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	/*
	* Copyright (c) 2010, 2013, Oracle and/or its affiliates. All rights reserved.
	* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
	*
	* This code is free software; you can redistribute it and/or modify it
	* under the terms of the GNU General Public License version 2 only, as
	* published by the Free Software Foundation. Oracle designates this
	* particular file as subject to the "Classpath" exception as provided
	* by Oracle in the LICENSE file that accompanied this code.
	*
	* This code is distributed in the hope that it will be useful, but WITHOUT
	* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
	* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
	* version 2 for more details (a copy is included in the LICENSE file that
	* accompanied this code).
	*
	* You should have received a copy of the GNU General Public License version
	* 2 along with this work; if not, write to the Free Software Foundation,
	* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
	*
	* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
	* or visit www.oracle.com if you need additional information or have any
	* questions.
	*/

	package java.lang;

	import java.util.WeakHashMap;
	import java.lang.ref.WeakReference;
	import java.util.concurrent.atomic.AtomicInteger;

	import jdk.internal.misc.Unsafe;

	import static java.lang.ClassValue.ClassValueMap.probeHomeLocation;
	import static java.lang.ClassValue.ClassValueMap.probeBackupLocations;

	/**
	* Lazily associate a computed value with (potentially) every type.
	* For example, if a dynamic language needs to construct a message dispatch
	* table for each class encountered at a message send call site,
	* it can use a {@code ClassValue} to cache information needed to
	* perform the message send quickly, for each class encountered.
	* @author John Rose, JSR 292 EG
	* @since 1.7
	*/
	public abstract class ClassValue<T> {
	/**
	* Sole constructor. (For invocation by subclass constructors, typically
	* implicit.)
	*/
	protected ClassValue() {
	}

	/**
	* Computes the given class's derived value for this {@code ClassValue}.
	* <p>
	* This method will be invoked within the first thread that accesses
	* the value with the {@link #get get} method.
	* <p>
	* Normally, this method is invoked at most once per class,
	* but it may be invoked again if there has been a call to
	* {@link #remove remove}.
	* <p>
	* If this method throws an exception, the corresponding call to {@code get}
	* will terminate abnormally with that exception, and no class value will be recorded.
	*
	* @param type the type whose class value must be computed
	* @return the newly computed value associated with this {@code ClassValue}, for the given class or interface
	* @see #get
	* @see #remove
	*/
	protected abstract T computeValue(Class<?> type);

	/**
	* Returns the value for the given class.
	* If no value has yet been computed, it is obtained by
	* an invocation of the {@link #computeValue computeValue} method.
	* <p>
	* The actual installation of the value on the class
	* is performed atomically.
	* At that point, if several racing threads have
	* computed values, one is chosen, and returned to
	* all the racing threads.
	* <p>
	* The {@code type} parameter is typically a class, but it may be any type,
	* such as an interface, a primitive type (like {@code int.class}), or {@code void.class}.
	* <p>
	* In the absence of {@code remove} calls, a class value has a simple
	* state diagram: uninitialized and initialized.
	* When {@code remove} calls are made,
	* the rules for value observation are more complex.
	* See the documentation for {@link #remove remove} for more information.
	*
	* @param type the type whose class value must be computed or retrieved
	* @return the current value associated with this {@code ClassValue}, for the given class or interface
	* @throws NullPointerException if the argument is null
	* @see #remove
	* @see #computeValue
	*/
	public T get(Class<?> type) {
	// non-racing this.hashCodeForCache : final int
	Entry<?>[] cache;
	Entry<T> e = probeHomeLocation(cache = getCacheCarefully(type), this);
	// racing e : current value <=> stale value from current cache or from stale cache
	// invariant: e is null or an Entry with readable Entry.version and Entry.value
	if (match(e))
	// invariant: No false positive matches. False negatives are OK if rare.
	// The key fact that makes this work: if this.version == e.version,
	// then this thread has a right to observe (final) e.value.
	return e.value();
	// The fast path can fail for any of these reasons:
	// 1. no entry has been computed yet
	// 2. hash code collision (before or after reduction mod cache.length)
	// 3. an entry has been removed (either on this type or another)
	// 4. the GC has somehow managed to delete e.version and clear the reference
	return getFromBackup(cache, type);
	}

