#region License
/*
 * HttpUtility.cs
 *
 * This code is derived from System.Net.HttpUtility.cs of Mono
 * (http://www.mono-project.com).
 *
 * The MIT License
 *
 * Copyright (c) 2005-2009 Novell, Inc. (http://www.novell.com)
 * Copyright (c) 2012-2014 sta.blockhead
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 */
#endregion

#region Authors
/*
 * Authors:
 * - Patrik Torstensson <Patrik.Torstensson@labs2.com>
 * - Wictor Wil√©n (decode/encode functions) <wictor@ibizkit.se>
 * - Tim Coleman <tim@timcoleman.com>
 * - Gonzalo Paniagua Javier <gonzalo@ximian.com>
 */
#endregion

using System;
using System.Collections;
using System.Collections.Generic;
using System.Collections.Specialized;
using System.Globalization;
using System.IO;
using System.Text;

namespace WebSocketSharp.Net
{
  internal sealed class HttpUtility
  {
    #region Private Static Fields

    private static Dictionary<string, char> _entities;
    private static char []                  _hexChars = "0123456789abcdef".ToCharArray ();
    private static object                   _sync = new object ();

    #endregion

    #region Private Static Properties

    private static Dictionary<string, char> Entities {
      get {
        lock (_sync) {
          if (_entities == null)
            initEntities ();

          return _entities;
        }
      }
    }

    #endregion

    #region Private Methods

    private static int getChar (byte [] bytes, int offset, int length)
    {
      var value = 0;
      var end = length + offset;
      for (int i = offset; i < end; i++) {
        var current = getInt (bytes [i]);
        if (current == -1)
          return -1;

        value = (value << 4) + current;
      }

      return value;
    }

    private static int getChar (string s, int offset, int length)
    {
      var value = 0;
      var end = length + offset;
      for (int i = offset; i < end; i++) {
        var c = s [i];
        if (c > 127)
          return -1;

        var current = getInt ((byte) c);
        if (current == -1)
          return -1;

        value = (value << 4) + current;
      }

      return value;
    }

    private static char [] getChars (MemoryStream buffer, Encoding encoding)
    {
      return encoding.GetChars (buffer.GetBuffer (), 0, (int) buffer.Length);
    }

    private static int getInt (byte b)
    {
      var c = (char) b;
      return c >= '0' && c <= '9'
             ? c - '0'
             : c >= 'a' && c <= 'f'
               ? c - 'a' + 10
               : c >= 'A' && c <= 'F'
                 ? c - 'A' + 10
                 : -1;
    }

