private byte[] FetchBlock(long index)
{
if (_blockCache.ContainsKey(index))
{
return(_blockCache[index]);
}
var result = new byte[_blockLength];
var position = _blockLength + index * _blockLength;
if (position == _fileStream.Length)
{
return(result);
}
_fileStream.Seek(position, SeekOrigin.Begin);
_fileStream.BlockingRead(result);
_blockCache[index] = result;
return(result);
}
private void Vacuum()
{
// here we get all free page indexes and allocations
var freePageIndexes = _pagemap.FreePageIndexes;
var freePageData = freePageIndexes.Select(freePageIndex =>
new Tuple <long, long>(_pagemap.GetPageAllocation(freePageIndex), freePageIndex)).ToList();
// sort allocations
freePageData.Sort((p1, p2) => p1.Item1.CompareTo(p2.Item1));
var reallocatedPageBytes = new byte[_pageSize];
// reallocate pages at the end of storage file
while (freePageData.Any())
{
long lastPageIndex = _pagemap.ReadLastPageIndex();
// compute last page allocation
long lastPageAllocation = _storageStream.Length - _pageSize;
if (freePageData.All(t => t.Item1 != lastPageAllocation))
{
// last page is occupied
// read its content
_storageStream.Seek(-_pageSize, SeekOrigin.End);
_storageStream.BlockingRead(reallocatedPageBytes);
var firstFreePage = freePageData[0];
var newPageAllocation = firstFreePage.Item1;
freePageData.RemoveAt(0);
// write a page content to new place
_storageStream.Seek(newPageAllocation, SeekOrigin.Begin);
_storageStream.Write(reallocatedPageBytes, 0, _pageSize);
// write an allocation marker of reallocated page
_pagemap.WritePageAllocation(lastPageIndex, newPageAllocation);
// write an allocation marker of first free page
_pagemap.WritePageAllocation(firstFreePage.Item2, -1);
// write a disk-to-virtual entry of reallocated page
_pagemap.WritePageIndex(lastPageIndex, newPageAllocation);
}
else
{
// last page is empty
_pagemap.WritePageAllocation(lastPageIndex, -1);
freePageData.RemoveAll(t => t.Item1 == lastPageAllocation);
}
// truncate pagemap
_pagemap.TruncateLastPageIndex();
// truncate storage file
_storageStream.SetLength(_storageStream.Length - _pageSize);
}
// clear free page indexes
_pagemap.ClearFreePageMarkers();
Flush(_storageStream);
_pagemap.Flush();
}
