/**
* Finds a free block, and returns its offset.
* This method will extend the file if needed, and if doing
* so, allocate new FAT blocks to Address the extra space.
*/
public override int GetFreeBlock()
{
int sectorsPerSBAT = _filesystem.GetBigBlockSizeDetails().GetBATEntriesPerBlock();
// First up, do we have any spare ones?
int offset = 0;
for (int i = 0; i < _sbat_blocks.Count; i++)
{
// Check this one
BATBlock sbat = _sbat_blocks[i];
if (sbat.HasFreeSectors)
{
// Claim one of them and return it
for (int j = 0; j < sectorsPerSBAT; j++)
{
int sbatValue = sbat.GetValueAt(j);
if (sbatValue == POIFSConstants.UNUSED_BLOCK)
{
// Bingo
return(offset + j);
}
}
}
// Move onto the next SBAT
offset += sectorsPerSBAT;
}
// If we Get here, then there aren't any
// free sectors in any of the SBATs
// So, we need to extend the chain and add another
// Create a new BATBlock
BATBlock newSBAT = BATBlock.CreateEmptyBATBlock(_filesystem.GetBigBlockSizeDetails(), false);
int batForSBAT = _filesystem.GetFreeBlock();
newSBAT.OurBlockIndex = batForSBAT;
// Are we the first SBAT?
if (_header.SBATCount == 0)
{
_header.SBATStart = batForSBAT;
_header.SBATBlockCount = 1;
}
else
{
// Find the end of the SBAT stream, and add the sbat in there
ChainLoopDetector loopDetector = _filesystem.GetChainLoopDetector();
int batOffset = _header.SBATStart;
while (true)
{
loopDetector.Claim(batOffset);
int nextBat = _filesystem.GetNextBlock(batOffset);
if (nextBat == POIFSConstants.END_OF_CHAIN)
{
break;
}
batOffset = nextBat;
}
// Add it in at the end
_filesystem.SetNextBlock(batOffset, batForSBAT);
// And update the count
_header.SBATBlockCount = _header.SBATCount + 1;
}
// Finish allocating
_filesystem.SetNextBlock(batForSBAT, POIFSConstants.END_OF_CHAIN);
_sbat_blocks.Add(newSBAT);
// Return our first spot
return(offset);
}
