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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 2016 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.lint;

import com.google.javascript.jscomp.AbstractCompiler;
import com.google.javascript.jscomp.CompilerPass;
import com.google.javascript.jscomp.DiagnosticType;
import com.google.javascript.jscomp.NodeTraversal;
import com.google.javascript.rhino.Node;

/**
 * Check for unused labels blocks. This can help catching errors like:
 *
 *  *   () => {a: 2}  // Returns undefined, not an Object
 * 
 *
 * Inspired by ESLint
 * (https://github.com/eslint/eslint/blob/master/lib/rules/no-unused-labels.js)
 */
public final class CheckUnusedLabels implements NodeTraversal.Callback, CompilerPass {
  public static final DiagnosticType UNUSED_LABEL = DiagnosticType.disabled(
      "JSC_UNUSED_LABEL", "Unused label {0}.");

  private static class LabelContext {
    private final String name;
    private final LabelContext parent;
    private boolean used;

    private LabelContext(String name, LabelContext parent) {
      this.name = name;
      this.parent = parent;
    }
  }

  private final AbstractCompiler compiler;
  private LabelContext currentContext;

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

  @Override
  public void process(Node externs, Node root) {
    NodeTraversal.traverse(compiler, root, this);
  }

  @Override
  public final boolean shouldTraverse(NodeTraversal t, Node n, Node parent) {
    switch (n.getToken()) {
      case BREAK:
      case CONTINUE:
        if (n.hasChildren()) {
          LabelContext temp = currentContext;
          while (temp != null) {
            if (temp.name.equals(n.getFirstChild().getString())) {
              temp.used = true;
              break;
            }
            temp = temp.parent;
          }
        }
        return false;
      case LABEL:
        currentContext = new LabelContext(n.getFirstChild().getString(), currentContext);
        break;
      default:
        break;
    }
    return true;
  }

  @Override
  public void visit(NodeTraversal t, Node n, Node parent) {
    if (n.isLabel() && currentContext != null) {
      if (!currentContext.used) {
        t.report(n, UNUSED_LABEL, n.getFirstChild().getString());
      }
      currentContext = currentContext.parent;
    }
  }
}