/// <summary>
/// Uses BinarySearch to locate the index of the block that contains start
/// </summary>
/// <param name="cache"></param>
/// <param name="start"></param>
/// <param name="match">True if match found. If this is false, the index returned is where the item would appear if it existed.</param>
/// <returns>Index</returns>
private int FindIndexOfBlockAtOffset(SparseMemoryStream cache, long start, out bool match)
{
if (cache.MemoryBlockCollection.Count == 0)
{
match = false;
return(0);
}
// use BinarySearch to locate blocks of interest quickly
if (_comparisonBlock == null)
{
_comparisonBlock = new MemoryStreamBlock(null, start);
}
else
{
_comparisonBlock.Offset = start;
}
int index = cache.MemoryBlockCollection.BinarySearch(_comparisonBlock);
if (index < 0) // no match
{
// ~index represents the place at which the block we asked for
// would appear if it existed
index = ~index;
match = false;
}
else
{
match = true;
}
return(index);
}
private int ReadFromCache(SparseMemoryStream cache, long start, int count, byte[] buffer, int bufferOffset)
{
#if DEBUG
// debug only check for valid parameters, as we generally expect callers to verify them
PackagingUtilities.VerifyStreamReadArgs(this, buffer, bufferOffset, count);
#endif
Debug.Assert(cache != null);
Debug.Assert(start >= 0);
IList <MemoryStreamBlock> collection = cache.MemoryBlockCollection;
checked
{
// use BinarySearch to locate blocks of interest quickly
bool match; // exact match?
int index = FindIndexOfBlockAtOffset(cache, start, out match);
// if match was found, read from it
int bytesRead = 0;
if (match)
{
MemoryStreamBlock memStreamBlock = collection[index];
long overlapBlockOffset;
long overlapBlockSize;
// we have got an overlap which can be used to satisfy the read request,
// at least partially
PackagingUtilities.CalculateOverlap(memStreamBlock.Offset, memStreamBlock.Stream.Length,
start, count,
out overlapBlockOffset, out overlapBlockSize);
if (overlapBlockSize > 0)
{
// overlap must be starting at the start as we know for sure that
// memStreamBlock.Offset <= start
Debug.Assert(overlapBlockOffset == start);
memStreamBlock.Stream.Seek(overlapBlockOffset - memStreamBlock.Offset, SeekOrigin.Begin);
// we know that memStream will return as much data as we requested
// even if this logic changes we do not have to return everything
// a partially complete read is acceptable here
bytesRead = memStreamBlock.Stream.Read(buffer, bufferOffset, (int)overlapBlockSize);
}
}
return(bytesRead);
}
}
/// <summary>
/// Uses BinarySearch to locate the index of the block that contains start
/// </summary>
/// <param name="cache"></param>
/// <param name="start"></param>
/// <param name="match">True if match found. If this is false, the index returned is where the item would appear if it existed.</param>
/// <returns>Index</returns>
private int FindIndexOfBlockAtOffset(SparseMemoryStream cache, long start, out bool match)
{
if (cache.MemoryBlockCollection.Count == 0)
{
match = false;
return 0;
}
// use BinarySearch to locate blocks of interest quickly
if (_comparisonBlock == null)
_comparisonBlock = new MemoryStreamBlock(null, start);
else
_comparisonBlock.Offset = start;
int index = cache.MemoryBlockCollection.BinarySearch(_comparisonBlock);
if (index < 0) // no match
{
// ~index represents the place at which the block we asked for
// would appear if it existed
index = ~index;
match = false;
}
else
{
match = true;
}
return index;
}
