private ContainerEntry FindContainerAtDepth(long position, int desiredDepth)
{
// Find the first container ending after position.
ContainerEntry firstAfter = _containerIndex.FirstEndingAfter(position);
int firstAfterDepth = _containerIndex.Depth(firstAfter);
// Find the ancestor at the desired depth. If there is one, and it contains position, we can use it
ContainerEntry firstAtDepth = AncestorAtDepth(firstAfter, firstAfterDepth, desiredDepth);
if (firstAtDepth.Contains(position))
{
return(firstAtDepth);
}
// If nothing deep enough was indexed, we must find the nearest point of known depth and walk the BION
long seekToPosition;
int seekToDepth;
// The nearest point is...
if (firstAfter.StartByteOffset < position)
{
// The start of the first container ending after position, if it starts before position
seekToPosition = firstAfter.StartByteOffset;
seekToDepth = firstAfterDepth - 1;
}
else if (firstAfter.Index > 0)
{
// The end of the container before that, if it started after position
ContainerEntry lastBefore = _containerIndex[firstAfter.Index - 1];
seekToPosition = lastBefore.EndByteOffset;
seekToDepth = _containerIndex.Depth(lastBefore) - 1;
}
else
{
// The start of the document, if the first container was found
seekToPosition = 0;
seekToDepth = 0;
}
// Seek to that position and read until we find a container at the right depth which contains position
_bionReader.Seek(seekToPosition);
long lastContainerStart = -1;
long lastContainerEnd = -1;
while (_bionReader.BytesRead < position + 3)
{
_bionReader.Read();
if (_bionReader.Depth + seekToDepth == desiredDepth)
{
lastContainerStart = _bionReader.BytesRead - 1;
_bionReader.SkipRest();
lastContainerEnd = _bionReader.BytesRead;
}
}
ContainerEntry found = new ContainerEntry(lastContainerStart, lastContainerEnd, -1);
if (!found.Contains(position))
{
return(ContainerEntry.Empty);
}
return(found);
}