org.eclipse.jgit/src/org/eclipse/jgit/lib/OffsetCache.java - jgit - Git at Google

 /*
  * Copyright (C) 2009, 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
  *
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or
  * 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.lib;

 import java.io.IOException;
 import java.lang.ref.ReferenceQueue;
 import java.lang.ref.SoftReference;
 import java.util.Random;
 import java.util.concurrent.atomic.AtomicLong;
 import java.util.concurrent.atomic.AtomicReferenceArray;
 import java.util.concurrent.locks.ReentrantLock;

 /**
  * Least frequently used cache for objects specified by PackFile positions.
  * <p>
  * This cache maps a <code>({@link PackFile},position)</code> tuple to an Object.
  * <p>
  * This cache is suitable for objects that are "relative expensive" to compute
  * from the underlying PackFile, given some known position in that file.
  * <p>
  * Whenever a cache miss occurs, {@link #load(PackFile, long)} is invoked by
  * exactly one thread for the given <code>(PackFile,position)</code> key tuple.
  * This is ensured by an array of locks, with the tuple hashed to a lock
  * instance.
  * <p>
  * During a miss, older entries are evicted from the cache so long as
  * {@link #isFull()} returns true.
  * <p>
  * Its too expensive during object access to be 100% accurate with a least
  * recently used (LRU) algorithm. Strictly ordering every read is a lot of
  * overhead that typically doesn't yield a corresponding benefit to the
  * application.
  * <p>
  * This cache implements a loose LRU policy by randomly picking a window
  * comprised of roughly 10% of the cache, and evicting the oldest accessed entry
  * within that window.
  * <p>
  * Entities created by the cache are held under SoftReferences, permitting the
  * Java runtime's garbage collector to evict entries when heap memory gets low.
  * Most JREs implement a loose least recently used algorithm for this eviction.
  * <p>
  * The internal hash table does not expand at runtime, instead it is fixed in
  * size at cache creation time. The internal lock table used to gate load
  * invocations is also fixed in size.
  * <p>
  * The key tuple is passed through to methods as a pair of parameters rather
  * than as a single Object, thus reducing the transient memory allocations of
  * callers. It is more efficient to avoid the allocation, as we can't be 100%
  * sure that a JIT would be able to stack-allocate a key tuple.
  * <p>
  * This cache has an implementation rule such that:
  * <ul>
  * <li>{@link #load(PackFile, long)} is invoked by at most one thread at a time
  * for a given <code>(PackFile,position)</code> tuple.</li>
  * <li>For every <code>load()</code> invocation there is exactly one
  * {@link #createRef(PackFile, long, Object)} invocation to wrap a SoftReference
  * around the cached entity.</li>
  * <li>For every Reference created by <code>createRef()</code> there will be
  * exactly one call to {@link #clear(Ref)} to cleanup any resources associated
  * with the (now expired) cached entity.</li>
  * </ul>
  * <p>
  * Therefore, it is safe to perform resource accounting increments during the
  * {@link #load(PackFile, long)} or {@link #createRef(PackFile, long, Object)}
  * methods, and matching decrements during {@link #clear(Ref)}. Implementors may
  * need to override {@link #createRef(PackFile, long, Object)} in order to embed
  * additional accounting information into an implementation specific
  * {@link OffsetCache.Ref} subclass, as the cached entity may have already been
  * evicted by the JRE's garbage collector.
  * <p>
  * To maintain higher concurrency workloads, during eviction only one thread
  * performs the eviction work, while other threads can continue to insert new
  * objects in parallel. This means that the cache can be temporarily over limit,
  * especially if the nominated eviction thread is being starved relative to the
  * other threads.
  *
  * @param <V>
  *            type of value stored in the cache.
  * @param <R>
  *            type of {@link OffsetCache.Ref} subclass used by the cache.
  */
 abstract class OffsetCache<V, R extends OffsetCache.Ref<V>> {
 	private static final Random rng = new Random();

 	/** ReferenceQueue that {@link #createRef(PackFile, long, Object)} must use. */
 	protected final ReferenceQueue<V> queue;

 	/** Number of entries in {@link #table}. */
 	private final int tableSize;

