private static JsonToken GetUnquotedStringToken(
     BsonJsonBuffer buffer
 )
 {
     // opening letter or $ has already been read
     var start = buffer.Position - 1;
     while (true) {
         var c = buffer.Read();
         switch (c) {
             case ':':
             case ',':
             case '}':
             case ']':
             case -1:
                 buffer.UnRead(c);
                 return new JsonToken(JsonTokenType.UnquotedString, buffer.Substring(start, buffer.Position - start));
             default:
                 if (c == '$' || char.IsLetterOrDigit((char) c)) {
                     // continue
                 } else if (char.IsWhiteSpace((char) c)) {
                     buffer.UnRead(c);
                     return new JsonToken(JsonTokenType.UnquotedString, buffer.Substring(start, buffer.Position - start));
                 } else {
                     throw new FileFormatException(FormatMessage("Invalid JSON unquoted string", buffer, start));
                 }
                 break;
         }
     }
 }
        private static JsonToken GetNumberToken(
            BsonJsonBuffer buffer,
            int c // first character
        )
        {
            // leading digit or '-' has already been read
            var start = buffer.Position - 1;
            NumberState state;
            switch (c) {
                case '-': state = NumberState.SawLeadingMinus; break;
                case '0': state = NumberState.SawLeadingZero; break;
                default: state = NumberState.SawIntegerDigits; break;
            }
            var type = JsonTokenType.Integer; // assume integer until proved otherwise

            while (true) {
                c = buffer.Read();
                switch (state) {
                    case NumberState.SawLeadingMinus:
                        switch (c) {
                            case '0':
                                state = NumberState.SawLeadingZero;
                                break;
                            default:
                                if (char.IsDigit((char) c)) {
                                    state = NumberState.SawIntegerDigits;
                                } else {
                                    state = NumberState.Invalid;
                                }
                                break;
                        }
                        break;
                    case NumberState.SawLeadingZero:
                        switch (c) {
                            case '.':
                                state = NumberState.SawDecimalPoint;
                                break;
                            case 'e':
                            case 'E':
                                state = NumberState.SawExponentLetter;
                                break;
                            case ',':
                            case '}':
                            case ']':
                            case -1:
                                state = NumberState.Done;
                                break;
                            default:
                                if (char.IsWhiteSpace((char) c)) {
                                    state = NumberState.Done;
                                } else {
                                    state = NumberState.Invalid;
                                }
                                break;
                        }
                        break;
                    case NumberState.SawIntegerDigits:
                        switch (c) {
                            case '.':
                                state = NumberState.SawDecimalPoint;
                                break;
                            case 'e':
                            case 'E':
                                state = NumberState.SawExponentLetter;
                                break;
                            case ',':
                            case '}':
                            case ']':
                            case -1:
                                state = NumberState.Done;
                                break;
                            default:
                                if (char.IsDigit((char) c)) {
                                    state = NumberState.SawIntegerDigits;
                                } else if (char.IsWhiteSpace((char) c)) {
                                    state = NumberState.Done;
                                } else {
                                    state = NumberState.Invalid;
                                }
                                break;
                        }
                        break;
                    case NumberState.SawDecimalPoint:
                        type = JsonTokenType.FloatingPoint;
                        if (char.IsDigit((char) c)) {
                            state = NumberState.SawFractionDigits;
                        } else {
                            state = NumberState.Invalid;
                        }
                        break;
                    case NumberState.SawFractionDigits:
                        switch (c) {
                            case 'e':
                            case 'E':
                                state = NumberState.SawExponentLetter;
                                break;
                            case ',':
                            case '}':
                            case ']':
                            case -1:
                                state = NumberState.Done;
                                break;
                            default:
                                if (char.IsDigit((char) c)) {
                                    state = NumberState.SawFractionDigits;
                                } else if (char.IsWhiteSpace((char) c)) {
                                    state = NumberState.Done;
                                } else {
                                    state = NumberState.Invalid;
                                }
                                break;
                        }
                        break;
                    case NumberState.SawExponentLetter:
                        type = JsonTokenType.FloatingPoint;
                        switch (c) {
                            case '+':
                            case '-':
                                state = NumberState.SawExponentSign;
                                break;
                            default:
                                if (char.IsDigit((char) c)) {
                                    state = NumberState.SawExponentDigits;
                                } else {
                                    state = NumberState.Invalid;
                                }
                                break;
                        }
                        break;
                    case NumberState.SawExponentSign:
                        if (char.IsDigit((char) c)) {
                            state = NumberState.SawExponentDigits;
                        } else {
                            state = NumberState.Invalid;
                        }
                        break;
                    case NumberState.SawExponentDigits:
                        switch (c) {
                            case ',':
                            case '}':
                            case ']':
                            case -1:
                                state = NumberState.Done;
                                break;
                            default:
                                if (char.IsDigit((char) c)) {
                                    state = NumberState.SawExponentDigits;
                                } else if (char.IsWhiteSpace((char) c)) {
                                    state = NumberState.Done;
                                } else {
                                    state = NumberState.Invalid;
                                }
                                break;
                        }
                        break;
                }

                switch (state) {
                    case NumberState.Done:
                        buffer.UnRead(c);
                        return new JsonToken(type, buffer.Substring(start, buffer.Position - start));
                    case NumberState.Invalid:
                        throw new FileFormatException(FormatMessage("Invalid JSON number", buffer, start));
                }
            }
        }
        private static JsonToken GetRegularExpressionToken(
            BsonJsonBuffer buffer
        )
        {
            // opening slash has already been read
            var start = buffer.Position - 1;
            var state = RegularExpressionState.InPattern;
            while (true) {
                var c = buffer.Read();
                switch (state) {
                    case RegularExpressionState.InPattern:
                        switch (c) {
                            case '/': state = RegularExpressionState.InOptions; break;
                            case '\\': state = RegularExpressionState.InEscapeSequence; break;
                            default: state = RegularExpressionState.InPattern; break;
                        }
                        break;
                    case RegularExpressionState.InEscapeSequence:
                        state = RegularExpressionState.InPattern;
                        break;
                    case RegularExpressionState.InOptions:
                        switch (c) {
                            case 'g':
                            case 'i':
                            case 'm':
                                state = RegularExpressionState.InOptions;
                                break;
                            case ',':
                            case '}':
                            case ']':
                            case -1:
                                state = RegularExpressionState.Done;
                                break;
                            default:
                                if (char.IsWhiteSpace((char) c)) {
                                    state = RegularExpressionState.Done;
                                } else {
                                    state = RegularExpressionState.Invalid;
                                }
                                break;
                        }
                        break;
                }

                switch (state) {
                    case RegularExpressionState.Done:
                        buffer.UnRead(c);
                        var count = buffer.Position - start;
                        return new JsonToken(JsonTokenType.RegularExpression, buffer.Substring(start, count));
                    case RegularExpressionState.Invalid:
                        throw new FileFormatException(FormatMessage("Invalid JSON regular expression", buffer, start));
                }
            }
        }
 private static string FormatMessage(
     string message,
     BsonJsonBuffer buffer,
     int start
 )
 {
     var length = 20;
     string snippet;
     if (buffer.Position + length >= buffer.Length) {
         snippet = buffer.Substring(start);
     } else {
         snippet = buffer.Substring(start, length) + "...";
     }
     return string.Format("{0}: '{1}'", message, snippet);
 }