverses a parse tree recursively.
   */
  public void traverse(Node root) {
    try {
      initTraversal(root);
      curNode = root;
      pushScope(root);
      // null parent ensures that the shallow callbacks will traverse root
      traverseBranch(root, null);
      popScope();
    } catch (Error | Exception unexpectedException) {
      throwUnexpectedException(unexpectedException);
    }
  }

  /** Traverses using the SyntacticScopeCreator */
  public static void traverse(AbstractCompiler compiler, Node root, Callback cb) {
    NodeTraversal t = new NodeTraversal(compiler, cb, new SyntacticScopeCreator(compiler));
    t.traverse(root);
  }

  /** Traverses in post order. */
  public static void traversePostOrder(
      AbstractCompiler compiler, Node root, AbstractPostOrderCallbackInterface cb) {
    traverse(compiler, root, makePostOrderCallback(cb));
  }

  void traverseRoots(Node externs, Node root) {
    try {
      Node scopeRoot = externs.getParent();
      checkNotNull(scopeRoot);

      initTraversal(scopeRoot);
      curNode = scopeRoot;
      pushScope(scopeRoot);

      traverseBranch(externs, scopeRoot);
      checkState(root.getParent() == scopeRoot);
      traverseBranch(root, scopeRoot);

      popScope();
    } catch (Error | Exception unexpectedException) {
      throwUnexpectedException(unexpectedException);
    }
  }

  public static void traverseRoots(
      AbstractCompiler compiler, Callback cb, Node externs, Node root) {
    NodeTraversal t = new NodeTraversal(compiler, cb, new SyntacticScopeCreator(compiler));
    t.traverseRoots(externs, root);
  }

  private static final String MISSING_SOURCE = "[source unknown]";

  private String formatNodePosition(Node n) {
    String sourceFileName = getBestSourceFileName(n);
    if (sourceFileName == null) {
      return MISSING_SOURCE + "\n";
    }

    int lineNumber = n.getLineno();
    int columnNumber = n.getCharno();
    String src = compiler.getSourceLine(sourceFileName, lineNumber);
    if (src == null) {
      src = MISSING_SOURCE;
    }
    return sourceFileName + ":" + lineNumber + ":" + columnNumber + "\n"
        + src + "\n";
  }

  /**
   * Traverses a parse tree recursively with a scope, starting with the given
   * root. This should only be used in the global scope or module scopes. Otherwise, use
   * {@link #traverseAtScope}.
   */
  void traverseWithScope(Node root, AbstractScope s) {
    checkState(s.isGlobal() || s.isModuleScope(), s);
    try {
      initTraversal(root);
      curNode = root;
      pushScope(s);
      traverseBranch(root, null);
      popScope();
    } catch (Error | Exception unexpectedException) {
      throwUnexpectedException(unexpectedException);
    }
  }

  /**
   * Traverses a parse tree recursively with a scope, starting at that scope's root. Omits children
   * of the scope root that are traversed in the outer scope (specifically, non-bleeding function
   * and class name nodes, class extends clauses, and computed property keys).
   */
  void traverseAtScope(AbstractScope s) {
    Node n = s.getRootNode();
    initTraversal(n);
    curNode = n;
    Deque> parentScopes = new ArrayDeque<>();
    AbstractScope temp = s.getParent();
    while (temp != null) {
      parentScopes.push(temp);
      temp = temp.getParent();
    }
    while (!parentScopes.isEmpty()) {
      pushScope(parentScopes.pop(), true);
    }
    if (n.isFunction()) {
      if (callback.shouldTraverse(this, n, null)) {
        pushScope(s);

        Node fnName = n.getFirstChild();
        Node args = fnName.getNext();
        Node body = args.getNext();
        if (!NodeUtil.isFunctionDeclaration(n)) {
          // Only traverse the function name if it's a bleeding function expression name.
          traverseBranch(fnName, n);
        }
        traverseBranch(args, n);
        traverseBranch(body, n);

        popScope();
        callback.visit(this, n, null);
      }
    } else if (n.isClass()) {
      if (callback.shouldTraverse(this, n, null)) {
        pushScope(s);

