// We should drop the current source entry from the window,
// it is somehow invalid for us to work with.
private bool IsBetterDelta(DeltaWindowEntry src, TemporaryBuffer.Heap resDelta)
{
if (bestDelta == null)
{
return(true);
}
// If both delta sequences are the same length, use the one
// that has a shorter delta chain since it would be faster
// to access during reads.
//
if (resDelta.Length() == bestDelta.Length())
{
return(src.Depth() < window[bestSlot].Depth());
}
return(resDelta.Length() < bestDelta.Length());
}
private static int DeltaSizeLimit(DeltaWindowEntry res, int maxDepth, DeltaWindowEntry
src)
{
// Ideally the delta is at least 50% of the original size,
// but we also want to account for delta header overhead in
// the pack file (to point to the delta base) so subtract off
// some of those header bytes from the limit.
//
int limit = res.Size() / 2 - 20;
// Distribute the delta limit over the entire chain length.
// This is weighted such that deeper items in the chain must
// be even smaller than if they were earlier in the chain, as
// they cost significantly more to unpack due to the increased
// number of recursive unpack calls.
//
int remainingDepth = maxDepth - src.Depth();
return((limit * remainingDepth) / maxDepth);
}
// We should drop the current source entry from the window,
// it is somehow invalid for us to work with.
private bool IsBetterDelta(DeltaWindowEntry src, TemporaryBuffer.Heap resDelta)
{
if (bestDelta == null)
{
return true;
}
// If both delta sequences are the same length, use the one
// that has a shorter delta chain since it would be faster
// to access during reads.
//
if (resDelta.Length() == bestDelta.Length())
{
return src.Depth() < window[bestSlot].Depth();
}
return resDelta.Length() < bestDelta.Length();
}
/// <exception cref="System.IO.IOException"></exception>
private int Delta(DeltaWindowEntry src, int srcSlot)
{
// Objects must use only the same type as their delta base.
// If we are looking at something where that isn't true we
// have exhausted everything of the correct type and should
// move on to the next thing to examine.
//
if (src.Type() != res.Type())
{
KeepInWindow();
return NEXT_RES;
}
// Only consider a source with a short enough delta chain.
if (src.Depth() > resMaxDepth)
{
return NEXT_SRC;
}
// Estimate a reasonable upper limit on delta size.
int msz = DeltaSizeLimit(res, resMaxDepth, src);
if (msz <= 8)
{
return NEXT_SRC;
}
// If we have to insert a lot to make this work, find another.
if (res.Size() - src.Size() > msz)
{
return NEXT_SRC;
}
// If the sizes are radically different, this is a bad pairing.
if (res.Size() < src.Size() / 16)
{
return NEXT_SRC;
}
DeltaIndex srcIndex;
try
{
srcIndex = Index(src);
}
catch (LargeObjectException)
{
// If the source is too big to work on, skip it.
DropFromWindow(srcSlot);
return NEXT_SRC;
}
catch (IOException notAvailable)
{
if ([email protected]())
{
// This is an edge that is suddenly not available.
DropFromWindow(srcSlot);
return NEXT_SRC;
}
else
{
throw;
}
}
byte[] resBuf;
try
{
resBuf = Buffer(res);
}
catch (LargeObjectException)
{
// If its too big, move on to another item.
return NEXT_RES;
}
// If we already have a delta for the current object, abort
// encoding early if this new pairing produces a larger delta.
if (bestDelta != null && bestDelta.Length() < msz)
{
msz = (int)bestDelta.Length();
}
TemporaryBuffer.Heap delta = new TemporaryBuffer.Heap(msz);
try
{
if (!srcIndex.Encode(delta, resBuf, msz))
{
return NEXT_SRC;
}
}
catch (IOException)
{
// This only happens when the heap overflows our limit.
return NEXT_SRC;
}
if (IsBetterDelta(src, delta))
{
bestDelta = delta;
bestSlot = srcSlot;
}
return NEXT_SRC;
}
private static int DeltaSizeLimit(DeltaWindowEntry res, int maxDepth, DeltaWindowEntry
src)
{
// Ideally the delta is at least 50% of the original size,
// but we also want to account for delta header overhead in
// the pack file (to point to the delta base) so subtract off
// some of those header bytes from the limit.
//
int limit = res.Size() / 2 - 20;
// Distribute the delta limit over the entire chain length.
// This is weighted such that deeper items in the chain must
// be even smaller than if they were earlier in the chain, as
// they cost significantly more to unpack due to the increased
// number of recursive unpack calls.
//
int remainingDepth = maxDepth - src.Depth();
return (limit * remainingDepth) / maxDepth;
}
/// <exception cref="System.IO.IOException"></exception>
private int Delta(DeltaWindowEntry src, int srcSlot)
{
// Objects must use only the same type as their delta base.
// If we are looking at something where that isn't true we
// have exhausted everything of the correct type and should
// move on to the next thing to examine.
//
if (src.Type() != res.Type())
{
KeepInWindow();
return(NEXT_RES);
}
// Only consider a source with a short enough delta chain.
if (src.Depth() > resMaxDepth)
{
return(NEXT_SRC);
}
// Estimate a reasonable upper limit on delta size.
int msz = DeltaSizeLimit(res, resMaxDepth, src);
if (msz <= 8)
{
return(NEXT_SRC);
}
// If we have to insert a lot to make this work, find another.
if (res.Size() - src.Size() > msz)
{
return(NEXT_SRC);
}
// If the sizes are radically different, this is a bad pairing.
if (res.Size() < src.Size() / 16)
{
return(NEXT_SRC);
}
DeltaIndex srcIndex;
try
{
srcIndex = Index(src);
}
catch (LargeObjectException)
{
// If the source is too big to work on, skip it.
DropFromWindow(srcSlot);
return(NEXT_SRC);
}
catch (IOException notAvailable)
{
if ([email protected]())
{
// This is an edge that is suddenly not available.
DropFromWindow(srcSlot);
return(NEXT_SRC);
}
else
{
throw;
}
}
byte[] resBuf;
try
{
resBuf = Buffer(res);
}
catch (LargeObjectException)
{
// If its too big, move on to another item.
return(NEXT_RES);
}
// If we already have a delta for the current object, abort
// encoding early if this new pairing produces a larger delta.
if (bestDelta != null && bestDelta.Length() < msz)
{
msz = (int)bestDelta.Length();
}
TemporaryBuffer.Heap delta = new TemporaryBuffer.Heap(msz);
try
{
if (!srcIndex.Encode(delta, resBuf, msz))
{
return(NEXT_SRC);
}
}
catch (IOException)
{
// This only happens when the heap overflows our limit.
return(NEXT_SRC);
}
if (IsBetterDelta(src, delta))
{
bestDelta = delta;
bestSlot = srcSlot;
}
return(NEXT_SRC);
}