 	/** Access clock for loose LRU. */
 	private final AtomicLong clock;

 	/** Hash bucket directory; entries are chained below. */
 	private final AtomicReferenceArray<Entry<V>> table;

 	/** Locks to prevent concurrent loads for same (PackFile,position). */
 	private final Lock[] locks;

 	/** Lock to elect the eviction thread after a load occurs. */
 	private final ReentrantLock evictLock;

 	/** Number of {@link #table} buckets to scan for an eviction window. */
 	private final int evictBatch;

 	/**
 	 * Create a new cache with a fixed size entry table and lock table.
 	 *
 	 * @param tSize
 	 *            number of entries in the entry hash table.
 	 * @param lockCount
 	 *            number of entries in the lock table. This is the maximum
 	 *            concurrency rate for creation of new objects through
 	 *            {@link #load(PackFile, long)} invocations.
 	 */
 	OffsetCache(final int tSize, final int lockCount) {
 		if (tSize < 1)
 			throw new IllegalArgumentException("tSize must be >= 1");
 		if (lockCount < 1)
 			throw new IllegalArgumentException("lockCount must be >= 1");

 		queue = new ReferenceQueue<V>();
 		tableSize = tSize;
 		clock = new AtomicLong(1);
 		table = new AtomicReferenceArray<Entry<V>>(tableSize);
 		locks = new Lock[lockCount];
 		for (int i = 0; i < locks.length; i++)
 			locks[i] = new Lock();
 		evictLock = new ReentrantLock();

 		int eb = (int) (tableSize * .1);
 		if (64 < eb)
 			eb = 64;
 		else if (eb < 4)
 			eb = 4;
 		if (tableSize < eb)
 			eb = tableSize;
 		evictBatch = eb;
 	}

 	/**
 	 * Lookup a cached object, creating and loading it if it doesn't exist.
 	 *
 	 * @param pack
 	 *            the pack that "contains" the cached object.
 	 * @param position
 	 *            offset within <code>pack</code> of the object.
 	 * @return the object reference.
 	 * @throws IOException
 	 *             the object reference was not in the cache and could not be
 	 *             obtained by {@link #load(PackFile, long)}.
 	 */
 	V getOrLoad(final PackFile pack, final long position) throws IOException {
 		final int slot = slot(pack, position);
 		final Entry<V> e1 = table.get(slot);
 		V v = scan(e1, pack, position);
 		if (v != null)
 			return v;

 		synchronized (lock(pack, position)) {
 			Entry<V> e2 = table.get(slot);
 			if (e2 != e1) {
 				v = scan(e2, pack, position);
 				if (v != null)
 					return v;
 			}

 			v = load(pack, position);
 			final Ref<V> ref = createRef(pack, position, v);
 			hit(ref);
 			for (;;) {
 				final Entry<V> n = new Entry<V>(clean(e2), ref);
 				if (table.compareAndSet(slot, e2, n))
 					break;
 				e2 = table.get(slot);
 			}
 		}

 		if (evictLock.tryLock()) {
 			try {
 				gc();
 				evict();
 			} finally {
 				evictLock.unlock();
 			}
 		}

 		return v;
 	}

 	private V scan(Entry<V> n, final PackFile pack, final long position) {
 		for (; n != null; n = n.next) {
 			final Ref<V> r = n.ref;
 			if (r.pack == pack && r.position == position) {
 				final V v = r.get();
 				if (v != null) {
 					hit(r);
 					return v;
 				}
 				n.kill();
 				break;
 			}
 		}
 		return null;
 	}

 	private void hit(final Ref<V> r) {
 		// We don't need to be 100% accurate here. Its sufficient that at least
 		// one thread performs the increment. Any other concurrent access at
 		// exactly the same time can simply use the same clock value.
 		//
 		// Consequently we attempt the set, but we don't try to recover should
 		// it fail. This is why we don't use getAndIncrement() here.
 		//
 		final long c = clock.get();
 		clock.compareAndSet(c, c + 1);
 		r.lastAccess = c;
 	}

