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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 2014 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.collect.HashBasedTable;
import com.google.javascript.jscomp.parsing.parser.FeatureSet;
import com.google.javascript.jscomp.parsing.parser.FeatureSet.Feature;
import com.google.javascript.rhino.Node;
import java.util.HashSet;
import java.util.Map;
import java.util.Set;

/**
 * Renames variables declared in function bodies so they don't shadow any variable referenced in the
 * param list.
 *
 * This transformation prevents name collisions when executable code from the param list is moved
 * into the function body.
 */
public final class Es6RenameVariablesInParamLists
    implements NodeTraversal.ScopedCallback, CompilerPass {

  private final AbstractCompiler compiler;
  private static final FeatureSet transpiledFeatures =
      FeatureSet.BARE_MINIMUM.with(Feature.DEFAULT_PARAMETERS, Feature.COMPUTED_PROPERTIES);

  public Es6RenameVariablesInParamLists(AbstractCompiler compiler) {
    this.compiler = compiler;
  }

  @Override
  public boolean shouldTraverse(NodeTraversal t, Node n, Node parent) {
    return true;
  }

  @Override
  public void enterScope(NodeTraversal t) {}

  @Override
  public void exitScope(NodeTraversal t) {
    Node block = t.getScopeRoot();
    Node function = block.getParent();
    if (!block.isBlock() || !function.isFunction()) {
      return;
    }

    Node paramList = function.getSecondChild();
    final CollectReferences collector = new CollectReferences();
    NodeTraversal.traverse(compiler, paramList, new NodeTraversal.AbstractPreOrderCallback() {
      @Override
      public final boolean shouldTraverse(NodeTraversal t, Node n, Node parent) {
        if (parent == null) {
          return true;
        }

        if ((parent.isDefaultValue() && n == parent.getLastChild())
            || (parent.isComputedProp() && n == parent.getFirstChild())) {
          NodeTraversal.traverse(compiler, n, collector);
          return false;
        }
        return true;
      }
    });

    Scope blockScope = t.getScope();
    HashBasedTable renameTable = HashBasedTable.create();
    Map renameRow = renameTable.row(block);
    for (Var var : blockScope.getVarIterable()) {
      String oldName = var.getName();
      if (collector.currFuncReferences.contains(oldName)) {
        renameRow.computeIfAbsent(
            oldName, (x) -> oldName + "$" + compiler.getUniqueNameIdSupplier().get());
      }
    }

    // TODO(nickreid): This could probably be reowrked into a single traversal if the renameTable
    // were updated dynamically during enterScope.
    NodeTraversal.builder()
        .setCompiler(compiler)
        .setScopeCreator(t.getScopeCreator())
        .setCallback(new Es6RenameReferences(renameTable))
        .traverseAtScope(blockScope);
  }

  @Override
  public void visit(NodeTraversal t, Node block, Node function) {}

  @Override
  public void process(Node externs, Node root) {
    TranspilationPasses.processTranspile(compiler, root, transpiledFeatures, this);
  }

  /**
   * Collects all references in a naive way.
   */
  private static class CollectReferences extends NodeTraversal.AbstractPostOrderCallback {

    private final Set currFuncReferences = new HashSet<>();

    @Override
    public void visit(NodeTraversal t, Node n, Node parent) {
      if (!NodeUtil.isReferenceName(n)) {
        return;
      }
      currFuncReferences.add(n.getString());
    }
  }
}