        Node className = n.getFirstChild();
        Node body = n.getLastChild();

        if (NodeUtil.isClassExpression(n)) {
          // Only traverse the class name if it's a bleeding class expression name.
          traverseBranch(className, n);
        }
        // Omit the extends node, which is in the outer scope. Computed property keys are already
        // excluded by handleClassMembers.
        traverseBranch(body, n);

        popScope();
        callback.visit(this, n, null);
      }
    } else if (n.isBlock()) {
      if (callback.shouldTraverse(this, n, null)) {
        pushScope(s);

        // traverseBranch is not called here to avoid re-creating the block scope.
        traverseChildren(n);

        popScope();
        callback.visit(this, n, null);
      }
    } else if (NodeUtil.isAnyFor(n)) {
      if (callback.shouldTraverse(this, n, null)) {
        pushScope(s);

        Node forAssignmentParam = n.getFirstChild();
        Node forIterableParam = forAssignmentParam.getNext();
        Node forBodyScope = forIterableParam.getNext();
        traverseBranch(forAssignmentParam, n);
        traverseBranch(forIterableParam, n);
        traverseBranch(forBodyScope, n);

        popScope();
        callback.visit(this, n, null);
      }
    } else if (n.isSwitch()) {
      if (callback.shouldTraverse(this, n, null)) {
        pushScope(s);

        traverseChildren(n);

        popScope();
        callback.visit(this, n, null);
      }
    } else {
      checkState(s.isGlobal() || s.isModuleScope(), "Expected global or module scope. Got:", s);
      traverseWithScope(n, s);
    }
  }

  private void traverseScopeRoot(Node scopeRoot) {
    try {
      initTraversal(scopeRoot);
      curNode = scopeRoot;
      initScopeRoots(scopeRoot.getParent());
      traverseBranch(scopeRoot, scopeRoot.getParent());
    } catch (Error | Exception unexpectedException) {
      throwUnexpectedException(unexpectedException);
    }
  }

  /**
   * Traverses *just* the contents of provided scope nodes (and optionally scopes nested within
   * them) but will fall back on traversing the entire AST from root if a null scope nodes list is
   * provided.
   * @param root If scopeNodes is null, this method will just traverse 'root' instead. If scopeNodes
   *     is not null, this parameter is ignored.
   */
  public static void traverseScopeRoots(
      AbstractCompiler compiler,
      @Nullable Node root,
      @Nullable List scopeNodes,
      final Callback cb,
      final boolean traverseNested) {
    traverseScopeRoots(compiler, root, scopeNodes, cb, null, traverseNested);
  }

  /**
   * Traverses *just* the contents of provided scope nodes (and optionally scopes nested within
   * them) but will fall back on traversing the entire AST from root if a null scope nodes list is
   * provided. Also allows for a callback to notify when starting on one of the provided scope
   * nodes.
   * @param root If scopeNodes is null, this method will just traverse 'root' instead. If scopeNodes
   *     is not null, this parameter is ignored.
   */
  public static void traverseScopeRoots(
      AbstractCompiler compiler,
      @Nullable Node root,
      @Nullable List scopeNodes,
      final Callback cb,
      @Nullable final ChangeScopeRootCallback changeCallback,
      final boolean traverseNested) {
    if (scopeNodes == null) {
      NodeTraversal.traverse(compiler, root, cb);
    } else {
      MemoizedScopeCreator scopeCreator =
          new MemoizedScopeCreator(new SyntacticScopeCreator(compiler));

      for (final Node scopeNode : scopeNodes) {
        traverseSingleScopeRoot(
            compiler, cb, changeCallback, traverseNested, scopeCreator, scopeNode);
      }
    }
  }

  private static void traverseSingleScopeRoot(
      AbstractCompiler compiler,
      final Callback cb,
      @Nullable ChangeScopeRootCallback changeCallback,
      final boolean traverseNested,
      MemoizedScopeCreator scopeCreator,
      final Node scopeNode) {
    if (changeCallback != null) {
      changeCallback.enterChangeScopeRoot(compiler, scopeNode);
    }

    ScopedCallback scb = new ScopedCallback() {
      boolean insideScopeNode = false;

      @Override
      public boolean shouldTraverse(NodeTraversal t, Node n, Node parent) {
        if (scopeNode == n) {
          insideScopeNode = true;
        }
        return (traverseNested || scopeNode == n || !NodeUtil.isChangeScopeRoot(n))
            && cb.shouldTraverse(t, n, parent);
      }