 	private void evict() {
 		while (isFull()) {
 			int ptr = rng.nextInt(tableSize);
 			Entry<V> old = null;
 			int slot = 0;
 			for (int b = evictBatch - 1; b >= 0; b--, ptr++) {
 				if (tableSize <= ptr)
 					ptr = 0;
 				for (Entry<V> e = table.get(ptr); e != null; e = e.next) {
 					if (e.dead)
 						continue;
 					if (old == null || e.ref.lastAccess < old.ref.lastAccess) {
 						old = e;
 						slot = ptr;
 					}
 				}
 			}
 			if (old != null) {
 				old.kill();
 				gc();
 				final Entry<V> e1 = table.get(slot);
 				table.compareAndSet(slot, e1, clean(e1));
 			}
 		}
 	}

 	/**
 	 * Clear every entry from the cache.
 	 *<p>
 	 * This is a last-ditch effort to clear out the cache, such as before it
 	 * gets replaced by another cache that is configured differently. This
 	 * method tries to force every cached entry through {@link #clear(Ref)} to
 	 * ensure that resources are correctly accounted for and cleaned up by the
 	 * subclass. A concurrent reader loading entries while this method is
 	 * running may cause resource accounting failures.
 	 */
 	void removeAll() {
 		for (int s = 0; s < tableSize; s++) {
 			Entry<V> e1;
 			do {
 				e1 = table.get(s);
 				for (Entry<V> e = e1; e != null; e = e.next)
 					e.kill();
 			} while (!table.compareAndSet(s, e1, null));
 		}
 		gc();
 	}

 	/**
 	 * Clear all entries related to a single file.
 	 * <p>
 	 * Typically this method is invoked during {@link PackFile#close()}, when we
 	 * know the pack is never going to be useful to us again (for example, it no
 	 * longer exists on disk). A concurrent reader loading an entry from this
 	 * same pack may cause the pack to become stuck in the cache anyway.
 	 *
 	 * @param pack
 	 *            the file to purge all entries of.
 	 */
 	void removeAll(final PackFile pack) {
 		for (int s = 0; s < tableSize; s++) {
 			final Entry<V> e1 = table.get(s);
 			boolean hasDead = false;
 			for (Entry<V> e = e1; e != null; e = e.next) {
 				if (e.ref.pack == pack) {
 					e.kill();
 					hasDead = true;
 				} else if (e.dead)
 					hasDead = true;
 			}
 			if (hasDead)
 				table.compareAndSet(s, e1, clean(e1));
 		}
 		gc();
 	}

 	/**
 	 * Materialize an object that doesn't yet exist in the cache.
 	 * <p>
 	 * This method is invoked by {@link #getOrLoad(PackFile, long)} when the
 	 * specified entity does not yet exist in the cache. Internal locking
 	 * ensures that at most one thread can call this method for each unique
 	 * <code>(pack,position)</code>, but multiple threads can call this method
 	 * concurrently for different <code>(pack,position)</code> tuples.
 	 *
 	 * @param pack
 	 *            the file to materialize the entry from.
 	 * @param position
 	 *            offset within the file of the entry.
 	 * @return the materialized object. Must never be null.
 	 * @throws IOException
 	 *             the method was unable to materialize the object for this
 	 *             input pair. The usual reasons would be file corruption, file
 	 *             not found, out of file descriptors, etc.
 	 */
 	protected abstract V load(PackFile pack, long position) throws IOException;

 	/**
 	 * Construct a Ref (SoftReference) around a cached entity.
 	 * <p>
 	 * Implementing this is only necessary if the subclass is performing
 	 * resource accounting during {@link #load(PackFile, long)} and
 	 * {@link #clear(Ref)} requires some information to update the accounting.
 	 * <p>
 	 * Implementors <b>MUST</b> ensure that the returned reference uses the
 	 * {@link #queue} ReferenceQueue, otherwise {@link #clear(Ref)} will not be
 	 * invoked at the proper time.
 	 *
 	 * @param pack
 	 *            the file to materialize the entry from.
 	 * @param position
 	 *            offset within the file of the entry.
 	 * @param v
 	 *            the object returned by {@link #load(PackFile, long)}.
 	 * @return a soft reference subclass wrapped around <code>v</code>.
 	 */
 	@SuppressWarnings("unchecked")
 	protected R createRef(final PackFile pack, final long position, final V v) {
 		return (R) new Ref<V>(pack, position, v, queue);
 	}

