Browse Source
PackWriter now produces new deltas if there is not a suitable delta
available for reuse from an existing pack file. This permits JGit to
send less data on the wire by sending a delta relative to an object
the other side already has, instead of sending the whole object.
The delta searching algorithm is similar in style to what C Git
uses, but apparently has some differences (see below for more on).
Briefly, objects that should be considered for delta compression are
pushed onto a list. This list is then sorted by a rough similarity
score, which is derived from the path name the object was discovered
at in the repository during object counting. The list is then
walked in order.
At each position in the list, up to $WINDOW objects prior to it
are attempted as delta bases. Each object in the window is tried,
and the shortest delta instruction sequence selects the base object.
Some rough rules are used to prevent pathological behavior during
this matching phase, like skipping pairings of objects that are
not similar enough in size.
PackWriter intentionally excludes commits and annotated tags from
this new delta search phase. In the JGit repository only 28 out
of 2600+ commits can be delta compressed by C Git. As the commit
count tends to be a fair percentage of the total number of objects
in the repository, and they generally do not delta compress well,
skipping over them can improve performance with little increase in
the output pack size.
Because this implementation was rebuilt from scratch based on my own
memory of how the packing algorithm has evolved over the years in
C Git, PackWriter, DeltaWindow, and DeltaEncoder don't use exactly
the same rules everywhere, and that leads JGit to produce different
(but logically equivalent) pack files.
Repository | Pack Size (bytes) | Packing Time
| JGit - CGit = Difference | JGit / CGit
-----------+----------------------------------+-----------------
git | 25094348 - 24322890 = +771458 | 59.434s / 59.133s
jgit | 5669515 - 5709046 = - 39531 | 6.654s / 6.806s
linux-2.6 | 389M - 386M = +3M | 20m02s / 18m01s
For the above tests pack.threads was set to 1, window size=10,
delta depth=50, and delta and object reuse was disabled for both
implementations. Both implementations were reading from an already
fully packed repository on local disk. The running time reported
is after 1 warm-up run of the tested implementation.
PackWriter is writing 771 KiB more data on git.git, 3M more on
linux-2.6, but is actually 39.5 KiB smaller on jgit.git. Being
larger by less than 0.7% on linux-2.6 isn't bad, nor is taking an
extra 2 minutes to pack. On the running time side, JGit is at a
major disadvantage because linux-2.6 doesn't fit into the default
WindowCache of 20M, while C Git is able to mmap the entire pack and
have it available instantly in physical memory (assuming hot cache).
CGit also has a feature where it caches deltas that were created
during the compression phase, and uses those cached deltas during
the writing phase. PackWriter does not implement this (yet),
and therefore must create every delta twice. This could easily
account for the increased running time we are seeing.
Change-Id: I6292edc66c2e95fbe45b519b65fdb3918068889c
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
stable-0.9
Shawn O. Pearce
15 years ago
3 changed files with 622 additions and 1 deletions
@ -0,0 +1,369 @@ |
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/* |
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* Copyright (C) 2010, Google Inc. |
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* and other copyright owners as documented in the project's IP log. |
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* |
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* This program and the accompanying materials are made available |
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* under the terms of the Eclipse Distribution License v1.0 which |
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* accompanies this distribution, is reproduced below, and is |
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* available at http://www.eclipse.org/org/documents/edl-v10.php
|
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* |
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* All rights reserved. |
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* |
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* Redistribution and use in source and binary forms, with or |
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* without modification, are permitted provided that the following |
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* conditions are met: |
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* |
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* - Redistributions of source code must retain the above copyright |
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* notice, this list of conditions and the following disclaimer. |
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* |
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* - Redistributions in binary form must reproduce the above |
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* copyright notice, this list of conditions and the following |
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* disclaimer in the documentation and/or other materials provided |
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* with the distribution. |
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* |
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* - Neither the name of the Eclipse Foundation, Inc. nor the |
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* names of its contributors may be used to endorse or promote |
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* products derived from this software without specific prior |
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* written permission. |
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* |
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* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND |
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* CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, |
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* INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES |
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* OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE |
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* ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR |
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* CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, |
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* SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT |
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* NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; |
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* LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER |
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* CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, |
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* STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) |
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* ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF |
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* ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. |
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*/ |
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package org.eclipse.jgit.storage.pack; |
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import java.io.IOException; |
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import org.eclipse.jgit.errors.IncorrectObjectTypeException; |
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import org.eclipse.jgit.errors.LargeObjectException; |
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import org.eclipse.jgit.errors.MissingObjectException; |
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import org.eclipse.jgit.lib.ObjectReader; |
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import org.eclipse.jgit.lib.ProgressMonitor; |
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import org.eclipse.jgit.util.TemporaryBuffer; |
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class DeltaWindow { |
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private static final int NEXT_RES = 0; |
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private static final int NEXT_SRC = 1; |
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private final PackWriter writer; |
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private final ObjectReader reader; |
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private final DeltaWindowEntry[] window; |
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/** Maximum depth we should create for any delta chain. */ |
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private final int maxDepth; |
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// The object we are currently considering needs a lot of state:
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/** Position of {@link #res} within {@link #window} array. */ |
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private int resSlot; |
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/** |
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* Maximum delta chain depth the current object can have. |
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* <p> |
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* This can be smaller than {@link #maxDepth}. |
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*/ |
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private int resMaxDepth; |
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/** Window entry of the object we are currently considering. */ |
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private DeltaWindowEntry res; |
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/** If we have a delta for {@link #res}, this is the shortest found yet. */ |
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private TemporaryBuffer.Heap bestDelta; |
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/** If we have {@link #bestDelta}, the window position it was created by. */ |
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private int bestSlot; |
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DeltaWindow(PackWriter pw, ObjectReader or) { |
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writer = pw; |
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reader = or; |
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// C Git increases the window size supplied by the user by 1.