    private static void initEntities ()
    {
      // Build the dictionary of HTML entity references.
      // This list comes from the HTML 4.01 W3C recommendation.
      _entities = new Dictionary<string, char> ();
      _entities.Add ("nbsp", '\u00A0');
      _entities.Add ("iexcl", '\u00A1');
      _entities.Add ("cent", '\u00A2');
      _entities.Add ("pound", '\u00A3');
      _entities.Add ("curren", '\u00A4');
      _entities.Add ("yen", '\u00A5');
      _entities.Add ("brvbar", '\u00A6');
      _entities.Add ("sect", '\u00A7');
      _entities.Add ("uml", '\u00A8');
      _entities.Add ("copy", '\u00A9');
      _entities.Add ("ordf", '\u00AA');
      _entities.Add ("laquo", '\u00AB');
      _entities.Add ("not", '\u00AC');
      _entities.Add ("shy", '\u00AD');
      _entities.Add ("reg", '\u00AE');
      _entities.Add ("macr", '\u00AF');
      _entities.Add ("deg", '\u00B0');
      _entities.Add ("plusmn", '\u00B1');
      _entities.Add ("sup2", '\u00B2');
      _entities.Add ("sup3", '\u00B3');
      _entities.Add ("acute", '\u00B4');
      _entities.Add ("micro", '\u00B5');
      _entities.Add ("para", '\u00B6');
      _entities.Add ("middot", '\u00B7');
      _entities.Add ("cedil", '\u00B8');
      _entities.Add ("sup1", '\u00B9');
      _entities.Add ("ordm", '\u00BA');
      _entities.Add ("raquo", '\u00BB');
      _entities.Add ("frac14", '\u00BC');
      _entities.Add ("frac12", '\u00BD');
      _entities.Add ("frac34", '\u00BE');
      _entities.Add ("iquest", '\u00BF');
      _entities.Add ("Agrave", '\u00C0');
      _entities.Add ("Aacute", '\u00C1');
      _entities.Add ("Acirc", '\u00C2');
      _entities.Add ("Atilde", '\u00C3');
      _entities.Add ("Auml", '\u00C4');
      _entities.Add ("Aring", '\u00C5');
      _entities.Add ("AElig", '\u00C6');
      _entities.Add ("Ccedil", '\u00C7');
      _entities.Add ("Egrave", '\u00C8');
      _entities.Add ("Eacute", '\u00C9');
      _entities.Add ("Ecirc", '\u00CA');
      _entities.Add ("Euml", '\u00CB');
      _entities.Add ("Igrave", '\u00CC');
      _entities.Add ("Iacute", '\u00CD');
      _entities.Add ("Icirc", '\u00CE');
      _entities.Add ("Iuml", '\u00CF');
      _entities.Add ("ETH", '\u00D0');
      _entities.Add ("Ntilde", '\u00D1');
      _entities.Add ("Ograve", '\u00D2');
      _entities.Add ("Oacute", '\u00D3');
      _entities.Add ("Ocirc", '\u00D4');
      _entities.Add ("Otilde", '\u00D5');
      _entities.Add ("Ouml", '\u00D6');
      _entities.Add ("times", '\u00D7');
      _entities.Add ("Oslash", '\u00D8');
      _entities.Add ("Ugrave", '\u00D9');
      _entities.Add ("Uacute", '\u00DA');
      _entities.Add ("Ucirc", '\u00DB');
      _entities.Add ("Uuml", '\u00DC');
      _entities.Add ("Yacute", '\u00DD');
      _entities.Add ("THORN", '\u00DE');
      _entities.Add ("szlig", '\u00DF');
      _entities.Add ("agrave", '\u00E0');
      _entities.Add ("aacute", '\u00E1');
      _entities.Add ("acirc", '\u00E2');
      _entities.Add ("atilde", '\u00E3');
      _entities.Add ("auml", '\u00E4');
      _entities.Add ("aring", '\u00E5');
      _entities.Add ("aelig", '\u00E6');
      _entities.Add ("ccedil", '\u00E7');
      _entities.Add ("egrave", '\u00E8');
      _entities.Add ("eacute", '\u00E9');
      _entities.Add ("ecirc", '\u00EA');
      _entities.Add ("euml", '\u00EB');
      _entities.Add ("igrave", '\u00EC');
      _entities.Add ("iacute", '\u00ED');
      _entities.Add ("icirc", '\u00EE');
      _entities.Add ("iuml", '\u00EF');
      _entities.Add ("eth", '\u00F0');
      _entities.Add ("ntilde", '\u00F1');
      _entities.Add ("ograve", '\u00F2');
      _entities.Add ("oacute", '\u00F3');
      _entities.Add ("ocirc", '\u00F4');
      _entities.Add ("otilde", '\u00F5');
      _entities.Add ("ouml", '\u00F6');
      _entities.Add ("divide", '\u00F7');
      _entities.Add ("oslash", '\u00F8');
      _entities.Add ("ugrave", '\u00F9');
      _entities.Add ("uacute", '\u00FA');
      _entities.Add ("ucirc", '\u00FB');
      _entities.Add ("uuml", '\u00FC');
      _entities.Add ("yacute", '\u00FD');
      _entities.Add ("thorn", '\u00FE');
      _entities.Add ("yuml", '\u00FF');
      _entities.Add ("fnof", '\u0192');
      _entities.Add ("Alpha", '\u0391');
      _entities.Add ("Beta", '\u0392');
      _entities.Add ("Gamma", '\u0393');
      _entities.Add ("Delta", '\u0394');
      _entities.Add ("Epsilon", '\u0395');
      _entities.Add ("Zeta", '\u0396');
      _entities.Add ("Eta", '\u0397');
      _entities.Add ("Theta", '\u0398');
      _entities.Add ("Iota", '\u0399');
      _entities.Add ("Kappa", '\u039A');
      _entities.Add ("Lambda", '\u039B');
      _entities.Add ("Mu", '\u039C');
      _entities.Add ("Nu", '\u039D');
      _entities.Add ("Xi", '\u039E');
      _entities.Add ("Omicron", '\u039F');
      _entities.Add ("Pi", '\u03A0');
      _entities.Add ("Rho", '\u03A1');
      _entities.Add ("Sigma", '\u03A3');
      _entities.Add ("Tau", '\u03A4');
      _entities.Add ("Upsilon", '\u03A5');
      _entities.Add ("Phi", '\u03A6');
      _entities.Add ("Chi", '\u03A7');
      _entities.Add ("Psi", '\u03A8');
      _entities.Add ("Omega", '\u03A9');
      _entities.Add ("alpha", '\u03B1');
      _entities.Add ("beta", '\u03B2');
