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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.collect.ImmutableMap;
import com.google.common.escape.ArrayBasedCharEscaper;
import com.google.common.escape.Escaper;
import java.io.IOException;

/**
 * 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. https://github.com/google/guava/issues/1620
 */
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();

  private static final JavaScriptEscaper JAVASCRIPT_ESCAPER = new JavaScriptEscaper();

  /**
   * 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;
  }

  /**
   * Uses {@link #javascriptEscaper()} to append the escaped transformation of {@code c} to {@code
   * to}. Using this method can be more memory efficient than calling {@link
   * Escaper#escape(String)}.
   */
  public static void appendWithJavascriptEscaper(CharSequence c, Appendable to) throws IOException {
    JAVASCRIPT_ESCAPER.appendTo(c, to);
  }

  /**
   * An Escaper for javascript strings. Turns all non-ASCII characters into ASCII javascript escape
   * sequences.
   */
  private static final class JavaScriptEscaper extends ArrayBasedCharEscaper {
    private static final ImmutableMap JS_MAP =
        ImmutableMap.builder()
            .put('\'', "\\x27")
            .put('"', "\\x22")
            .put('<', "\\x3c")
            .put('=', "\\x3d")
            .put('>', "\\x3e")
            .put('&', "\\x26")
            .put('\b', "\\b")
            .put('\t', "\\t")
            .put('\n', "\\n")
            .put('\f', "\\f")
            .put('\r', "\\r")
            .put('\\', "\\\\")
            .buildOrThrow();

    JavaScriptEscaper() {
      super(JS_MAP, 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);
    }

    /**
     * Append the escaped transformation of {@code c} to {@code to}.
     *
     * {@link Escaper#escape(String)} has to build and return a new string, which is redundant if
     * we're just going to pass that string on to an {@link Appendable} anyway. This method just
     * appends the possibly-transformed characters one at a time to avoid the double memory
     * allocation.
     */
    void appendTo(CharSequence c, Appendable to) throws IOException {
      int l = c.length();
      for (int i = 0; i < l; i++) {
        char unescaped = c.charAt(i);
        // #escape(char) is protected so this must be done in a subclass.
        char[] escaped = escape(unescaped);
        if (escaped != null) {
          for (char value : escaped) {
            to.append(value);
          }
        } else {
          to.append(unescaped);
        }
      }
    }
  }

  // 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;
  }
}