System.Net.HttpListenerRequest.Helpers.UrlDecodeStringFromStringInternal C# (CSharp) Method

            private static string UrlDecodeStringFromStringInternal(string s, Encoding e)
            {
                int count = s.Length;
                UrlDecoder helper = new UrlDecoder(count, e);

                // go through the string's chars collapsing %XX and %uXXXX and
                // appending each char as char, with exception of %XX constructs
                // that are appended as bytes

                for (int pos = 0; pos < count; pos++)
                {
                    char ch = s[pos];

                    if (ch == '+')
                    {
                        ch = ' ';
                    }
                    else if (ch == '%' && pos < count - 2)
                    {
                        if (s[pos + 1] == 'u' && pos < count - 5)
                        {
                            int h1 = HexToInt(s[pos + 2]);
                            int h2 = HexToInt(s[pos + 3]);
                            int h3 = HexToInt(s[pos + 4]);
                            int h4 = HexToInt(s[pos + 5]);

                            if (h1 >= 0 && h2 >= 0 && h3 >= 0 && h4 >= 0)
                            {   // valid 4 hex chars
                                ch = (char)((h1 << 12) | (h2 << 8) | (h3 << 4) | h4);
                                pos += 5;

                                // only add as char
                                helper.AddChar(ch);
                                continue;
                            }
                        }
                        else
                        {
                            int h1 = HexToInt(s[pos + 1]);
                            int h2 = HexToInt(s[pos + 2]);

                            if (h1 >= 0 && h2 >= 0)
                            {     // valid 2 hex chars
                                byte b = (byte)((h1 << 4) | h2);
                                pos += 2;

                                // don't add as char
                                helper.AddByte(b);
                                continue;
                            }
                        }
                    }

                    if ((ch & 0xFF80) == 0)
                        helper.AddByte((byte)ch); // 7 bit have to go as bytes because of Unicode
                    else
                        helper.AddChar(ch);
                }

                return helper.GetString();
            }