      _entities.Add ("gamma", '\u03B3');
      _entities.Add ("delta", '\u03B4');
      _entities.Add ("epsilon", '\u03B5');
      _entities.Add ("zeta", '\u03B6');
      _entities.Add ("eta", '\u03B7');
      _entities.Add ("theta", '\u03B8');
      _entities.Add ("iota", '\u03B9');
      _entities.Add ("kappa", '\u03BA');
      _entities.Add ("lambda", '\u03BB');
      _entities.Add ("mu", '\u03BC');
      _entities.Add ("nu", '\u03BD');
      _entities.Add ("xi", '\u03BE');
      _entities.Add ("omicron", '\u03BF');
      _entities.Add ("pi", '\u03C0');
      _entities.Add ("rho", '\u03C1');
      _entities.Add ("sigmaf", '\u03C2');
      _entities.Add ("sigma", '\u03C3');
      _entities.Add ("tau", '\u03C4');
      _entities.Add ("upsilon", '\u03C5');
      _entities.Add ("phi", '\u03C6');
      _entities.Add ("chi", '\u03C7');
      _entities.Add ("psi", '\u03C8');
      _entities.Add ("omega", '\u03C9');
      _entities.Add ("thetasym", '\u03D1');
      _entities.Add ("upsih", '\u03D2');
      _entities.Add ("piv", '\u03D6');
      _entities.Add ("bull", '\u2022');
      _entities.Add ("hellip", '\u2026');
      _entities.Add ("prime", '\u2032');
      _entities.Add ("Prime", '\u2033');
      _entities.Add ("oline", '\u203E');
      _entities.Add ("frasl", '\u2044');
      _entities.Add ("weierp", '\u2118');
      _entities.Add ("image", '\u2111');
      _entities.Add ("real", '\u211C');
      _entities.Add ("trade", '\u2122');
      _entities.Add ("alefsym", '\u2135');
      _entities.Add ("larr", '\u2190');
      _entities.Add ("uarr", '\u2191');
      _entities.Add ("rarr", '\u2192');
      _entities.Add ("darr", '\u2193');
      _entities.Add ("harr", '\u2194');
      _entities.Add ("crarr", '\u21B5');
      _entities.Add ("lArr", '\u21D0');
      _entities.Add ("uArr", '\u21D1');
      _entities.Add ("rArr", '\u21D2');
      _entities.Add ("dArr", '\u21D3');
      _entities.Add ("hArr", '\u21D4');
      _entities.Add ("forall", '\u2200');
      _entities.Add ("part", '\u2202');
      _entities.Add ("exist", '\u2203');
      _entities.Add ("empty", '\u2205');
      _entities.Add ("nabla", '\u2207');
      _entities.Add ("isin", '\u2208');
      _entities.Add ("notin", '\u2209');
      _entities.Add ("ni", '\u220B');
      _entities.Add ("prod", '\u220F');
      _entities.Add ("sum", '\u2211');
      _entities.Add ("minus", '\u2212');
      _entities.Add ("lowast", '\u2217');
      _entities.Add ("radic", '\u221A');
      _entities.Add ("prop", '\u221D');
      _entities.Add ("infin", '\u221E');
      _entities.Add ("ang", '\u2220');
      _entities.Add ("and", '\u2227');
      _entities.Add ("or", '\u2228');
      _entities.Add ("cap", '\u2229');
      _entities.Add ("cup", '\u222A');
      _entities.Add ("int", '\u222B');
      _entities.Add ("there4", '\u2234');
      _entities.Add ("sim", '\u223C');
      _entities.Add ("cong", '\u2245');
      _entities.Add ("asymp", '\u2248');
      _entities.Add ("ne", '\u2260');
      _entities.Add ("equiv", '\u2261');
      _entities.Add ("le", '\u2264');
      _entities.Add ("ge", '\u2265');
      _entities.Add ("sub", '\u2282');
      _entities.Add ("sup", '\u2283');
      _entities.Add ("nsub", '\u2284');
      _entities.Add ("sube", '\u2286');
      _entities.Add ("supe", '\u2287');
      _entities.Add ("oplus", '\u2295');
      _entities.Add ("otimes", '\u2297');
      _entities.Add ("perp", '\u22A5');
      _entities.Add ("sdot", '\u22C5');
      _entities.Add ("lceil", '\u2308');
      _entities.Add ("rceil", '\u2309');
      _entities.Add ("lfloor", '\u230A');
      _entities.Add ("rfloor", '\u230B');
      _entities.Add ("lang", '\u2329');
      _entities.Add ("rang", '\u232A');
      _entities.Add ("loz", '\u25CA');
      _entities.Add ("spades", '\u2660');
      _entities.Add ("clubs", '\u2663');
      _entities.Add ("hearts", '\u2665');
      _entities.Add ("diams", '\u2666');
      _entities.Add ("quot", '\u0022');
      _entities.Add ("amp", '\u0026');
      _entities.Add ("lt", '\u003C');
      _entities.Add ("gt", '\u003E');
      _entities.Add ("OElig", '\u0152');
      _entities.Add ("oelig", '\u0153');
      _entities.Add ("Scaron", '\u0160');
      _entities.Add ("scaron", '\u0161');
      _entities.Add ("Yuml", '\u0178');
      _entities.Add ("circ", '\u02C6');
      _entities.Add ("tilde", '\u02DC');
      _entities.Add ("ensp", '\u2002');
      _entities.Add ("emsp", '\u2003');
      _entities.Add ("thinsp", '\u2009');
      _entities.Add ("zwnj", '\u200C');
      _entities.Add ("zwj", '\u200D');
      _entities.Add ("lrm", '\u200E');
      _entities.Add ("rlm", '\u200F');
      _entities.Add ("ndash", '\u2013');
      _entities.Add ("mdash", '\u2014');
      _entities.Add ("lsquo", '\u2018');
      _entities.Add ("rsquo", '\u2019');
      _entities.Add ("sbquo", '\u201A');
      _entities.Add ("ldquo", '\u201C');
      _entities.Add ("rdquo", '\u201D');
      _entities.Add ("bdquo", '\u201E');
      _entities.Add ("dagger", '\u2020');
      _entities.Add ("Dagger", '\u2021');
      _entities.Add ("permil", '\u2030');
      _entities.Add ("lsaquo", '\u2039');
      _entities.Add ("rsaquo", '\u203A');
      _entities.Add ("euro", '\u20AC');
    }