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// We don't know why it does this, but if the user asks for
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// window=10, it actually processes with window=11. Because
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// the window size has the largest direct impact on the final
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// pack file size, we match this odd behavior here to give us
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// a better chance of producing a similar sized pack as C Git.
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//
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// We would prefer to directly honor the user's request since
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// PackWriter has a minimum of 2 for the window size, but then
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// users might complain that JGit is creating a bigger pack file.
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//
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window = new DeltaWindowEntry[pw.getDeltaSearchWindowSize() + 1]; |
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for (int i = 0; i < window.length; i++) |
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window[i] = new DeltaWindowEntry(); |
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maxDepth = pw.getMaxDeltaDepth(); |
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} |
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void search(ProgressMonitor monitor, ObjectToPack[] toSearch, int off, |
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int cnt) throws IOException { |
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for (int end = off + cnt; off < end; off++) { |
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monitor.update(1); |
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res = window[resSlot]; |
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res.set(toSearch[off]); |
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if (res.object.isDoNotDelta()) { |
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// PackWriter marked edge objects with the do-not-delta flag.
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// They are the only ones that appear in toSearch with it set,
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// but we don't actually want to make a delta for them, just
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// need to push them into the window so they can be read by
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// other objects coming through.
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//
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keepInWindow(); |
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} else { |
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// Search for a delta for the current window slot.
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//
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search(); |
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} |
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} |
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} |
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private void search() throws IOException { |
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// TODO(spearce) If the object is used as a base for other
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// objects in this pack we should limit the depth we create
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// for ourselves to be the remainder of our longest dependent
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// chain and the configured maximum depth. This can happen
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// when the dependents are being reused out a pack, but we
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// cannot be because we are near the edge of a thin pack.
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//
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resMaxDepth = maxDepth; |
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// Loop through the window backwards, considering every entry.
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// This lets us look at the bigger objects that came before.
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//
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for (int srcSlot = prior(resSlot); srcSlot != resSlot; srcSlot = prior(srcSlot)) { |
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DeltaWindowEntry src = window[srcSlot]; |
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if (src.empty()) |
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break; |
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if (delta(src, srcSlot) == NEXT_RES) { |
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bestDelta = null; |
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return; |
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} |
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} |
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// We couldn't find a suitable delta for this object, but it may
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// still be able to act as a base for another one.
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//
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if (bestDelta == null) { |
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keepInWindow(); |
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return; |
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} |
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// Select this best matching delta as the base for the object.
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//
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ObjectToPack srcObj = window[bestSlot].object; |
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ObjectToPack resObj = res.object; |
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if (srcObj.isDoNotDelta()) { |
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// The source (the delta base) is an edge object outside of the
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// pack. Its part of the common base set that the peer already
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// has on hand, so we don't want to send it. We have to store
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// an ObjectId and *NOT* an ObjectToPack for the base to ensure
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// the base isn't included in the outgoing pack file.
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//
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resObj.setDeltaBase(srcObj.copy()); |
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} else { |
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// The base is part of the pack we are sending, so it should be
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// a direct pointer to the base.
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//
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resObj.setDeltaBase(srcObj); |
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} |
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resObj.setDeltaDepth(srcObj.getDeltaDepth() + 1); |
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resObj.clearReuseAsIs(); |
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// Discard the cached best result, otherwise it leaks.