      @Override
      public void visit(NodeTraversal t, Node n, Node parent) {
        if (scopeNode == n) {
          insideScopeNode = false;
        }
        cb.visit(t, n, parent);
      }

      @Override
      public void enterScope(NodeTraversal t) {
        if (insideScopeNode && cb instanceof ScopedCallback) {
          ((ScopedCallback) cb).enterScope(t);
        }
      }

      @Override
      public void exitScope(NodeTraversal t) {
        if (insideScopeNode && cb instanceof ScopedCallback) {
          ((ScopedCallback) cb).exitScope(t);
        }
      }
    };

    NodeTraversal t = new NodeTraversal(compiler, scb, scopeCreator);
    t.traverseScopeRoot(scopeNode);
  }

  /**
   * Traverse a function out-of-band of normal traversal.
   *
   * @param node The function node.
   * @param scope The scope the function is contained in. Does not fire enter/exit
   *     callback events for this scope.
   */
  public void traverseFunctionOutOfBand(Node node, AbstractScope scope) {
    checkNotNull(scope);
    checkState(node.isFunction(), node);
    checkNotNull(scope.getRootNode());
    initTraversal(node);
    curNode = node.getParent();
    pushScope(scope, true /* quietly */);
    traverseBranch(node, curNode);
    popScope(true /* quietly */);
  }

  /**
   * Traverses an inner node recursively with a refined scope. An inner node may
   * be any node with a non {@code null} parent (i.e. all nodes except the
   * root).
   *
   * @param node the node to traverse
   * @param parent the node's parent, it may not be {@code null}
   * @param refinedScope the refined scope of the scope currently at the top of
   *     the scope stack or in trivial cases that very scope or {@code null}
   */
  void traverseInnerNode(Node node, Node parent, AbstractScope refinedScope) {
    checkNotNull(parent);
    initTraversal(node);
    if (refinedScope != null && getAbstractScope() != refinedScope) {
      curNode = node;
      pushScope(refinedScope);
      traverseBranch(node, parent);
      popScope();
    } else {
      traverseBranch(node, parent);
    }
  }

  public AbstractCompiler getCompiler() {
    return compiler;
  }

  /**
   * Gets the current line number, or zero if it cannot be determined. The line
   * number is retrieved lazily as a running time optimization.
   */
  public int getLineNumber() {
    Node cur = curNode;
    while (cur != null) {
      int line = cur.getLineno();
      if (line >= 0) {
        return line;
      }
      cur = cur.getParent();
    }
    return 0;
  }

  /**
   * Gets the current char number, or zero if it cannot be determined. The line
   * number is retrieved lazily as a running time optimization.
   */
  public int getCharno() {
    Node cur = curNode;
    while (cur != null) {
      int line = cur.getCharno();
      if (line >= 0) {
        return line;
      }
      cur = cur.getParent();
    }
    return 0;
  }

  /**
   * Gets the current input source name.
   *
   * @return A string that may be empty, but not null
   */
  public String getSourceName() {
    return sourceName;
  }

  /**
   * Gets the current input source.
   */
  public CompilerInput getInput() {
    if (compilerInput == null && inputId != null) {
      compilerInput = compiler.getInput(inputId);
    }
    return compilerInput;
  }

  /**
   * Gets the current input module.
   */
  public JSModule getModule() {
    CompilerInput input = getInput();
    return input == null ? null : input.getModule();
  }

  /** Returns the node currently being traversed. */
  public Node getCurrentNode() {
    return curNode;
  }

  /**
   * Traversal for passes that work only on changed functions.
   * Suppose a loopable pass P1 uses this traversal.
   * Then, if a function doesn't change between two runs of P1, it won't look at
   * the function the second time.
   * (We're assuming that P1 runs to a fixpoint, o/w we may miss optimizations.)
   *
   * 
Most changes are reported with calls to Compiler.reportCodeChange(), which
   * doesn't know which scope changed. We keep track of the current scope by
   * calling Compiler.setScope inside pushScope and popScope.
   * The automatic tracking can be wrong in rare cases when a pass changes scope
   * w/out causing a call to pushScope or popScope.
   *
   * Passes that do cross-scope modifications call
   * Compiler.reportChangeToEnclosingScope(Node n).
   */
  public static void traverseChangedFunctions(
      final AbstractCompiler compiler, final ChangeScopeRootCallback callback) {
    final Node jsRoot = compiler.getJsRoot();
    NodeTraversal.traverse(compiler, jsRoot,
        new AbstractPreOrderCallback() {
          @Override
          public final boolean shouldTraverse(NodeTraversal t, Node n, Node parent) {
            if (NodeUtil.isChangeScopeRoot(n) && compiler.hasScopeChanged(n)) {
              callback.enterChangeScopeRoot(compiler, n);
            }
            return true;
          }
        });
  }