    private static bool notEncoded (char c)
    {
      return c == '!' ||
             c == '\'' ||
             c == '(' ||
             c == ')' ||
             c == '*' ||
             c == '-' ||
             c == '.' ||
             c == '_';
    }

    private static void urlEncodeChar (char c, Stream result, bool isUnicode)
    {
      if (c > 255) {
        // FIXME: What happens when there is an internal error?
        //if (!isUnicode)
        //  throw new ArgumentOutOfRangeException ("c", c, "c must be less than 256.");

        result.WriteByte ((byte) '%');
        result.WriteByte ((byte) 'u');

        var i = (int) c;
        var idx = i >> 12;
        result.WriteByte ((byte) _hexChars [idx]);

        idx = (i >> 8) & 0x0F;
        result.WriteByte ((byte) _hexChars [idx]);

        idx = (i >> 4) & 0x0F;
        result.WriteByte ((byte) _hexChars [idx]);

        idx = i & 0x0F;
        result.WriteByte ((byte) _hexChars [idx]);

        return;
      }

      if (c > ' ' && notEncoded (c)) {
        result.WriteByte ((byte) c);
        return;
      }

      if (c == ' ') {
        result.WriteByte ((byte) '+');
        return;
      }

      if ((c < '0') ||
          (c < 'A' && c > '9') ||
          (c > 'Z' && c < 'a') ||
          (c > 'z')) {
        if (isUnicode && c > 127) {
          result.WriteByte ((byte) '%');
          result.WriteByte ((byte) 'u');
          result.WriteByte ((byte) '0');
          result.WriteByte ((byte) '0');
        }
        else {
          result.WriteByte ((byte) '%');
        }

        var idx = ((int) c) >> 4;
        result.WriteByte ((byte) _hexChars [idx]);

        idx = ((int) c) & 0x0F;
        result.WriteByte ((byte) _hexChars [idx]);
      }
      else {
        result.WriteByte ((byte) c);
      }
    }

    private static void urlPathEncodeChar (char c, Stream result)
    {
      if (c < 33 || c > 126) {
        var bytes = Encoding.UTF8.GetBytes (c.ToString ());
        foreach (var b in bytes) {
          result.WriteByte ((byte) '%');

          var i = ((int) b) >> 4;
          result.WriteByte ((byte) _hexChars [i]);

          i = ((int) b) & 0x0F;
          result.WriteByte ((byte) _hexChars [i]);
        }
      }
      else if (c == ' ') {
        result.WriteByte ((byte) '%');
        result.WriteByte ((byte) '2');
        result.WriteByte ((byte) '0');
      }
      else {
        result.WriteByte ((byte) c);
      }
    }

    private static void writeCharBytes (IList buffer, char c, Encoding encoding)
    {
      if (c > 255)
        foreach (var b in encoding.GetBytes (new [] { c }))
          buffer.Add (b);
      else
        buffer.Add ((byte) c);
    }