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//
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bestDelta = null; |
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// If this should be the end of a chain, don't keep
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// it in the window. Just move on to the next object.
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//
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if (resObj.getDeltaDepth() == maxDepth) |
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return; |
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shuffleBaseUpInPriority(); |
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keepInWindow(); |
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} |
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private int delta(final DeltaWindowEntry src, final int srcSlot) |
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throws IOException { |
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// Objects must use only the same type as their delta base.
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// If we are looking at something where that isn't true we
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// have exhausted everything of the correct type and should
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// move on to the next thing to examine.
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//
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if (src.type() != res.type()) { |
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keepInWindow(); |
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return NEXT_RES; |
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} |
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// Only consider a source with a short enough delta chain.
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if (src.depth() > resMaxDepth) |
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return NEXT_SRC; |
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// Estimate a reasonable upper limit on delta size.
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int msz = deltaSizeLimit(res, resMaxDepth, src); |
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if (msz <= 8) |
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return NEXT_SRC; |
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// If we have to insert a lot to make this work, find another.
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if (res.size() - src.size() > msz) |
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return NEXT_SRC; |
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// If the sizes are radically different, this is a bad pairing.
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if (res.size() < src.size() / 16) |
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return NEXT_SRC; |
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DeltaIndex srcIndex; |
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try { |
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srcIndex = index(src); |
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} catch (LargeObjectException tooBig) { |
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// If the source is too big to work on, skip it.
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dropFromWindow(srcSlot); |
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return NEXT_SRC; |
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} catch (IOException notAvailable) { |
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if (src.object.isDoNotDelta()) { |
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// This is an edge that is suddenly not available.
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dropFromWindow(srcSlot); |
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return NEXT_SRC; |
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} else { |
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throw notAvailable; |
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} |
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} |
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byte[] resBuf; |
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try { |
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resBuf = buffer(res); |
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} catch (LargeObjectException tooBig) { |
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// If its too big, move on to another item.
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return NEXT_RES; |
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} |
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// If we already have a delta for the current object, abort
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// encoding early if this new pairing produces a larger delta.
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if (bestDelta != null && bestDelta.length() < msz) |
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msz = (int) bestDelta.length(); |
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TemporaryBuffer.Heap delta = new TemporaryBuffer.Heap(msz); |
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try { |
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if (!srcIndex.encode(delta, resBuf, msz)) |
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return NEXT_SRC; |
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} catch (IOException deltaTooBig) { |
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// This only happens when the heap overflows our limit.
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return NEXT_SRC; |
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} |
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if (isBetterDelta(src, delta)) { |
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bestDelta = delta; |
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bestSlot = srcSlot; |
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} |
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return NEXT_SRC; |
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} |
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private void shuffleBaseUpInPriority() { |
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// Shuffle the entire window so that the best match we just used
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// is at our current index, and our current object is at the index
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// before it. Slide any entries in between to make space.
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//
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window[resSlot] = window[bestSlot]; |
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DeltaWindowEntry next = res; |
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int slot = prior(resSlot); |
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for (; slot != bestSlot; slot = prior(slot)) { |
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DeltaWindowEntry e = window[slot]; |
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window[slot] = next; |
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next = e; |
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} |
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window[slot] = next; |
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} |
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private void keepInWindow() { |
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if (++resSlot == window.length) |
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resSlot = 0; |
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} |
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private int prior(int slot) { |
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if (slot == 0) |
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return window.length - 1; |
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return slot - 1; |
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} |
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private void dropFromWindow(@SuppressWarnings("unused") int srcSlot) { |
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// We should drop the current source entry from the window,
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// it is somehow invalid for us to work with.
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} |
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private boolean isBetterDelta(DeltaWindowEntry src, |
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TemporaryBuffer.Heap resDelta) { |
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if (bestDelta == null) |
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return true; |
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// If both delta sequences are the same length, use the one
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// that has a shorter delta chain since it would be faster
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// to access during reads.
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//
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if (resDelta.length() == bestDelta.length()) |
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return src.depth() < window[bestSlot].depth(); |
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return resDelta.length() < bestDelta.length(); |
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} |
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private static int deltaSizeLimit(DeltaWindowEntry res, int maxDepth, |
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DeltaWindowEntry src) { |
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// Ideally the delta is at least 50% of the original size,
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// but we also want to account for delta header overhead in
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// the pack file (to point to the delta base) so subtract off
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// some of those header bytes from the limit.
