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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;

/**
 * A simple utility for shortening identifiers in a stable way. Generates
 * short substitution strings deterministically, using a compact
 * (1 to 6 characters in length) repesentation of a 32-bit hash of the key.
 * The string is suitable to be used as a JavaScript or CSS identifier.
 * Collisions are possible but unlikely, depending on the underlying hash algorithm used.
 *
 * This substitution scheme uses case-sensitive names for maximum
 * compression. Digits are also allowed in all but the first character of a
 * class name. There are a few characters allowed by the CSS grammar that we
 * choose not to use (e.g. the underscore and hyphen), to keep names simple.
 *
 * Xid should maintain as minimal dependencies as possible to ease its
 * integration with other tools, such as server side HTML generators.
 */
public class Xid {

  /** Possible first chars in an identifier */
  private static final String START_CHARS =
      "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";

  /** Possible non-first chars in an identifier */
  private static final String CHARS = START_CHARS + "0123456789";

  private static final int START_RADIX = START_CHARS.length();
  private static final int RADIX = CHARS.length();

  private final HashFunction hasher;

  /**
   * Strategy for selecting the underlying hash code function to be used by Xid.
   */
  public interface HashFunction {
    int hashCode(String value);
  }

  private static final HashFunction DEFAULT = new HashFunction() {
    @Override public int hashCode(String value) {
      return value.hashCode();
    }
  };

  public Xid() {
    this.hasher = DEFAULT;
  }

  public Xid(HashFunction hasher) {
    this.hasher = hasher;
  }

  /**
   * Gets the string that should be substituted for {@code key}. The same value will be
   * consistently returned for any particular {@code key}.
   *
   * @param key  the text to be replaced (never null)
   * @return the value to substitute for {@code key}
   */
  public String get(String key) {
    return toString(getAsInt(key));
  }

  /**
   * Returns the underlying integer representation of the given key.
   */
  public int getAsInt(String key) {
    return this.hasher.hashCode(key);
  }

  /**
   * Converts a 32-bit integer to a unique short string whose first character
   * is in {@link #START_CHARS} and whose subsequent characters, if any, are
   * in {@link #CHARS}. The result is 1-6 characters in length.
   */
  static String toString(int i) {
    char[] buf = new char[6];
    int len = 0;

    long l = i - (long) Integer.MIN_VALUE;
    buf[len++] = START_CHARS.charAt((int) (l % START_RADIX));
    i = (int) (l / START_RADIX);

    while (i > 0) {
      buf[len++] = CHARS.charAt(i % RADIX);
      i /= RADIX;
    }

    return new String(buf, 0, len);
  }
}