 	/**
 	 * Update accounting information now that an object has left the cache.
 	 * <p>
 	 * This method is invoked exactly once for the combined
 	 * {@link #load(PackFile, long)} and
 	 * {@link #createRef(PackFile, long, Object)} invocation pair that was used
 	 * to construct and insert an object into the cache.
 	 *
 	 * @param ref
 	 *            the reference wrapped around the object. Implementations must
 	 *            be prepared for <code>ref.get()</code> to return null.
 	 */
 	protected void clear(final R ref) {
 		// Do nothing by default.
 	}

 	/**
 	 * Determine if the cache is full and requires eviction of entries.
 	 * <p>
 	 * By default this method returns false. Implementors may override to
 	 * consult with the accounting updated by {@link #load(PackFile, long)},
 	 * {@link #createRef(PackFile, long, Object)} and {@link #clear(Ref)}.
 	 *
 	 * @return true if the cache is still over-limit and requires eviction of
 	 *         more entries.
 	 */
 	protected boolean isFull() {
 		return false;
 	}

 	@SuppressWarnings("unchecked")
 	private void gc() {
 		R r;
 		while ((r = (R) queue.poll()) != null) {
 			// Sun's Java 5 and 6 implementation have a bug where a Reference
 			// can be enqueued and dequeued twice on the same reference queue
 			// due to a race condition within ReferenceQueue.enqueue(Reference).
 			//
 			// http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6837858
 			//
 			// We CANNOT permit a Reference to come through us twice, as it will
 			// skew the resource counters we maintain. Our canClear() check here
 			// provides a way to skip the redundant dequeues, if any.
 			//
 			if (r.canClear()) {
 				clear(r);

 				boolean found = false;
 				final int s = slot(r.pack, r.position);
 				final Entry<V> e1 = table.get(s);
 				for (Entry<V> n = e1; n != null; n = n.next) {
 					if (n.ref == r) {
 						n.dead = true;
 						found = true;
 						break;
 					}
 				}
 				if (found)
 					table.compareAndSet(s, e1, clean(e1));
 			}
 		}
 	}

 	/**
 	 * Compute the hash code value for a <code>(PackFile,position)</code> tuple.
 	 * <p>
 	 * For example, <code>return packHash + (int) (position >>> 4)</code>.
 	 * Implementors must override with a suitable hash (for example, a different
 	 * right shift on the position).
 	 *
 	 * @param packHash
 	 *            hash code for the file being accessed.
 	 * @param position
 	 *            position within the file being accessed.
 	 * @return a reasonable hash code mixing the two values.
 	 */
 	protected abstract int hash(int packHash, long position);

 	private int slot(final PackFile pack, final long position) {
 		return (hash(pack.hash, position) >>> 1) % tableSize;
 	}

 	private Lock lock(final PackFile pack, final long position) {
 		return locks[(hash(pack.hash, position) >>> 1) % locks.length];
 	}

 	private static <V> Entry<V> clean(Entry<V> top) {
 		while (top != null && top.dead) {
 			top.ref.enqueue();
 			top = top.next;
 		}
 		if (top == null)
 			return null;
 		final Entry<V> n = clean(top.next);
 		return n == top.next ? top : new Entry<V>(n, top.ref);
 	}

 	private static class Entry<V> {
 		/** Next entry in the hash table's chain list. */
 		final Entry<V> next;

 		/** The referenced object. */
 		final Ref<V> ref;

 		/**
 		 * Marked true when ref.get() returns null and the ref is dead.
 		 * <p>
 		 * A true here indicates that the ref is no longer accessible, and that
 		 * we therefore need to eventually purge this Entry object out of the
 		 * bucket's chain.
 		 */
 		volatile boolean dead;