  private void handleScript(Node n, Node parent) {
    if (Thread.interrupted()) {
      throw new RuntimeException(new InterruptedException());
    }
    setChangeScope(n);
    setInputId(n.getInputId(), getSourceName(n));

    curNode = n;
    curScript = n;
    if (callback.shouldTraverse(this, n, parent)) {
      traverseChildren(n);
      curNode = n;
      callback.visit(this, n, parent);
    }
    setChangeScope(null);
  }

  private void handleFunction(Node n, Node parent) {
    Node changeScope = this.currentChangeScope;
    setChangeScope(n);
    curNode = n;
    if (callback.shouldTraverse(this, n, parent)) {
      traverseFunction(n, parent);
      curNode = n;
      callback.visit(this, n, parent);
    }
    setChangeScope(changeScope);
  }

  /** Traverses a module. */
  private void handleModule(Node n, Node parent) {
    pushScope(n);
    curNode = n;
    if (callback.shouldTraverse(this, n, parent)) {
      curNode = n;
      traverseChildren(n);
      callback.visit(this, n, parent);
    }
    popScope();
  }

  /** Traverses a branch. */
  private void traverseBranch(Node n, Node parent) {
    switch (n.getToken()) {
      case SCRIPT:
        handleScript(n, parent);
        return;
      case FUNCTION:
        handleFunction(n, parent);
        return;
      case MODULE_BODY:
        handleModule(n, parent);
        return;
      case CLASS:
        handleClass(n, parent);
        return;
      case CLASS_MEMBERS:
        handleClassMembers(n, parent);
        return;
      default:
        break;
    }

    curNode = n;
    if (!callback.shouldTraverse(this, n, parent)) {
      return;
    }

    if (NodeUtil.createsBlockScope(n)) {
      pushScope(n);
      traverseChildren(n);
      popScope();
    } else {
      traverseChildren(n);
    }

    curNode = n;
    callback.visit(this, n, parent);
  }

  /** Traverses a function. */
  private void traverseFunction(Node n, Node parent) {
    final Node fnName = n.getFirstChild();
    // NOTE: If a function declaration is the root of a traversal, then we will treat it as a
    // function expression (since 'parent' is null, even though 'n' actually has a parent node) and
    // traverse the function name before entering the scope, rather than afterwards. Removing the
    // null check for 'parent' seems safe, but causes a rare crash when traverseScopeRoots is called
    // by PeepholeOptimizationsPass on a function that somehow doesn't actually have a parent at all
    // (presumably because it's already been removed from the AST?) so that doesn't actually work.
    // This does not actually change anything, though, since rooting a traversal at a function node
    // causes the function scope to be entered twice (unless using #traverseAtScope, which doesn't
    // call this method), so that the name node is just always traversed inside the scope anyway.
    boolean isFunctionDeclaration = parent != null && NodeUtil.isFunctionDeclaration(n);

    if (isFunctionDeclaration) {
      // Function declarations are in the scope containing the declaration.
      traverseBranch(fnName, n);
    }

    curNode = n;
    pushScope(n);

    if (!isFunctionDeclaration) {
      // Function expression names are only accessible within the function
      // scope.
      traverseBranch(fnName, n);
    }

    final Node args = fnName.getNext();
    final Node body = args.getNext();

    // Args
    traverseBranch(args, n);

