JPEG parser

    class JpegParser
    {
        const UInt16 SOI = 0xffd8,
                     TEM = 0xff01,
                     RSTmin = 0xffd0,
                     RSTmax = 0xffd7,
                     EOI = 0xffd9,
                     MARKmin = 0xff01,
                     MARKmax = 0xfffe;

        byte[] data;
        long startIndex;
        public JpegParser(byte[] data, long startIndex)
        {
            this.data = data;
            this.startIndex = startIndex;
        }

        public bool HasJpegHeader()
        {
            if (data.LongLength < startIndex + 4)
                return false;

            var soi = GetBigEndianInt16(startIndex);  // Start of Image (SOI) marker (FFD8)
            int jfif = GetBigEndianInt16(startIndex + 2); // JFIF marker (FFE0)

            return soi == 0xffd8 && jfif == 0xffe0;
        }

        UInt16 GetBigEndianInt16(long index)
        {
            return (UInt16)(data[index] << 8 | data[index + 1]);
        }

        // exception means bad image format
        public long? SkipJpegHeader()
        {
            try
            {
                long currIndex = startIndex;

                // read out soi
                if (GetBigEndianInt16(currIndex) != 0xffd8)
                    return null;
                currIndex += 2;

                while (currIndex < data.LongLength)
                {
                    var next = GetBigEndianInt16(currIndex);
                    currIndex += 2;

                    if (next == SOI)
                        return null;

                    if (next == TEM)
                        continue;

                    if (next >= RSTmin && next <= RSTmax)
                        continue;

                    if (next == EOI)
                        return currIndex;

                    if (next >= MARKmin && next <= MARKmax)
                    {
                        // we found a marker, skip the section
                        currIndex += GetBigEndianInt16(currIndex);
                        continue;
                    }

                    currIndex -= 1;
                }
                return null;
            }
            catch (IndexOutOfRangeException)
            {
                return null;
            }
        }
    }