    #endregion

    #region Internal Methods

    internal static Uri CreateRequestUrl (
      string requestUri, string host, bool websocketRequest, bool secure)
    {
      if (requestUri == null || requestUri.Length == 0 || host == null || host.Length == 0)
        return null;

      string scheme = null;
      string path = null;
      if (requestUri.StartsWith ("/")) {
        path = requestUri;
      }
      else if (requestUri.MaybeUri ()) {
        Uri uri;
        var valid = Uri.TryCreate (requestUri, UriKind.Absolute, out uri) &&
                    (((scheme = uri.Scheme).StartsWith ("http") && !websocketRequest) ||
                     (scheme.StartsWith ("ws") && websocketRequest));

        if (!valid)
          return null;

        host = uri.Authority;
        path = uri.PathAndQuery;
      }
      else if (requestUri == "*") {
      }
      else {
        // As authority form
        host = requestUri;
      }

      if (scheme == null)
        scheme = (websocketRequest ? "ws" : "http") + (secure ? "s" : String.Empty);

      var colon = host.IndexOf (':');
      if (colon == -1)
        host = String.Format ("{0}:{1}", host, scheme == "http" || scheme == "ws" ? 80 : 443);

      var url = String.Format ("{0}://{1}{2}", scheme, host, path);

      Uri res;
      if (!Uri.TryCreate (url, UriKind.Absolute, out res))
        return null;

      return res;
    }

    internal static NameValueCollection ParseQueryStringInternally (string query, Encoding encoding)
    {
      int len;
      if (query == null || (len = query.Length) == 0 || (len == 1 && query [0] == '?'))
        return new NameValueCollection (1);

      if (query [0] == '?')
        query = query.Substring (1);

      var res = new QueryStringCollection ();
      var components = query.Split ('&');
      foreach (var component in components) {
        var i = component.IndexOf ('=');
        if (i > -1) {
          var name = UrlDecode (component.Substring (0, i), encoding);
          var val = component.Length > i + 1
                    ? UrlDecode (component.Substring (i + 1), encoding)
                    : String.Empty;

          res.Add (name, val);
        }
        else {
          res.Add (null, UrlDecode (component, encoding));
        }
      }

      return res;
    }

    internal static string UrlDecodeInternally (
      byte [] bytes, int offset, int count, Encoding encoding)
    {
      var output = new StringBuilder ();
      using (var acc = new MemoryStream ()) {
        var end = count + offset;

        int xchar;
        for (int i = offset; i < end; i++) {
          if (bytes [i] == '%' && i + 2 < count && bytes [i + 1] != '%') {
            if (bytes [i + 1] == (byte) 'u' && i + 5 < end) {
              if (acc.Length > 0) {
                output.Append (getChars (acc, encoding));
                acc.SetLength (0);
              }

              xchar = getChar (bytes, i + 2, 4);
              if (xchar != -1) {
                output.Append ((char) xchar);
                i += 5;

                continue;
              }
            }
            else if ((xchar = getChar (bytes, i + 1, 2)) != -1) {
              acc.WriteByte ((byte) xchar);
              i += 2;

              continue;
            }
          }

          if (acc.Length > 0) {
            output.Append (getChars (acc, encoding));
            acc.SetLength (0);
          }

          if (bytes [i] == '+')
            output.Append (' ');
          else
            output.Append ((char) bytes [i]);
        }

        if (acc.Length > 0)
          output.Append (getChars (acc, encoding));
      }

      return output.ToString ();
    }

    internal static byte [] UrlDecodeToBytesInternally (byte [] bytes, int offset, int count)
    {
      using (var res = new MemoryStream ()) {
        var end = offset + count;
        for (int i = offset; i < end; i++) {
          var c = (char) bytes [i];
          if (c == '+') {
            c = ' ';
          }
          else if (c == '%' && i < end - 2) {
            var xchar = getChar (bytes, i + 1, 2);
            if (xchar != -1) {
              c = (char) xchar;
              i += 2;
            }
          }

          res.WriteByte ((byte) c);
        }

        res.Close ();
        return res.ToArray ();
      }
    }

    internal static byte [] UrlEncodeToBytesInternally (byte [] bytes, int offset, int count)
    {
      using (var res = new MemoryStream ()) {
        var end = offset + count;
        for (int i = offset; i < end; i++)
          urlEncodeChar ((char) bytes [i], res, false);

