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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;

import com.google.common.base.Preconditions;
import com.google.common.collect.ImmutableList;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;

/**
 * Contains information on default externs files. Provide {@link DefaultExterns#prepareExterns} to
 * filter externs source files.
 */
public final class DefaultExterns {
  private DefaultExterns() {}

  // Core language externs. When the environment is CUSTOM, only these externs will be included.
  private static final ImmutableList BUILTIN_LANG_EXTERNS =
      ImmutableList.of("es3.js", "es5.js", "es6.js", "es6_collections.js");

  // Ordered browser externs. Externs not included in this list are added last.
  private static final ImmutableList BROWSER_EXTERN_DEP_ORDER =
      ImmutableList.of(
          "intl.js",
          "w3c_event.js",
          "w3c_event3.js",
          "gecko_event.js",
          "ie_event.js",
          "webkit_event.js",
          "w3c_device_sensor_event.js",
          "w3c_dom1.js",
          "w3c_dom2.js",
          "w3c_dom3.js",
          "w3c_dom4.js",
          "gecko_dom.js",
          "ie_dom.js",
          "webkit_dom.js",
          "w3c_css.js",
          "gecko_css.js",
          "ie_css.js",
          "webkit_css.js",
          "w3c_touch_event.js",
          "webstorage.js",
          "whatwg_console.js",
          "nonstandard_console.js",
          // w3d_rtc.js must be evaluated before nonstandard_rtc.js
          "w3c_rtc.js");

  /**
   * Filters and orders the passed externs for the specified environment.
   *
   * @param env The environment being used.
   * @param externs Flat tilename to source externs map. Must be mutable and will be modified.
   * @return Ordered list of externs.
   */
  public static List prepareExterns(CompilerOptions.Environment env,
        Map externs) {
    List out = new ArrayList<>();

    for (String key : BUILTIN_LANG_EXTERNS) {
      Preconditions.checkState(externs.containsKey(key), "Externs must contain builtin: %s", key);
      out.add(externs.remove(key));
    }

    if (env == CompilerOptions.Environment.BROWSER) {
      for (String key : BROWSER_EXTERN_DEP_ORDER) {
        Preconditions.checkState(externs.containsKey(key),
            "Externs must contain builtin for env %s: %s", env, key);
        out.add(externs.remove(key));
      }

      out.addAll(externs.values());
    }

    return out;
  }

}