    // Body
    // ES6 "arrow" function may not have a block as a body.
    traverseBranch(body, n);

    popScope();
  }

  /**
   * Traverses a class. Note that we traverse some of the child nodes slightly out of order to
   * ensure children are visited in the correct scope. The following children are in the outer
   * scope: (1) the 'extends' clause, (2) any computed method keys, (3) the class name for class
   * declarations only (class expression names are traversed in the class scope). This requires that
   * we visit the extends node (second child) and any computed member keys (grandchildren of the
   * last, body, child) before visiting the name (first child) or body (last child).
   */
  private void handleClass(Node n, Node parent) {
    this.curNode = n;
    if (!callback.shouldTraverse(this, n, parent)) {
      return;
    }

    final Node className = n.getFirstChild();
    final Node extendsClause = className.getNext();
    final Node body = extendsClause.getNext();

    boolean isClassExpression = NodeUtil.isClassExpression(n);

    traverseBranch(extendsClause, n);

    for (Node child = body.getFirstChild(); child != null;) {
      Node next = child.getNext(); // see traverseChildren
      if (child.isComputedProp()) {
        traverseBranch(child.getFirstChild(), child);
      }
      child = next;
    }

    if (!isClassExpression) {
      // Class declarations are in the scope containing the declaration.
      traverseBranch(className, n);
    }

    curNode = n;
    pushScope(n);

    if (isClassExpression) {
      // Class expression names are only accessible within the function
      // scope.
      traverseBranch(className, n);
    }

    // Body
    traverseBranch(body, n);

    popScope();

    this.curNode = n;
    callback.visit(this, n, parent);
  }

  /** Traverse class members, excluding keys of computed props. */
  private void handleClassMembers(Node n, Node parent) {
    this.curNode = n;
    if (!callback.shouldTraverse(this, n, parent)) {
      return;
    }

    for (Node child = n.getFirstChild(); child != null;) {
      Node next = child.getNext(); // see traverseChildren
      if (child.isComputedProp()) {
        curNode = n;
        if (callback.shouldTraverse(this, child, n)) {
          traverseBranch(child.getLastChild(), child);
          curNode = n;
          callback.visit(this, child, n);
        }
      } else {
        traverseBranch(child, n);
      }
      child = next;
    }

    this.curNode = n;
    callback.visit(this, n, parent);
  }

  private void traverseChildren(Node n) {
    for (Node child = n.getFirstChild(); child != null; ) {
      // child could be replaced, in which case our child node
      // would no longer point to the true next
      Node next = child.getNext();
      traverseBranch(child, n);
      child = next;
    }
  }

  /** Examines the functions stack for the last instance of a function node. When possible, prefer
   *  this method over NodeUtil.getEnclosingFunction() because this in general looks at less nodes.
   */
  public Node getEnclosingFunction() {
    Node root = getCfgRoot();
    return root.isFunction() ? root : null;
  }

  /** Sets the given node as the current scope and pushes the relevant frames on the CFG stacks. */
  private void recordScopeRoot(Node node) {
    if (NodeUtil.isValidCfgRoot(node)) {
      cfgs.push(node);
    }
  }

  /** Creates a new scope (e.g. when entering a function). */
  private void pushScope(Node node) {
    checkNotNull(curNode);
    checkNotNull(node);
    scopeRoots.add(node);
    recordScopeRoot(node);
    if (scopeCallback != null) {
      scopeCallback.enterScope(this);
    }
  }

  /** Creates a new scope (e.g. when entering a function). */
  private void pushScope(AbstractScope s) {
    pushScope(s, false);
  }

  /**
   * Creates a new scope (e.g. when entering a function).
   * @param quietly Don't fire an enterScope callback.
   */
  private void pushScope(AbstractScope s, boolean quietly) {
    checkNotNull(curNode);
    scopes.push(s);
    recordScopeRoot(s.getRootNode());
    if (!quietly && scopeCallback != null) {
      scopeCallback.enterScope(this);
    }
  }

  private void popScope() {
    popScope(false);
  }

  /**
   * Pops back to the previous scope (e.g. when leaving a function).
   * @param quietly Don't fire the exitScope callback.
   */
  private void popScope(boolean quietly) {
    if (!quietly && scopeCallback != null) {
      scopeCallback.exitScope(this);
    }
    Node scopeRoot;
    int roots = scopeRoots.size();
    if (roots > 0) {
      scopeRoot = scopeRoots.remove(roots - 1);
    } else {
      scopeRoot = scopes.pop().getRootNode();
    }
    if (NodeUtil.isValidCfgRoot(scopeRoot)) {
      cfgs.pop();
    }
  }

  /** Gets the current scope. */
  public AbstractScope getAbstractScope() {
    AbstractScope scope = scopes.peek();

    // NOTE(dylandavidson): Use for-each loop to avoid slow ArrayList#get performance.
    for (Node scopeRoot : scopeRoots) {
      scope = scopeCreator.createScope(scopeRoot, scope);
      scopes.push(scope);
    }
    scopeRoots.clear();
    return scope;
  }

