public override void GetBlock(BlockArguments args)
{
int blockDataLength = m_blockDataLength;
long pos = args.Position;
if (IsDisposed || m_dataIoSession.IsDisposed)
{
throw new ObjectDisposedException(GetType().FullName);
}
if (pos < 0)
{
throw new ArgumentOutOfRangeException("position", "cannot be negative");
}
if (pos >= (long)blockDataLength * (uint.MaxValue - 1))
{
throw new ArgumentOutOfRangeException("position", "position reaches past the end of the file.");
}
uint physicalBlockIndex;
uint indexPosition;
if (pos <= uint.MaxValue) //64-bit divide is 2 times slower
{
indexPosition = ((uint)pos / (uint)blockDataLength);
}
else
{
indexPosition = (uint)((ulong)pos / (ulong)blockDataLength); //64-bit signed divide is twice as slow as 64-bit unsigned.
}
args.FirstPosition = (long)indexPosition * blockDataLength;
args.Length = blockDataLength;
if (args.IsWriting)
{
throw new Exception("File is read only");
}
else
{
//Reading
if (indexPosition >= m_stream.m_subFile.DataBlockCount)
{
throw new ArgumentOutOfRangeException("position", "position reaches past the end of the file.");
}
physicalBlockIndex = m_stream.m_subFile.DirectBlock + indexPosition;
m_dataIoSession.Read(physicalBlockIndex, BlockType.DataBlock, indexPosition);
args.FirstPointer = (IntPtr)m_dataIoSession.Pointer;
args.SupportsWriting = false;
}
}
/// <summary>
/// This uses the (blockIndex,offset) values to determine what the next block index is.
/// This also has consistency checks to determine if the file is inconsistent (potentially corruption)
/// </summary>
/// <param name="blockIndex">the index of the block to read</param>
/// <param name="offset">the offset inside the block to use to determine the next index block</param>
/// <param name="blockType">the value 1-4 which tell what indirect block this is</param>
/// <param name="blockBaseIndex">the lowest virtual address that can be referenced from this indirect block</param>
/// <returns></returns>
private uint GetBlockIndexValue(uint blockIndex, int offset, BlockType blockType, uint blockBaseIndex)
{
DiskIoSession buffer = m_ioSessions.SourceIndex;
if (blockIndex == 0)
{
return(0);
}
//Skip the redundant read if this block is still cached.
if (!buffer.IsValid || buffer.BlockIndex != blockIndex)
{
buffer.Read(blockIndex, blockType, blockBaseIndex);
}
return(*(uint *)(buffer.Pointer + (offset << 2)));
}
