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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 2011 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.debugging.sourcemap;

import java.io.IOException;

public class Util {

  private static final char[] HEX_CHARS
      = { '0', '1', '2', '3', '4', '5', '6', '7',
          '8', '9', 'a', 'b', 'c', 'd', 'e', 'f' };

  /**
   * Escapes the given string to a double quoted (") JavaScript/JSON string
   */
  static String escapeString(String s) {
    final char quote = '"';
    final String doublequoteEscape = "\\\"";
    final String singlequoteEscape = "\'";
    final String backslashEscape = "\\\\";

    StringBuilder sb = new StringBuilder(s.length() + 2);
    sb.append(quote);

    final class UnescapedRegion {
      int unescapedRegionStart = 0;
      int unescapedRegionEnd = 0;

      void appendUnescaped() {
        if (unescapedRegionStart != unescapedRegionEnd) {
          // Note: we want to use the "append(String)" override
          // for performance reasons and the ErrorProne suggestion
          // to use the CharSequence override is inferior for that.
          sb.append(s.substring(unescapedRegionStart, unescapedRegionEnd));
        }
        unescapedRegionStart = unescapedRegionEnd;
      }

      void incrementForNormalChar() {
        unescapedRegionEnd++;
      }

      void incrementForEscapedChar() {
        if (unescapedRegionStart != unescapedRegionEnd) {
          throw new IllegalStateException();
        }
        unescapedRegionStart++;
        unescapedRegionEnd++;
      }

      void appendForEscapedChar(String escaped) {
        this.appendUnescaped();
        this.incrementForEscapedChar();
        sb.append(escaped);
      }
    }

    UnescapedRegion region = new UnescapedRegion();

    int length = s.length();
    for (int i = 0; i < length; i++) {
      char c = s.charAt(i);
      switch (c) {
        case '\n':
          region.appendForEscapedChar("\\n");
          break;
        case '\r':
          region.appendForEscapedChar("\\r");
          break;
        case '\t':
          region.appendForEscapedChar("\\t");
          break;
        case '\\':
          region.appendForEscapedChar(backslashEscape);
          break;
        case '\"':
          region.appendForEscapedChar(doublequoteEscape);
          break;
        case '\'':
          region.appendForEscapedChar(singlequoteEscape);
          break;
        case '>':
          // Unicode-escape the '>' in '-->' and ']]>'
          if (i >= 2
              && ((s.charAt(i - 1) == '-' && s.charAt(i - 2) == '-')
                  || (s.charAt(i - 1) == ']' && s.charAt(i - 2) == ']'))) {
            region.appendForEscapedChar("\\u003e");
          } else {
            region.incrementForNormalChar();
          }
          break;
        case '<':
          // Unicode-escape the '<' in ' 0x1f && c <= 0x7f) {
            region.incrementForNormalChar();
          } else {
            // Other characters can be misinterpreted by some JS parsers,
            // or perhaps mangled by proxies along the way,
            // so we play it safe and Unicode escape them.
            region.appendUnescaped();
            region.incrementForEscapedChar();
            appendHexJavaScriptRepresentation(sb, c);
          }
      }
    }
    region.appendUnescaped();
    sb.append(quote);
    return sb.toString();
  }

  /** @see #appendHexJavaScriptRepresentation(int, Appendable) */
  public static void appendHexJavaScriptRepresentation(StringBuilder sb, char c) {
    try {
      appendHexJavaScriptRepresentation(c, sb);
    } catch (IOException ex) {
      // StringBuilder does not throw IOException.
      throw new RuntimeException(ex);
    }
  }

  /**
   * Returns a JavaScript representation of the character in a hex escaped
   * format.
   * @param codePoint The code point to append.
   * @param out The buffer to which the hex representation should be appended.
   */
  private static void appendHexJavaScriptRepresentation(
      int codePoint, Appendable out)
      throws IOException {
    if (Character.isSupplementaryCodePoint(codePoint)) {
      // Handle supplementary Unicode values which are not representable in
      // JavaScript.  We deal with these by escaping them as two 4B sequences
      // so that they will round-trip properly when sent from Java to JavaScript
      // and back.
      char[] surrogates = Character.toChars(codePoint);
      appendHexJavaScriptRepresentation(surrogates[0], out);
      appendHexJavaScriptRepresentation(surrogates[1], out);
      return;
    }
    out.append("\\u")
        .append(HEX_CHARS[(codePoint >>> 12) & 0xf])
        .append(HEX_CHARS[(codePoint >>> 8) & 0xf])
        .append(HEX_CHARS[(codePoint >>> 4) & 0xf])
        .append(HEX_CHARS[codePoint & 0xf]);
  }

  private Util() {}
}