/// <summary>
/// Reads data from the stream.
/// </summary>
/// <param name="buffer">The buffer to fill.</param>
/// <param name="offset">The buffer offset to start from.</param>
/// <param name="count">The number of bytes to read.</param>
/// <returns>The number of bytes read.</returns>
public override int Read(byte[] buffer, int offset, int count)
{
int numRead;
if (_diffStream == null)
{
_baseStream.Position = _position;
numRead = _baseStream.Read(buffer, offset, count);
}
else
{
if (_position > _diffStream.Length)
{
throw new IOException("Attempt to read beyond end of file");
}
int toRead = (int)Math.Min(count, _diffStream.Length - _position);
// If the read is within the base stream's range, then touch it first to get the
// (potentially) stale data.
if (_position < _baseStream.Length)
{
int baseToRead = (int)Math.Min(toRead, _baseStream.Length - _position);
_baseStream.Position = _position;
int totalBaseRead = 0;
while (totalBaseRead < baseToRead)
{
totalBaseRead += _baseStream.Read(buffer, offset + totalBaseRead, baseToRead - totalBaseRead);
}
}
// Now overlay any data from the overlay stream (if any)
IEnumerable <StreamExtent> overlayExtents = StreamExtent.Intersect(_diffExtents, new StreamExtent(_position, toRead));
foreach (var extent in overlayExtents)
{
_diffStream.Position = extent.Start;
int overlayNumRead = 0;
while (overlayNumRead < extent.Length)
{
overlayNumRead += _diffStream.Read(
buffer,
(int)(offset + (extent.Start - _position) + overlayNumRead),
(int)(extent.Length - overlayNumRead));
}
}
numRead = toRead;
}
_position += numRead;
return(numRead);
}
/// <summary>
/// Writes data to the stream at the current location.
/// </summary>
/// <param name="buffer">The data to write.</param>
/// <param name="offset">The first byte to write from buffer.</param>
/// <param name="count">The number of bytes to write.</param>
public override void Write(byte[] buffer, int offset, int count)
{
CheckFrozen();
if (_diffStream != null)
{
_diffStream.Position = _position;
_diffStream.Write(buffer, offset, count);
// Beware of Linq's delayed model - force execution now by placing into a list.
// Without this, large execution chains can build up (v. slow) and potential for stack overflow.
_diffExtents = new List <StreamExtent>(StreamExtent.Union(_diffExtents, new StreamExtent(_position, count)));
_position += count;
}
else
{
_baseStream.Position = _position;
_baseStream.Write(buffer, offset, count);
_position += count;
}
}
/// <summary>
/// Gets the parts of a stream that are stored, within a specified range.
/// </summary>
/// <param name="start">The offset of the first byte of interest.</param>
/// <param name="count">The number of bytes of interest.</param>
/// <returns>An enumeration of stream extents, indicating stored bytes.</returns>
public virtual IEnumerable <StreamExtent> GetExtentsInRange(long start, long count)
{
return(StreamExtent.Intersect(Extents, new StreamExtent[] { new StreamExtent(start, count) }));
}