        res.Close ();
        return res.ToArray ();
      }
    }

    internal static byte [] UrlEncodeUnicodeToBytesInternally (string s)
    {
      using (var res = new MemoryStream ()) {
        foreach (var c in s)
          urlEncodeChar (c, res, true);

        res.Close ();
        return res.ToArray ();
      }
    }

    #endregion

    #region Public Methods

    public static string HtmlAttributeEncode (string s)
    {
      if (s == null || s.Length == 0 || !s.Contains ('&', '"', '<', '>'))
        return s;

      var output = new StringBuilder ();
      foreach (var c in s)
        output.Append (
          c == '&'
          ? "&amp;"
          : c == '"'
            ? "&quot;"
            : c == '<'
              ? "&lt;"
              : c == '>'
                ? "&gt;"
                : c.ToString ()
        );

      return output.ToString ();
    }

    public static void HtmlAttributeEncode (string s, TextWriter output)
    {
      if (output == null)
        throw new ArgumentNullException ("output");

      output.Write (HtmlAttributeEncode (s));
    }

    /// <summary>
    /// Decodes an HTML-encoded <see cref="string"/> and returns the decoded <see cref="string"/>.
    /// </summary>
    /// <returns>
    /// A <see cref="string"/> that represents the decoded string.
    /// </returns>
    /// <param name="s">
    /// A <see cref="string"/> to decode.
    /// </param>
    public static string HtmlDecode (string s) 
    {
      if (s == null || s.Length == 0 || !s.Contains ('&'))
        return s;

      var entity = new StringBuilder ();
      var output = new StringBuilder ();

      // 0 -> nothing,
      // 1 -> right after '&'
      // 2 -> between '&' and ';' but no '#'
      // 3 -> '#' found after '&' and getting numbers
      var state = 0;
      var number = 0;
      var haveTrailingDigits = false;
      foreach (var c in s) {
        if (state == 0) {
          if (c == '&') {
            entity.Append (c);
            state = 1;
          }
          else {
            output.Append (c);
          }

          continue;
        }

        if (c == '&') {
          state = 1;
          if (haveTrailingDigits) {
            entity.Append (number.ToString (CultureInfo.InvariantCulture));
            haveTrailingDigits = false;
          }

          output.Append (entity.ToString ());
          entity.Length = 0;
          entity.Append ('&');

          continue;
        }

        if (state == 1) {
          if (c == ';') {
            state = 0;
            output.Append (entity.ToString ());
            output.Append (c);
            entity.Length = 0;
          }
          else {
            number = 0;
            if (c != '#')
              state = 2;
            else
              state = 3;

            entity.Append (c);
          }
        }
        else if (state == 2) {
          entity.Append (c);
          if (c == ';') {
            var key = entity.ToString ();
            if (key.Length > 1 && Entities.ContainsKey (key.Substring (1, key.Length - 2)))
              key = Entities [key.Substring (1, key.Length - 2)].ToString ();

            output.Append (key);
            state = 0;
            entity.Length = 0;
          }
        }
        else if (state == 3) {
          if (c == ';') {
            if (number > 65535) {
              output.Append ("&#");
              output.Append (number.ToString (CultureInfo.InvariantCulture));
              output.Append (";");
            }
            else {
              output.Append ((char) number);
            }

            state = 0;
            entity.Length = 0;
            haveTrailingDigits = false;
          }
          else if (Char.IsDigit (c)) {
            number = number * 10 + ((int) c - '0');
            haveTrailingDigits = true;
          }
          else {
            state = 2;
            if (haveTrailingDigits) {
              entity.Append (number.ToString (CultureInfo.InvariantCulture));
              haveTrailingDigits = false;
            }

            entity.Append (c);
          }
        }
      }

      if (entity.Length > 0)
        output.Append (entity.ToString ());
      else if (haveTrailingDigits)
        output.Append (number.ToString (CultureInfo.InvariantCulture));

      return output.ToString ();
    }

    /// <summary>
    /// Decodes an HTML-encoded <see cref="string"/> and sends the decoded <see cref="string"/>
    /// to the specified <see cref="TextWriter"/>.
    /// </summary>
    /// <param name="s">
    /// A <see cref="string"/> to decode.
    /// </param>
    /// <param name="output">
    /// A <see cref="TextWriter"/> that receives the decoded string.
    /// </param>
    public static void HtmlDecode (string s, TextWriter output)
    {
      if (output == null)
        throw new ArgumentNullException ("output");

      output.Write (HtmlDecode (s));
    }

    /// <summary>
    /// HTML-encodes a <see cref="string"/> and returns the encoded <see cref="string"/>.
    /// </summary>
    /// <returns>
    /// A <see cref="string"/> that represents the encoded string.
    /// </returns>
    /// <param name="s">
    /// A <see cref="string"/> to encode.
    /// </param>
    public static string HtmlEncode (string s)
    {
      if (s == null || s.Length == 0)
        return s;

