/// <summary>
/// This is the default resync_to_restart method for data source
/// managers to use if they don't have any better approach.
/// </summary>
/// <param name="cinfo">An instance of <see cref="jpeg_decompress_struct"/></param>
/// <param name="desired">The desired</param>
/// <returns><c>false</c> if suspension is required.</returns>
/// <remarks>That method assumes that no backtracking is possible.
/// Some data source managers may be able to back up, or may have
/// additional knowledge about the data which permits a more
/// intelligent recovery strategy; such managers would
/// presumably supply their own resync method.<br/><br/>
///
/// read_restart_marker calls resync_to_restart if it finds a marker other than
/// the restart marker it was expecting. (This code is *not* used unless
/// a nonzero restart interval has been declared.) cinfo.unread_marker is
/// the marker code actually found (might be anything, except 0 or FF).
/// The desired restart marker number (0..7) is passed as a parameter.<br/><br/>
///
/// This routine is supposed to apply whatever error recovery strategy seems
/// appropriate in order to position the input stream to the next data segment.
/// Note that cinfo.unread_marker is treated as a marker appearing before
/// the current data-source input point; usually it should be reset to zero
/// before returning.<br/><br/>
///
/// This implementation is substantially constrained by wanting to treat the
/// input as a data stream; this means we can't back up. Therefore, we have
/// only the following actions to work with:<br/>
/// 1. Simply discard the marker and let the entropy decoder resume at next
/// byte of file.<br/>
/// 2. Read forward until we find another marker, discarding intervening
/// data. (In theory we could look ahead within the current bufferload,
/// without having to discard data if we don't find the desired marker.
/// This idea is not implemented here, in part because it makes behavior
/// dependent on buffer size and chance buffer-boundary positions.)<br/>
/// 3. Leave the marker unread (by failing to zero cinfo.unread_marker).
/// This will cause the entropy decoder to process an empty data segment,
/// inserting dummy zeroes, and then we will reprocess the marker.<br/>
///
/// #2 is appropriate if we think the desired marker lies ahead, while #3 is
/// appropriate if the found marker is a future restart marker (indicating
/// that we have missed the desired restart marker, probably because it got
/// corrupted).<br/>
/// We apply #2 or #3 if the found marker is a restart marker no more than
/// two counts behind or ahead of the expected one. We also apply #2 if the
/// found marker is not a legal JPEG marker code (it's certainly bogus data).
/// If the found marker is a restart marker more than 2 counts away, we do #1
/// (too much risk that the marker is erroneous; with luck we will be able to
/// resync at some future point).<br/>
/// For any valid non-restart JPEG marker, we apply #3. This keeps us from
/// overrunning the end of a scan. An implementation limited to single-scan
/// files might find it better to apply #2 for markers other than EOI, since
/// any other marker would have to be bogus data in that case.</remarks>
public virtual bool resync_to_restart(jpeg_decompress_struct cinfo, int desired)
{
/* Always put up a warning. */
cinfo.WARNMS(J_MESSAGE_CODE.JWRN_MUST_RESYNC, cinfo.m_unread_marker, desired);
/* Outer loop handles repeated decision after scanning forward. */
int action = 1;
for ( ; ; )
{
if (cinfo.m_unread_marker < (int)JPEG_MARKER.SOF0)
{
/* invalid marker */
action = 2;
}
else if (cinfo.m_unread_marker < (int)JPEG_MARKER.RST0 ||
cinfo.m_unread_marker > (int)JPEG_MARKER.RST7)
{
/* valid non-restart marker */
action = 3;
}
else
{
if (cinfo.m_unread_marker == ((int)JPEG_MARKER.RST0 + ((desired + 1) & 7))
|| cinfo.m_unread_marker == ((int)JPEG_MARKER.RST0 + ((desired + 2) & 7)))
{
/* one of the next two expected restarts */
action = 3;
}
else if (cinfo.m_unread_marker == ((int)JPEG_MARKER.RST0 + ((desired - 1) & 7)) ||
cinfo.m_unread_marker == ((int)JPEG_MARKER.RST0 + ((desired - 2) & 7)))
{
/* a prior restart, so advance */
action = 2;
}
else
{
/* desired restart or too far away */
action = 1;
}
}
cinfo.TRACEMS(4, J_MESSAGE_CODE.JTRC_RECOVERY_ACTION, cinfo.m_unread_marker, action);
switch (action)
{
case 1:
/* Discard marker and let entropy decoder resume processing. */
cinfo.m_unread_marker = 0;
return true;
case 2:
/* Scan to the next marker, and repeat the decision loop. */
if (!cinfo.m_marker.next_marker())
return false;
break;
case 3:
/* Return without advancing past this marker. */
/* Entropy decoder will be forced to process an empty segment. */
return true;
}
}
}