  /**
   * Instantiate some, but not necessarily all, scopes from stored roots.
   *
   * 
NodeTraversal instantiates scopes lazily when getScope() or similar is called, by iterating
   * over a stored list of not-yet-instantiated scopeRoots.  When a not-yet-instantiated parent
   * scope is requested, it doesn't make sense to instantiate all pending scopes.  Instead,
   * we count the number that are needed to ensure the requested parent is instantiated and call
   * this function to instantiate only as many scopes as are needed, shifting their roots off the
   * queue, and returning the deepest scope actually created.
   */
  private AbstractScope instantiateScopes(int count) {
    checkArgument(count <= scopeRoots.size());
    AbstractScope scope = scopes.peek();

    for (int i = 0; i < count; i++) {
      scope = scopeCreator.createScope(scopeRoots.get(i), scope);
      scopes.push(scope);
    }
    scopeRoots.subList(0, count).clear();
    return scope;
  }

  public boolean isHoistScope() {
    return isHoistScopeRootNode(getScopeRoot());
  }

  public Node getClosestHoistScopeRoot() {
    int roots = scopeRoots.size();
    for (int i = roots; i > 0; i--) {
      Node rootNode = scopeRoots.get(i - 1);
      if (isHoistScopeRootNode(rootNode)) {
        return rootNode;
      }
    }

    return scopes.peek().getClosestHoistScope().getRootNode();
  }

  public AbstractScope getClosestContainerScope() {
    for (int i = scopeRoots.size(); i > 0; i--) {
      if (!NodeUtil.createsBlockScope(scopeRoots.get(i - 1))) {
        return instantiateScopes(i);
      }
    }
    return scopes.peek().getClosestContainerScope();
  }

  public AbstractScope getClosestHoistScope() {
    for (int i = scopeRoots.size(); i > 0; i--) {
      if (isHoistScopeRootNode(scopeRoots.get(i - 1))) {
        return instantiateScopes(i);
      }
    }
    return scopes.peek().getClosestHoistScope();
  }

  private static boolean isHoistScopeRootNode(Node n) {
    switch (n.getToken()) {
      case FUNCTION:
      case MODULE_BODY:
      case ROOT:
      case SCRIPT:
        return true;
      default:
        return NodeUtil.isFunctionBlock(n);
    }
  }

  public Scope getScope() {
    return getAbstractScope().untyped();
  }

  public TypedScope getTypedScope() {
    return getAbstractScope().typed();
  }

  /** Gets the control flow graph for the current JS scope. */
  @SuppressWarnings("unchecked")  // The type is always ControlFlowGraph
  public ControlFlowGraph getControlFlowGraph() {
    ControlFlowGraph result;
    Object o = cfgs.peek();
    if (o instanceof Node) {
      Node cfgRoot = (Node) o;
      ControlFlowAnalysis cfa = new ControlFlowAnalysis(compiler, false, true);
      cfa.process(null, cfgRoot);
      result = cfa.getCfg();
      cfgs.pop();
      cfgs.push(result);
    } else {
      result = (ControlFlowGraph) o;
    }
    return result;
  }

  /** Returns the current scope's root. */
  public Node getScopeRoot() {
    int roots = scopeRoots.size();
    if (roots > 0) {
      return scopeRoots.get(roots - 1);
    } else {
      AbstractScope s = scopes.peek();
      return s != null ? s.getRootNode() : null;
    }
  }

  @SuppressWarnings("unchecked") // The type is always ControlFlowGraph
  private Node getCfgRoot() {
    Node result;
    Object o = cfgs.peek();
    if (o instanceof Node) {
      result = (Node) o;
    } else {
      result = ((ControlFlowGraph) o).getEntry().getValue();
    }
    return result;
  }

  public ScopeCreator getScopeCreator() {
    return scopeCreator;
  }

  /**
   * Determines whether the traversal is currently in the global scope. Note that this returns false
   * in a global block scope.
   */
  public boolean inGlobalScope() {
    return getScopeDepth() == 0;
  }

  /**
   * Determines whether the traversal is currently in the global scope. Note that this returns false
   * in a global block scope.
   */
  public boolean inModuleScope() {
    return NodeUtil.isModuleScopeRoot(getScopeRoot());
  }

  public boolean inGlobalOrModuleScope() {
    return this.inGlobalScope() || inModuleScope();
  }