      var needEncode = false;
      foreach (var c in s) {
        if (c == '&' || c == '"' || c == '<' || c == '>' || c > 159) {
          needEncode = true;
          break;
        }
      }

      if (!needEncode)
        return s;

      var output = new StringBuilder ();
      foreach (var c in s) {
        if (c == '&') {
          output.Append ("&amp;");
        }
        else if (c == '"') {
          output.Append ("&quot;");
        }
        else if (c == '<') {
          output.Append ("&lt;");
        }
        else if (c == '>') {
          output.Append ("&gt;");
        }
        else {
          // MS starts encoding with &# from 160 and stops at 255.
          // We don't do that. One reason is the 65308/65310 unicode
          // characters that look like '<' and '>'.
          if (c > 159) {
            output.Append ("&#");
            output.Append (((int) c).ToString (CultureInfo.InvariantCulture));
            output.Append (";");
          }
          else {
            output.Append (c);
          }
        }
      }

      return output.ToString ();
    }

    /// <summary>
    /// HTML-encodes a <see cref="string"/> and sends the encoded <see cref="string"/>
    /// to the specified <see cref="TextWriter"/>.
    /// </summary>
    /// <param name="s">
    /// A <see cref="string"/> to encode.
    /// </param>
    /// <param name="output">
    /// A <see cref="TextWriter"/> that receives the encoded string.
    /// </param>
    public static void HtmlEncode (string s, TextWriter output)
    {
      if (output == null)
        throw new ArgumentNullException ("output");

      output.Write (HtmlEncode (s));
    }

    public static NameValueCollection ParseQueryString (string query)
    {
      if (query == null)
        throw new ArgumentNullException ("query");

      return ParseQueryStringInternally (query, Encoding.UTF8);
    }

    public static NameValueCollection ParseQueryString (string query, Encoding encoding)
    {
      if (query == null)
        throw new ArgumentNullException ("query");

      if (encoding == null)
        throw new ArgumentNullException ("encoding");

      return ParseQueryStringInternally (query, encoding);
    }

    public static string UrlDecode (string s)
    {
      return UrlDecode (s, Encoding.UTF8);
    }

    public static string UrlDecode (string s, Encoding encoding)
    {
      if (s == null || s.Length == 0 || !s.Contains ('%', '+'))
        return s;

      if (encoding == null)
        encoding = Encoding.UTF8;

      var len = s.Length;
      var buff = new List<byte> ();

      int xchar;
      for (int i = 0; i < len; i++) {
        var c = s [i];
        if (c == '%' && i + 2 < len && s [i + 1] != '%') {
          if (s [i + 1] == 'u' && i + 5 < len) {
            // Unicode hex sequence.
            xchar = getChar (s, i + 2, 4);
            if (xchar != -1) {
              writeCharBytes (buff, (char) xchar, encoding);
              i += 5;
            }
            else {
              writeCharBytes (buff, '%', encoding);
            }
          }
          else if ((xchar = getChar (s, i + 1, 2)) != -1) {
            writeCharBytes (buff, (char) xchar, encoding);
            i += 2;
          }
          else {
            writeCharBytes (buff, '%', encoding);
          }

          continue;
        }

        if (c == '+')
          writeCharBytes (buff, ' ', encoding);
        else
          writeCharBytes (buff, c, encoding);
      }

      return encoding.GetString (buff.ToArray ());
    }

    public static string UrlDecode (byte [] bytes, Encoding encoding)
    {
      if (bytes == null)
        return null;

      var len = bytes.Length;
      if (len == 0)
        return String.Empty;

      if (encoding == null)
        encoding = Encoding.UTF8;

      return UrlDecodeInternally (bytes, 0, len, encoding);
    }

    public static string UrlDecode (byte [] bytes, int offset, int count, Encoding encoding)
    {
      if (bytes == null)
        return null;

      var len = bytes.Length;
      if (len == 0 || count == 0)
        return String.Empty;

      if (offset < 0 || offset >= len)
        throw new ArgumentOutOfRangeException ("offset");

      if (count < 0 || count > len - offset)
        throw new ArgumentOutOfRangeException ("count");

      if (encoding == null)
        encoding = Encoding.UTF8;

      return UrlDecodeInternally (bytes, offset, count, encoding);
    }

    public static byte [] UrlDecodeToBytes (byte [] bytes)
    {
      int len;
      return bytes != null && (len = bytes.Length) > 0
             ? UrlDecodeToBytesInternally (bytes, 0, len)
             : bytes;
    }