 		Entry(final Entry<V> n, final Ref<V> r) {
 			next = n;
 			ref = r;
 		}

 		final void kill() {
 			dead = true;
 			ref.enqueue();
 		}
 	}

 	/**
 	 * A soft reference wrapped around a cached object.
 	 *
 	 * @param <V>
 	 *            type of the cached object.
 	 */
 	protected static class Ref<V> extends SoftReference<V> {
 		final PackFile pack;

 		final long position;

 		long lastAccess;

 		private boolean cleared;

 		protected Ref(final PackFile pack, final long position, final V v,
 				final ReferenceQueue<V> queue) {
 			super(v, queue);
 			this.pack = pack;
 			this.position = position;
 		}

 		final synchronized boolean canClear() {
 			if (cleared)
 				return false;
 			cleared = true;
 			return true;
 		}
 	}

 	private static final class Lock {
 		// Used only for its implicit monitor.
 	}
 }
	/*
	* Copyright (C) 2009, 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
	*
	* All rights reserved.
	*
	* Redistribution and use in source and binary forms, with or
	* 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.lib;

	import java.io.IOException;
	import java.lang.ref.ReferenceQueue;
	import java.lang.ref.SoftReference;
	import java.util.Random;
	import java.util.concurrent.atomic.AtomicLong;
	import java.util.concurrent.atomic.AtomicReferenceArray;
	import java.util.concurrent.locks.ReentrantLock;

	/**
	* Least frequently used cache for objects specified by PackFile positions.
	* <p>
	* This cache maps a <code>({@link PackFile},position)</code> tuple to an Object.
	* <p>
	* This cache is suitable for objects that are "relative expensive" to compute
	* from the underlying PackFile, given some known position in that file.
	* <p>
	* Whenever a cache miss occurs, {@link #load(PackFile, long)} is invoked by
	* exactly one thread for the given <code>(PackFile,position)</code> key tuple.
	* This is ensured by an array of locks, with the tuple hashed to a lock
	* instance.
	* <p>
	* During a miss, older entries are evicted from the cache so long as
	* {@link #isFull()} returns true.
	* <p>
	* Its too expensive during object access to be 100% accurate with a least
	* recently used (LRU) algorithm. Strictly ordering every read is a lot of
	* overhead that typically doesn't yield a corresponding benefit to the
	* application.
	* <p>
	* This cache implements a loose LRU policy by randomly picking a window
	* comprised of roughly 10% of the cache, and evicting the oldest accessed entry
	* within that window.
	* <p>
	* Entities created by the cache are held under SoftReferences, permitting the
	* Java runtime's garbage collector to evict entries when heap memory gets low.
	* Most JREs implement a loose least recently used algorithm for this eviction.
	* <p>
	* The internal hash table does not expand at runtime, instead it is fixed in
	* size at cache creation time. The internal lock table used to gate load
	* invocations is also fixed in size.
	* <p>
	* The key tuple is passed through to methods as a pair of parameters rather
	* than as a single Object, thus reducing the transient memory allocations of
	* callers. It is more efficient to avoid the allocation, as we can't be 100%
	* sure that a JIT would be able to stack-allocate a key tuple.
	* <p>
	* This cache has an implementation rule such that:
	* <ul>
	* <li>{@link #load(PackFile, long)} is invoked by at most one thread at a time
	* for a given <code>(PackFile,position)</code> tuple.</li>
	* <li>For every <code>load()</code> invocation there is exactly one
	* {@link #createRef(PackFile, long, Object)} invocation to wrap a SoftReference
	* around the cached entity.</li>
	* <li>For every Reference created by <code>createRef()</code> there will be
	* exactly one call to {@link #clear(Ref)} to cleanup any resources associated
	* with the (now expired) cached entity.</li>
	* </ul>
	* <p>
	* Therefore, it is safe to perform resource accounting increments during the
	* {@link #load(PackFile, long)} or {@link #createRef(PackFile, long, Object)}
	* methods, and matching decrements during {@link #clear(Ref)}. Implementors may
	* need to override {@link #createRef(PackFile, long, Object)} in order to embed
	* additional accounting information into an implementation specific
	* {@link OffsetCache.Ref} subclass, as the cached entity may have already been
	* evicted by the JRE's garbage collector.
	* <p>
	* To maintain higher concurrency workloads, during eviction only one thread
	* performs the eviction work, while other threads can continue to insert new
	* objects in parallel. This means that the cache can be temporarily over limit,
	* especially if the nominated eviction thread is being starved relative to the
	* other threads.
	*
	* @param <V>
	* type of value stored in the cache.
	* @param <R>
	* type of {@link OffsetCache.Ref} subclass used by the cache.
	*/
	abstract class OffsetCache<V, R extends OffsetCache.Ref<V>> {
	private static final Random rng = new Random();

