jgit/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/dfs/DeltaBaseCache.java

/*
 * Copyright (C) 2011, Google Inc.
 * and other copyright owners as documented in the project's IP log.
 *
 * This program and the accompanying materials are made available
 * 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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 * without modification, are permitted provided that the following
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 * - Redistributions of source code must retain the above copyright
 *   notice, this list of conditions and the following disclaimer.
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 * - Redistributions in binary form must reproduce the above
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 *   disclaimer in the documentation and/or other materials provided
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 * - Neither the name of the Eclipse Foundation, Inc. nor the
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package org.eclipse.jgit.internal.storage.dfs;

import java.lang.ref.SoftReference;

/**
 * Caches recently used objects for {@link DfsReader}.
 * <p>
 * This cache is not thread-safe. Each reader should have its own cache.
 */
final class DeltaBaseCache {
	private static final int TABLE_BITS = 10;

	private static final int MASK_BITS = 32 - TABLE_BITS;

	private static int hash(long position) {
		return (((int) position) << MASK_BITS) >>> MASK_BITS;
	}

	private int maxByteCount;

	private final Slot[] table;

	private Slot lruHead;

	private Slot lruTail;

	private int curByteCount;

	DeltaBaseCache(DfsReader reader) {
		this(reader.getOptions().getDeltaBaseCacheLimit());
	}

	DeltaBaseCache(int maxBytes) {
		maxByteCount = maxBytes;
		table = new Slot[1 << TABLE_BITS];
	}

	Entry get(DfsPackKey key, long position) {
		Slot e = table[hash(position)];
		for (; e != null; e = e.tableNext) {
			if (e.offset == position && key.equals(e.pack)) {
				Entry buf = e.data.get();
				if (buf != null) {
					moveToHead(e);
					return buf;
				}
				return null;
			}
		}
		return null;
	}

	void put(DfsPackKey key, long offset, int objectType, byte[] data) {
		if (data.length > maxByteCount)
			return; // Too large to cache.

		curByteCount += data.length;
		releaseMemory();

		int tableIdx = hash(offset);
		Slot e = new Slot(key, offset, data.length);
		e.data = new SoftReference<Entry>(new Entry(data, objectType));
		e.tableNext = table[tableIdx];
		table[tableIdx] = e;
		lruPushHead(e);
	}

	private void releaseMemory() {
		while (curByteCount > maxByteCount && lruTail != null) {
			Slot e = lruTail;
			curByteCount -= e.size;
			lruRemove(e);
			removeFromTable(e);
		}
	}

	private void removeFromTable(Slot e) {
		int tableIdx = hash(e.offset);
		Slot p = table[tableIdx];

		if (p == e) {
			table[tableIdx] = e.tableNext;
			return;
		}

		for (; p != null; p = p.tableNext) {
			if (p.tableNext == e) {
				p.tableNext = e.tableNext;
				return;
			}
		}

		throw new IllegalStateException(String.format(
				"entry for %s:%d not in table", //$NON-NLS-1$
				e.pack, Long.valueOf(e.offset)));
	}

	private void moveToHead(final Slot e) {
		if (e != lruHead) {
			lruRemove(e);
			lruPushHead(e);
		}
	}

	private void lruRemove(final Slot e) {
		Slot p = e.lruPrev;
		Slot n = e.lruNext;

		if (p != null) {
			p.lruNext = n;
		} else {
			lruHead = n;
		}

		if (n != null) {
			n.lruPrev = p;
		} else {
			lruTail = p;
		}
	}

	private void lruPushHead(final Slot e) {
		Slot n = lruHead;
		e.lruNext = n;
		if (n != null)
			n.lruPrev = e;
		else
			lruTail = e;

		e.lruPrev = null;
		lruHead = e;
	}

	int getMemoryUsed() {
		return curByteCount;
	}

	int getMemoryUsedByLruChainForTest() {
		int r = 0;
		for (Slot e = lruHead; e != null; e = e.lruNext) {
			r += e.size;
		}
		return r;
	}

	int getMemoryUsedByTableForTest() {
		int r = 0;
		for (int i = 0; i < table.length; i++) {
			for (Slot e = table[i]; e != null; e = e.tableNext) {
				r += e.size;
			}
		}
		return r;
	}

	static class Entry {
		final byte[] data;

		final int type;

		Entry(final byte[] aData, final int aType) {
			data = aData;
			type = aType;
		}
	}

	private static class Slot {
		final DfsPackKey pack;

		final long offset;

		final int size;

		Slot tableNext;

		Slot lruPrev;

		Slot lruNext;

		SoftReference<Entry> data;

		Slot(DfsPackKey key, long offset, int size) {
			this.pack = key;
			this.offset = offset;
			this.size = size;
		}
	}
}