/// <summary>
/// This loosely parses JPEG data so And returns a <code>Tuple<long, long></code> containing the first And last
/// offsets of the JPEG bytes within the file. It does Not parse data within the markers; it simply determines their
/// length where possible And skips over them. This means that it Is possible that it may misidentify the bounds of
/// faulty JPEGs.
/// </summary>
/// <param name="startOfJpeg">The offset of the first byte of the JPEG data.</param>
/// <param name="offset">Current file offset that will be updated.</param>
/// <returns>
/// A Tuple<long, long> containing the offsets of the first And last byte of the JPEG, Or null if no complete
/// JPEG was found.
/// </returns>
/// </summary>
private Tuple <long, long> WalkJpeg(long startOfJpeg, ref long offset)
{
var position = startOfJpeg;
if (_readable.ReadUInt16(position) != 0xD8FF)
{
return(null);
}
// TODO: explain += 2
position += 2;
// TODO: again... - 4
while (position < _readable.Capacity - 4)
{
var marker = FlipUShort(_readable.ReadUInt16(position));
var length = FlipUShort(_readable.ReadUInt16(position + 2));
if (marker == 0xFFFF)
{
// Skip filler byte
position += 1;
}
else if ((marker & 0xFF) >= 0xD0 && (marker & 0xFF) <= 0xD7)
{
// RST markers with no params
position += 2;
}
else if (marker == 0xFFDA)
{
// SOS marker; skip and then continue until EOI
position += length + 2;
marker = FlipUShort(_readable.ReadUInt16(position));
while (marker != 0xFFD9)
{
position++;
marker = _readable.ReadUInt16(position);
}
offset = position + 2;
return(new Tuple <long, long>(startOfJpeg, position + 1));
}
else if ((marker & 0xFF00) == 0xFF00)
{
// All other markers have lengths or are reserved. Skip over.
position += length + 2;
}
else
{
// No marker found. Invalid JPEG.
offset += 1;
break;
}
}
return(null);
}
