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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.jscomp.NodeTraversal.AbstractPostOrderCallback;
import com.google.javascript.rhino.Node;
import java.util.HashSet;
import java.util.Set;

/**
 * Check for duplicate case labels in a switch statement Eg: switch (foo) { case 1: case 1: }
 *
 * This is normally an indication of a programmer error.
 *
 * Inspired by ESLint
 * (https://github.com/eslint/eslint/blob/master/lib/rules/no-duplicate-case.js)
 */
public final class CheckDuplicateCase extends AbstractPostOrderCallback implements CompilerPass {
  public static final DiagnosticType DUPLICATE_CASE = DiagnosticType.warning(
      "JSC_DUPLICATE_CASE", "Duplicate case in a switch statement.");

  private final AbstractCompiler compiler;

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

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

  @Override
  public void visit(NodeTraversal t, Node n, Node parent) {
    if (n.isSwitch()) {
      Set cases = new HashSet<>();
      for (Node curr = n.getSecondChild(); curr != null; curr = curr.getNext()) {
        String source = compiler.toSource(curr.getFirstChild());
        if (!cases.add(source)) {
          t.report(curr, DUPLICATE_CASE);
        }
      }
    }
  }
}