	/** ReferenceQueue that {@link #createRef(PackFile, long, Object)} must use. */
	protected final ReferenceQueue<V> queue;

	/** Number of entries in {@link #table}. */
	private final int tableSize;

	/** Access clock for loose LRU. */
	private final AtomicLong clock;

	/** Hash bucket directory; entries are chained below. */
	private final AtomicReferenceArray<Entry<V>> table;

	/** Locks to prevent concurrent loads for same (PackFile,position). */
	private final Lock[] locks;

	/** Lock to elect the eviction thread after a load occurs. */
	private final ReentrantLock evictLock;

	/** Number of {@link #table} buckets to scan for an eviction window. */
	private final int evictBatch;

	/**
	* Create a new cache with a fixed size entry table and lock table.
	*
	* @param tSize
	* number of entries in the entry hash table.
	* @param lockCount
	* number of entries in the lock table. This is the maximum
	* concurrency rate for creation of new objects through
	* {@link #load(PackFile, long)} invocations.
	*/
	OffsetCache(final int tSize, final int lockCount) {
	if (tSize < 1)
	throw new IllegalArgumentException("tSize must be >= 1");
	if (lockCount < 1)
	throw new IllegalArgumentException("lockCount must be >= 1");

	queue = new ReferenceQueue<V>();
	tableSize = tSize;
	clock = new AtomicLong(1);
	table = new AtomicReferenceArray<Entry<V>>(tableSize);
	locks = new Lock[lockCount];
	for (int i = 0; i < locks.length; i++)
	locks[i] = new Lock();
	evictLock = new ReentrantLock();

	int eb = (int) (tableSize * .1);
	if (64 < eb)
	eb = 64;
	else if (eb < 4)
	eb = 4;
	if (tableSize < eb)
	eb = tableSize;
	evictBatch = eb;
	}

	/**
	* Lookup a cached object, creating and loading it if it doesn't exist.
	*
	* @param pack
	* the pack that "contains" the cached object.
	* @param position
	* offset within <code>pack</code> of the object.
	* @return the object reference.
	* @throws IOException
	* the object reference was not in the cache and could not be
	* obtained by {@link #load(PackFile, long)}.
	*/
	V getOrLoad(final PackFile pack, final long position) throws IOException {
	final int slot = slot(pack, position);
	final Entry<V> e1 = table.get(slot);
	V v = scan(e1, pack, position);
	if (v != null)
	return v;

	synchronized (lock(pack, position)) {
	Entry<V> e2 = table.get(slot);
	if (e2 != e1) {
	v = scan(e2, pack, position);
	if (v != null)
	return v;
	}

	v = load(pack, position);
	final Ref<V> ref = createRef(pack, position, v);
	hit(ref);
	for (;;) {
	final Entry<V> n = new Entry<V>(clean(e2), ref);
	if (table.compareAndSet(slot, e2, n))
	break;
	e2 = table.get(slot);
	}
	}

	if (evictLock.tryLock()) {
	try {
	gc();
	evict();
	} finally {
	evictLock.unlock();
	}
	}

	return v;
	}

	private V scan(Entry<V> n, final PackFile pack, final long position) {
	for (; n != null; n = n.next) {
	final Ref<V> r = n.ref;
	if (r.pack == pack && r.position == position) {
	final V v = r.get();
	if (v != null) {
	hit(r);
	return v;
	}
	n.kill();
	break;
	}
	}
	return null;
	}

	private void hit(final Ref<V> r) {
	// We don't need to be 100% accurate here. Its sufficient that at least
	// one thread performs the increment. Any other concurrent access at
	// exactly the same time can simply use the same clock value.
	//
	// Consequently we attempt the set, but we don't try to recover should
	// it fail. This is why we don't use getAndIncrement() here.
	//
	final long c = clock.get();
	clock.compareAndSet(c, c + 1);
	r.lastAccess = c;
	}