    public static byte [] UrlDecodeToBytes (string s)
    {
      return UrlDecodeToBytes (s, Encoding.UTF8);
    }

    public static byte [] UrlDecodeToBytes (string s, Encoding encoding)
    {
      if (s == null)
        return null;

      if (s.Length == 0)
        return new byte [0];

      if (encoding == null)
        encoding = Encoding.UTF8;

      var bytes = encoding.GetBytes (s);
      return UrlDecodeToBytesInternally (bytes, 0, bytes.Length);
    }

    public static byte [] UrlDecodeToBytes (byte [] bytes, int offset, int count)
    {
      int len;
      if (bytes == null || (len = bytes.Length) == 0)
        return bytes;

      if (count == 0)
        return new byte [0];

      if (offset < 0 || offset >= len)
        throw new ArgumentOutOfRangeException ("offset");

      if (count < 0 || count > len - offset )
        throw new ArgumentOutOfRangeException ("count");

      return UrlDecodeToBytesInternally (bytes, offset, count);
    }

    public static string UrlEncode (byte [] bytes)
    {
      int len;
      return bytes == null
             ? null
             : (len = bytes.Length) == 0
               ? String.Empty
               : Encoding.ASCII.GetString (UrlEncodeToBytesInternally (bytes, 0, len));
    }

    public static string UrlEncode (string s)
    {
      return UrlEncode (s, Encoding.UTF8);
    }
  
    public static string UrlEncode (string s, Encoding encoding)
    {
      int len;
      if (s == null || (len = s.Length) == 0)
        return s;

      var needEncode = false;
      foreach (var c in s) {
        if ((c < '0') || (c < 'A' && c > '9') || (c > 'Z' && c < 'a') || (c > 'z')) {
          if (notEncoded (c))
            continue;

          needEncode = true;
          break;
        }
      }

      if (!needEncode)
        return s;

      if (encoding == null)
        encoding = Encoding.UTF8;

      // Avoided GetByteCount call.
      var bytes = new byte [encoding.GetMaxByteCount (len)];
      var realLen = encoding.GetBytes (s, 0, len, bytes, 0);

      return Encoding.ASCII.GetString (UrlEncodeToBytesInternally (bytes, 0, realLen));
    }
  
    public static string UrlEncode (byte [] bytes, int offset, int count)
    {
      var encoded = UrlEncodeToBytes (bytes, offset, count);
      return encoded == null
             ? null
             : encoded.Length == 0
               ? String.Empty
               : Encoding.ASCII.GetString (encoded);
    }

    public static byte [] UrlEncodeToBytes (byte [] bytes)
    {
      int len;
      return bytes != null && (len = bytes.Length) > 0
             ? UrlEncodeToBytesInternally (bytes, 0, len)
             : bytes;
    }

    public static byte [] UrlEncodeToBytes (string s)
    {
      return UrlEncodeToBytes (s, Encoding.UTF8);
    }

    public static byte [] UrlEncodeToBytes (string s, Encoding encoding)
    {
      if (s == null)
        return null;

      if (s.Length == 0)
        return new byte [0];

      if (encoding == null)
        encoding = Encoding.UTF8;

      var bytes = encoding.GetBytes (s);
      return UrlEncodeToBytesInternally (bytes, 0, bytes.Length);
    }

    public static byte [] UrlEncodeToBytes (byte [] bytes, int offset, int count)
    {
      int len;
      if (bytes == null || (len = bytes.Length) == 0)
        return bytes;

      if (count == 0)
        return new byte [0];

      if (offset < 0 || offset >= len)
        throw new ArgumentOutOfRangeException ("offset");

      if (count < 0 || count > len - offset)
        throw new ArgumentOutOfRangeException ("count");

      return UrlEncodeToBytesInternally (bytes, offset, count);
    }

    public static string UrlEncodeUnicode (string s)
    {
      return s != null && s.Length > 0
             ? Encoding.ASCII.GetString (UrlEncodeUnicodeToBytesInternally (s))
             : s;
    }

    public static byte [] UrlEncodeUnicodeToBytes (string s)
    {
      return s == null
             ? null
             : s.Length == 0
               ? new byte [0]
               : UrlEncodeUnicodeToBytesInternally (s);
    }

    public static string UrlPathEncode (string s)
    {
      if (s == null || s.Length == 0)
        return s;

      using (var res = new MemoryStream ()) {
        foreach (var c in s)
          urlPathEncodeChar (c, res);

        res.Close ();
        return Encoding.ASCII.GetString (res.ToArray ());
      }
    }

    #endregion
  }
}