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//
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final int limit = res.size() / 2 - 20; |
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// Distribute the delta limit over the entire chain length.
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// This is weighted such that deeper items in the chain must
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// be even smaller than if they were earlier in the chain, as
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// they cost significantly more to unpack due to the increased
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// number of recursive unpack calls.
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//
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final int remainingDepth = maxDepth - src.depth(); |
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return (limit * remainingDepth) / maxDepth; |
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} |
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private DeltaIndex index(DeltaWindowEntry ent) |
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throws MissingObjectException, IncorrectObjectTypeException, |
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IOException, LargeObjectException { |
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DeltaIndex idx = ent.index; |
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if (idx == null) { |
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try { |
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idx = new DeltaIndex(buffer(ent)); |
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} catch (OutOfMemoryError noMemory) { |
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LargeObjectException e = new LargeObjectException(ent.object); |
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e.initCause(noMemory); |
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throw e; |
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} |
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ent.index = idx; |
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} |
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return idx; |
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} |
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private byte[] buffer(DeltaWindowEntry ent) throws MissingObjectException, |
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IncorrectObjectTypeException, IOException, LargeObjectException { |
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byte[] buf = ent.buffer; |
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if (buf == null) |
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ent.buffer = buf = writer.buffer(reader, ent.object); |
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return buf; |
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} |
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} |
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@ -0,0 +1,80 @@ |
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/* |
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* Copyright (C) 2010, Google Inc. |
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* and other copyright owners as documented in the project's IP log. |
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* |
||||||
|
* This program and the accompanying materials are made available |
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|
* under the terms of the Eclipse Distribution License v1.0 which |
||||||
|
* accompanies this distribution, is reproduced below, and is |
||||||
|
* available at http://www.eclipse.org/org/documents/edl-v10.php
|
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|
* |
||||||
|
* All rights reserved. |
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|
* |
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|
* Redistribution and use in source and binary forms, with or |
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|
* without modification, are permitted provided that the following |
||||||
|
* conditions are met: |
||||||
|
* |
||||||
|
* - Redistributions of source code must retain the above copyright |
||||||
|
* notice, this list of conditions and the following disclaimer. |
||||||
|
* |
||||||
|
* - Redistributions in binary form must reproduce the above |
||||||
|
* copyright notice, this list of conditions and the following |
||||||
|
* disclaimer in the documentation and/or other materials provided |
||||||
|
* with the distribution. |
||||||
|
* |
||||||
|
* - Neither the name of the Eclipse Foundation, Inc. nor the |
||||||
|
* names of its contributors may be used to endorse or promote |
||||||
|
* products derived from this software without specific prior |
||||||
|
* written permission. |
||||||
|
* |
||||||
|
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND |
||||||
|
* CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, |
||||||
|
* INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES |
||||||
|
* OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE |
||||||
|
* ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR |
||||||
|
* CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, |
||||||
|
* SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT |
||||||
|
* NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; |
||||||
|
* LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER |
||||||
|
* CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, |
||||||
|
* STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) |
||||||
|
* ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF |
||||||
|
* ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. |
||||||
|
*/ |
||||||
|
|
||||||
|
package org.eclipse.jgit.storage.pack; |
||||||
|
|
||||||
|
class DeltaWindowEntry { |
||||||
|
ObjectToPack object; |
||||||
|
|
||||||
|
/** Complete contents of this object. Lazily loaded. */ |
||||||
|
byte[] buffer; |
||||||
|
|
||||||
|
/** Index of this object's content, to encode other deltas. Lazily loaded. */ |
||||||
|
DeltaIndex index; |
||||||
|
|
||||||
|
void set(ObjectToPack object) { |
||||||
|
this.object = object; |
||||||
|
this.index = null; |
||||||
|
this.buffer = null; |
||||||
|
} |
||||||
|
|
||||||
|
/** @return current delta chain depth of this object. */ |
||||||
|
int depth() { |
||||||
|
return object.getDeltaDepth(); |
||||||
|
} |
||||||
|
|
||||||
|
/** @return type of the object in this window entry. */ |
||||||
|
int type() { |
||||||
|
return object.getType(); |
||||||
|
} |
||||||
|
|
||||||
|
/** @return estimated unpacked size of the object, in bytes . */ |
||||||
|
int size() { |
||||||
|
return object.getWeight(); |
||||||
|
} |
||||||
|
|
||||||
|
/** @return true if there is no object stored in this entry. */ |
||||||
|
boolean empty() { |
||||||
|
return object == null; |
||||||
|
} |
||||||
|
} |
||||||
Loading…
Reference in new issue