	private void evict() {
	while (isFull()) {
	int ptr = rng.nextInt(tableSize);
	Entry<V> old = null;
	int slot = 0;
	for (int b = evictBatch - 1; b >= 0; b--, ptr++) {
	if (tableSize <= ptr)
	ptr = 0;
	for (Entry<V> e = table.get(ptr); e != null; e = e.next) {
	if (e.dead)
	continue;
	if (old == null \|\| e.ref.lastAccess < old.ref.lastAccess) {
	old = e;
	slot = ptr;
	}
	}
	}
	if (old != null) {
	old.kill();
	gc();
	final Entry<V> e1 = table.get(slot);
	table.compareAndSet(slot, e1, clean(e1));
	}
	}
	}

	/**
	* Clear every entry from the cache.
	*<p>
	* This is a last-ditch effort to clear out the cache, such as before it
	* gets replaced by another cache that is configured differently. This
	* method tries to force every cached entry through {@link #clear(Ref)} to
	* ensure that resources are correctly accounted for and cleaned up by the
	* subclass. A concurrent reader loading entries while this method is
	* running may cause resource accounting failures.
	*/
	void removeAll() {
	for (int s = 0; s < tableSize; s++) {
	Entry<V> e1;
	do {
	e1 = table.get(s);
	for (Entry<V> e = e1; e != null; e = e.next)
	e.kill();
	} while (!table.compareAndSet(s, e1, null));
	}
	gc();
	}

	/**
	* Clear all entries related to a single file.
	* <p>
	* Typically this method is invoked during {@link PackFile#close()}, when we
	* know the pack is never going to be useful to us again (for example, it no
	* longer exists on disk). A concurrent reader loading an entry from this
	* same pack may cause the pack to become stuck in the cache anyway.
	*
	* @param pack
	* the file to purge all entries of.
	*/
	void removeAll(final PackFile pack) {
	for (int s = 0; s < tableSize; s++) {
	final Entry<V> e1 = table.get(s);
	boolean hasDead = false;
	for (Entry<V> e = e1; e != null; e = e.next) {
	if (e.ref.pack == pack) {
	e.kill();
	hasDead = true;
	} else if (e.dead)
	hasDead = true;
	}
	if (hasDead)
	table.compareAndSet(s, e1, clean(e1));
	}
	gc();
	}

	/**
	* Materialize an object that doesn't yet exist in the cache.
	* <p>
	* This method is invoked by {@link #getOrLoad(PackFile, long)} when the
	* specified entity does not yet exist in the cache. Internal locking
	* ensures that at most one thread can call this method for each unique
	* <code>(pack,position)</code>, but multiple threads can call this method
	* concurrently for different <code>(pack,position)</code> tuples.
	*
	* @param pack
	* the file to materialize the entry from.
	* @param position
	* offset within the file of the entry.
	* @return the materialized object. Must never be null.
	* @throws IOException
	* the method was unable to materialize the object for this
	* input pair. The usual reasons would be file corruption, file
	* not found, out of file descriptors, etc.
	*/
	protected abstract V load(PackFile pack, long position) throws IOException;

	/**
	* Construct a Ref (SoftReference) around a cached entity.
	* <p>
	* Implementing this is only necessary if the subclass is performing
	* resource accounting during {@link #load(PackFile, long)} and
	* {@link #clear(Ref)} requires some information to update the accounting.
	* <p>
	* Implementors <b>MUST</b> ensure that the returned reference uses the
	* {@link #queue} ReferenceQueue, otherwise {@link #clear(Ref)} will not be
	* invoked at the proper time.
	*
	* @param pack
	* the file to materialize the entry from.
	* @param position
	* offset within the file of the entry.
	* @param v
	* the object returned by {@link #load(PackFile, long)}.
	* @return a soft reference subclass wrapped around <code>v</code>.
	*/
	@SuppressWarnings("unchecked")
	protected R createRef(final PackFile pack, final long position, final V v) {
	return (R) new Ref<V>(pack, position, v, queue);
	}