  /** Determines whether the traversal is currently in the scope of the block of a function. */
  public boolean inFunctionBlockScope() {
    return NodeUtil.isFunctionBlock(getScopeRoot());
  }

  /**
   * Determines whether the hoist scope of the current traversal is global.
   */
  public boolean inGlobalHoistScope() {
    Node cfgRoot = getCfgRoot();
    checkState(
        cfgRoot.isScript()
            || cfgRoot.isRoot()
            || cfgRoot.isBlock()
            || cfgRoot.isFunction()
            || cfgRoot.isModuleBody(),
        cfgRoot);
    return cfgRoot.isScript() || cfgRoot.isRoot() || cfgRoot.isBlock();
  }

  /**
   * Determines whether the hoist scope of the current traversal is global.
   */
  public boolean inModuleHoistScope() {
    Node moduleRoot = getCfgRoot();
    if (moduleRoot.isFunction()) {
      // For wrapped modules, the function block is the module scope root.
      moduleRoot = moduleRoot.getLastChild();
    }
    return NodeUtil.isModuleScopeRoot(moduleRoot);
  }

  int getScopeDepth() {
    int sum = scopes.size() + scopeRoots.size();
    checkState(sum > 0);
    return sum - 1; // Use 0-based scope depth to be consistent within the compiler
  }

  /** Reports a diagnostic (error or warning) */
  public void report(Node n, DiagnosticType diagnosticType,
      String... arguments) {
    JSError error = JSError.make(n, diagnosticType, arguments);
    compiler.report(error);
  }

  public void reportCodeChange() {
    Node changeScope = this.currentChangeScope;
    checkNotNull(changeScope);
    checkState(NodeUtil.isChangeScopeRoot(changeScope), changeScope);
    compiler.reportChangeToChangeScope(changeScope);
  }

  public void reportCodeChange(Node n) {
    compiler.reportChangeToEnclosingScope(n);
  }

  private static String getSourceName(Node n) {
    String name = n.getSourceFileName();
    return nullToEmpty(name);
  }

  /**
   * Returns the SCRIPT node enclosing the current scope, or `null` if unknown
   *
   * 
e.g. returns null if {@link #traverseInnerNode(Node, Node, AbstractScope)} was used
   */
  @Nullable
  Node getCurrentScript() {
    return curScript;
  }

  /**
   * @param n The current change scope, should be null when the traversal is complete.
   */
  private void setChangeScope(Node n) {
    this.currentChangeScope = n;
  }

  private Node getEnclosingScript(Node n) {
    while (n != null && !n.isScript()) {
      n = n.getParent();
    }
    return n;
  }

  private void initTraversal(Node traversalRoot) {
    if (Thread.interrupted()) {
      throw new RuntimeException(new InterruptedException());
    }
    Node changeScope = NodeUtil.getEnclosingChangeScopeRoot(traversalRoot);
    setChangeScope(changeScope);
    Node script = getEnclosingScript(changeScope);
    if (script != null) {
      setInputId(script.getInputId(), script.getSourceFileName());
    } else {
      setInputId(null, "");
    }
    curScript = script;
  }

  /**
   * Prefills the scopeRoots stack up to a given spot in the AST. Allows for starting traversal at
   * any spot while still having correct scope state.
   */
  private void initScopeRoots(Node n) {
    Deque queuedScopeRoots = new ArrayDeque<>();
    while (n != null) {
      if (isScopeRoot(n)) {
        queuedScopeRoots.addFirst(n);
      }
      n = n.getParent();
    }
    for (Node queuedScopeRoot : queuedScopeRoots) {
      pushScope(queuedScopeRoot);
    }
  }

  private boolean isScopeRoot(Node n) {
    if (n.isRoot() && n.getParent() == null) {
      return true;
    } else if (n.isFunction()) {
      return true;
    } else if (NodeUtil.createsBlockScope(n)) {
      return true;
    }
    return false;
  }

  private void setInputId(InputId id, String sourceName) {
    inputId = id;
    this.sourceName = sourceName;
    compilerInput = null;
  }

  InputId getInputId() {
    return inputId;
  }

  private String getBestSourceFileName(Node n) {
    return n == null ? sourceName : n.getSourceFileName();
  }
}
 















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