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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 2014 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.deps;

import com.google.common.escape.ArrayBasedCharEscaper;
import com.google.common.escape.Escaper;

import java.util.HashMap;
import java.util.Map;

/**
 * A factory for Escaper instances used to escape strings for safe use in
 * various common programming languages.
 *
 * NOTE: This class is cribbed from the Guava libraries SourceCodeEscapers which
 * is not part of the current Guava release.
 */
public final class SourceCodeEscapers {
  private SourceCodeEscapers() {}

  // From: http://en.wikipedia.org/wiki/ASCII#ASCII_printable_characters
  private static final char PRINTABLE_ASCII_MIN = 0x20;  // ' '
  private static final char PRINTABLE_ASCII_MAX = 0x7E;  // '~'

  private static final char[] HEX_DIGITS = "0123456789abcdef".toCharArray();

  /**
   * Returns an {@link Escaper} instance that replaces non-ASCII characters
   * in a string with their equivalent Javascript UTF-16 escape sequences
   * "{@literal \}unnnn", "\xnn" or special replacement sequences "\b", "\t",
   * "\n", "\f", "\r" or "\\".
   *
   * Warning: This escaper is not suitable for JSON. JSON users
   * may wish to use GSON or
   * other high-level APIs when possible.
   */
  public static Escaper javascriptEscaper() {
    return JAVASCRIPT_ESCAPER;
  }

  /**
   * An Escaper for javascript strings. Turns all non-ASCII characters into
   * ASCII javascript escape sequences.
   */
  private static final Escaper JAVASCRIPT_ESCAPER;
  static {
    Map jsMap = new HashMap<>();
    jsMap.put('\'', "\\x27");
    jsMap.put('"',  "\\x22");
    jsMap.put('<',  "\\x3c");
    jsMap.put('=',  "\\x3d");
    jsMap.put('>',  "\\x3e");
    jsMap.put('&',  "\\x26");
    jsMap.put('\b', "\\b");
    jsMap.put('\t', "\\t");
    jsMap.put('\n', "\\n");
    jsMap.put('\f', "\\f");
    jsMap.put('\r', "\\r");
    jsMap.put('\\', "\\\\");
    JAVASCRIPT_ESCAPER = new ArrayBasedCharEscaper(
        jsMap, PRINTABLE_ASCII_MIN, PRINTABLE_ASCII_MAX) {
          @Override
          protected char[] escapeUnsafe(char c) {
            // Do two digit hex escape for value less than 0x100.
            if (c < 0x100) {
              char[] r = new char[4];
              r[3] = HEX_DIGITS[c & 0xF];
              c = (char) (c >>> 4);
              r[2] = HEX_DIGITS[c & 0xF];
              r[1] = 'x';
              r[0] = '\\';
              return r;
            }
            return asUnicodeHexEscape(c);
          }
    };
  }

  // Helper for common case of escaping a single char.
  private static char[] asUnicodeHexEscape(char c) {
    // Equivalent to String.format("\\u%04x", (int) c);
    char[] r = new char[6];
    r[0] = '\\';
    r[1] = 'u';
    r[5] = HEX_DIGITS[c & 0xF];
    c = (char) (c >>> 4);
    r[4] = HEX_DIGITS[c & 0xF];
    c = (char) (c >>> 4);
    r[3] = HEX_DIGITS[c & 0xF];
    c = (char) (c >>> 4);
    r[2] = HEX_DIGITS[c & 0xF];
    return r;
  }
}