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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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/*
 *
 * ***** BEGIN LICENSE BLOCK *****
 * Version: MPL 1.1/GPL 2.0
 *
 * The contents of this file are subject to the Mozilla Public License Version
 * 1.1 (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.mozilla.org/MPL/
 *
 * Software distributed under the License is distributed on an "AS IS" basis,
 * WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
 * for the specific language governing rights and limitations under the
 * License.
 *
 * The Original Code is Rhino code, released
 * May 6, 1999.
 *
 * The Initial Developer of the Original Code is
 * Netscape Communications Corporation.
 * Portions created by the Initial Developer are Copyright (C) 1997-1999
 * the Initial Developer. All Rights Reserved.
 *
 * Contributor(s):
 *   Google Inc.
 *
 * Alternatively, the contents of this file may be used under the terms of
 * the GNU General Public License Version 2 or later (the "GPL"), in which
 * case the provisions of the GPL are applicable instead of those above. If
 * you wish to allow use of your version of this file only under the terms of
 * the GPL and not to allow others to use your version of this file under the
 * MPL, indicate your decision by deleting the provisions above and replacing
 * them with the notice and other provisions required by the GPL. If you do
 * not delete the provisions above, a recipient may use your version of this
 * file under either the MPL or the GPL.
 *
 * ***** END LICENSE BLOCK ***** */

package com.google.javascript.rhino;

import static com.google.javascript.jscomp.base.JSCompObjects.identical;

import com.google.common.collect.Interner;
import com.google.common.collect.Interners;

/**
 * An interning pool for strings used by the Rhino package.
 *
 * As of 2021-03-16, this custom pool is measurably more performant than `String::intern`. Some
 * reasons for this are presented at https://shipilev.net/jvm/anatomy-quarks/10-string-intern/.
 */
public final class RhinoStringPool {

  /**
   * The threadsafe datastructure that backs this pool.
   *
   * 
We use weak-refs, rather than strong-refs, to prevent a memory leak in server-like
   * applications.
   */
  private static final Interner INTERNER = Interners.newWeakInterner();

  /**
   * Check if two strings are the same according to interning.
   *
   * 
The caller is responsible for ensuring that strings passed to this method are actually
   * interned. This method mainly exists to highlight where equality checks depend on interning for
   * correctness.
   *
   * Ideally we could use something like a branded-type, for interned strings, to verify correct
   * usage. However, Java doesn't support type-brands, and using like wrapper objects would
   * undermine performance. This also needs to be ergonomic enough that callers don't resort to
   * using `==` directly.
   */
  public static boolean uncheckedEquals(String a, String b) {
    return identical(a, b);
  }

  static String addOrGet(String s) {
    return INTERNER.intern(s);
  }

  private RhinoStringPool() {}
}