	/**
	* Update accounting information now that an object has left the cache.
	* <p>
	* This method is invoked exactly once for the combined
	* {@link #load(PackFile, long)} and
	* {@link #createRef(PackFile, long, Object)} invocation pair that was used
	* to construct and insert an object into the cache.
	*
	* @param ref
	* the reference wrapped around the object. Implementations must
	* be prepared for <code>ref.get()</code> to return null.
	*/
	protected void clear(final R ref) {
	// Do nothing by default.
	}

	/**
	* Determine if the cache is full and requires eviction of entries.
	* <p>
	* By default this method returns false. Implementors may override to
	* consult with the accounting updated by {@link #load(PackFile, long)},
	* {@link #createRef(PackFile, long, Object)} and {@link #clear(Ref)}.
	*
	* @return true if the cache is still over-limit and requires eviction of
	* more entries.
	*/
	protected boolean isFull() {
	return false;
	}

	@SuppressWarnings("unchecked")
	private void gc() {
	R r;
	while ((r = (R) queue.poll()) != null) {
	// Sun's Java 5 and 6 implementation have a bug where a Reference
	// can be enqueued and dequeued twice on the same reference queue
	// due to a race condition within ReferenceQueue.enqueue(Reference).
	//
	// http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6837858
	//
	// We CANNOT permit a Reference to come through us twice, as it will
	// skew the resource counters we maintain. Our canClear() check here
	// provides a way to skip the redundant dequeues, if any.
	//
	if (r.canClear()) {
	clear(r);

	boolean found = false;
	final int s = slot(r.pack, r.position);
	final Entry<V> e1 = table.get(s);
	for (Entry<V> n = e1; n != null; n = n.next) {
	if (n.ref == r) {
	n.dead = true;
	found = true;
	break;
	}
	}
	if (found)
	table.compareAndSet(s, e1, clean(e1));
	}
	}
	}

	/**
	* Compute the hash code value for a <code>(PackFile,position)</code> tuple.
	* <p>
	* For example, <code>return packHash + (int) (position >>> 4)</code>.
	* Implementors must override with a suitable hash (for example, a different
	* right shift on the position).
	*
	* @param packHash
	* hash code for the file being accessed.
	* @param position
	* position within the file being accessed.
	* @return a reasonable hash code mixing the two values.
	*/
	protected abstract int hash(int packHash, long position);

	private int slot(final PackFile pack, final long position) {
	return (hash(pack.hash, position) >>> 1) % tableSize;
	}

	private Lock lock(final PackFile pack, final long position) {
	return locks[(hash(pack.hash, position) >>> 1) % locks.length];
	}

	private static <V> Entry<V> clean(Entry<V> top) {
	while (top != null && top.dead) {
	top.ref.enqueue();
	top = top.next;
	}
	if (top == null)
	return null;
	final Entry<V> n = clean(top.next);
	return n == top.next ? top : new Entry<V>(n, top.ref);
	}

	private static class Entry<V> {
	/** Next entry in the hash table's chain list. */
	final Entry<V> next;

	/** The referenced object. */
	final Ref<V> ref;

	/**
	* Marked true when ref.get() returns null and the ref is dead.
	* <p>
	* A true here indicates that the ref is no longer accessible, and that
	* we therefore need to eventually purge this Entry object out of the
	* bucket's chain.
	*/
	volatile boolean dead;

	Entry(final Entry<V> n, final Ref<V> r) {
	next = n;
	ref = r;
	}

	final void kill() {
	dead = true;
	ref.enqueue();
	}
	}

	/**
	* A soft reference wrapped around a cached object.
	*
	* @param <V>
	* type of the cached object.
	*/
	protected static class Ref<V> extends SoftReference<V> {
	final PackFile pack;

	final long position;

	long lastAccess;

	private boolean cleared;

	protected Ref(final PackFile pack, final long position, final V v,
	final ReferenceQueue<V> queue) {
	super(v, queue);
	this.pack = pack;
	this.position = position;
	}

	final synchronized boolean canClear() {
	if (cleared)
	return false;
	cleared = true;
	return true;
	}
	}

	private static final class Lock {
	// Used only for its implicit monitor.
	}
	}