	/**
	* Removes the associated value for the given class.
	* If this value is subsequently {@linkplain #get read} for the same class,
	* its value will be reinitialized by invoking its {@link #computeValue computeValue} method.
	* This may result in an additional invocation of the
	* {@code computeValue} method for the given class.
	* <p>
	* In order to explain the interaction between {@code get} and {@code remove} calls,
	* we must model the state transitions of a class value to take into account
	* the alternation between uninitialized and initialized states.
	* To do this, number these states sequentially from zero, and note that
	* uninitialized (or removed) states are numbered with even numbers,
	* while initialized (or re-initialized) states have odd numbers.
	* <p>
	* When a thread {@code T} removes a class value in state {@code 2N},
	* nothing happens, since the class value is already uninitialized.
	* Otherwise, the state is advanced atomically to {@code 2N+1}.
	* <p>
	* When a thread {@code T} queries a class value in state {@code 2N},
	* the thread first attempts to initialize the class value to state {@code 2N+1}
	* by invoking {@code computeValue} and installing the resulting value.
	* <p>
	* When {@code T} attempts to install the newly computed value,
	* if the state is still at {@code 2N}, the class value will be initialized
	* with the computed value, advancing it to state {@code 2N+1}.
	* <p>
	* Otherwise, whether the new state is even or odd,
	* {@code T} will discard the newly computed value
	* and retry the {@code get} operation.
	* <p>
	* Discarding and retrying is an important proviso,
	* since otherwise {@code T} could potentially install
	* a disastrously stale value. For example:
	* <ul>
	* <li>{@code T} calls {@code CV.get(C)} and sees state {@code 2N}
	* <li>{@code T} quickly computes a time-dependent value {@code V0} and gets ready to install it
	* <li>{@code T} is hit by an unlucky paging or scheduling event, and goes to sleep for a long time
	* <li>...meanwhile, {@code T2} also calls {@code CV.get(C)} and sees state {@code 2N}
	* <li>{@code T2} quickly computes a similar time-dependent value {@code V1} and installs it on {@code CV.get(C)}
	* <li>{@code T2} (or a third thread) then calls {@code CV.remove(C)}, undoing {@code T2}'s work
	* <li> the previous actions of {@code T2} are repeated several times
	* <li> also, the relevant computed values change over time: {@code V1}, {@code V2}, ...
	* <li>...meanwhile, {@code T} wakes up and attempts to install {@code V0}; <em>this must fail</em>
	* </ul>
	* We can assume in the above scenario that {@code CV.computeValue} uses locks to properly
	* observe the time-dependent states as it computes {@code V1}, etc.
	* This does not remove the threat of a stale value, since there is a window of time
	* between the return of {@code computeValue} in {@code T} and the installation
	* of the new value. No user synchronization is possible during this time.
	*
	* @param type the type whose class value must be removed
	* @throws NullPointerException if the argument is null
	*/
	public void remove(Class<?> type) {
	ClassValueMap map = getMap(type);
	map.removeEntry(this);
	}

	// Possible functionality for JSR 292 MR 1
	/public/ void put(Class<?> type, T value) {
	ClassValueMap map = getMap(type);
	map.changeEntry(this, value);
	}

	/// --------
	/// Implementation...
	/// --------

	/** Return the cache, if it exists, else a dummy empty cache. */
	private static Entry<?>[] getCacheCarefully(Class<?> type) {
	// racing type.classValueMap{.cacheArray} : null => new Entry[X] <=> new Entry[Y]
	ClassValueMap map = type.classValueMap;
	if (map == null) return EMPTY_CACHE;
	Entry<?>[] cache = map.getCache();
	return cache;
	// invariant: returned value is safe to dereference and check for an Entry
	}

	/** Initial, one-element, empty cache used by all Class instances. Must never be filled. */
	private static final Entry<?>[] EMPTY_CACHE = { null };

	/**
	* Slow tail of ClassValue.get to retry at nearby locations in the cache,
	* or take a slow lock and check the hash table.
	* Called only if the first probe was empty or a collision.
	* This is a separate method, so compilers can process it independently.
	*/
	private T getFromBackup(Entry<?>[] cache, Class<?> type) {
	Entry<T> e = probeBackupLocations(cache, this);
	if (e != null)
	return e.value();
	return getFromHashMap(type);
	}

	// Hack to suppress warnings on the (T) cast, which is a no-op.
	@SuppressWarnings("unchecked")
	Entry<T> castEntry(Entry<?> e) { return (Entry<T>) e; }

	/** Called when the fast path of get fails, and cache reprobe also fails.
	*/
	private T getFromHashMap(Class<?> type) {
	// The fail-safe recovery is to fall back to the underlying classValueMap.
	ClassValueMap map = getMap(type);
	for (;;) {
	Entry<T> e = map.startEntry(this);
	if (!e.isPromise())
	return e.value();
	try {
	// Try to make a real entry for the promised version.
	e = makeEntry(e.version(), computeValue(type));
	} finally {
	// Whether computeValue throws or returns normally,
	// be sure to remove the empty entry.
	e = map.finishEntry(this, e);
	}
	if (e != null)
	return e.value();
	// else try again, in case a racing thread called remove (so e == null)
	}
	}

	/** Check that e is non-null, matches this ClassValue, and is live. */
	boolean match(Entry<?> e) {
	// racing e.version : null (blank) => unique Version token => null (GC-ed version)
	// non-racing this.version : v1 => v2 => ... (updates are read faithfully from volatile)
	return (e != null && e.get() == this.version);
	// invariant: No false positives on version match. Null is OK for false negative.
	// invariant: If version matches, then e.value is readable (final set in Entry.<init>)
	}

	/** Internal hash code for accessing Class.classValueMap.cacheArray. */
	final int hashCodeForCache = nextHashCode.getAndAdd(HASH_INCREMENT) & HASH_MASK;

	/** Value stream for hashCodeForCache. See similar structure in ThreadLocal. */
	private static final AtomicInteger nextHashCode = new AtomicInteger();

	/** Good for power-of-two tables. See similar structure in ThreadLocal. */
	private static final int HASH_INCREMENT = 0x61c88647;

	/** Mask a hash code to be positive but not too large, to prevent wraparound. */
	static final int HASH_MASK = (-1 >>> 2);

	/**
	* Private key for retrieval of this object from ClassValueMap.
	*/
	static class Identity {
	}
	/**
	* This ClassValue's identity, expressed as an opaque object.
	* The main object {@code ClassValue.this} is incorrect since
	* subclasses may override {@code ClassValue.equals}, which
	* could confuse keys in the ClassValueMap.
	*/
	final Identity identity = new Identity();

	/**
	* Current version for retrieving this class value from the cache.
	* Any number of computeValue calls can be cached in association with one version.
	* But the version changes when a remove (on any type) is executed.
	* A version change invalidates all cache entries for the affected ClassValue,
	* by marking them as stale. Stale cache entries do not force another call
	* to computeValue, but they do require a synchronized visit to a backing map.
	* <p>
	* All user-visible state changes on the ClassValue take place under
	* a lock inside the synchronized methods of ClassValueMap.
	* Readers (of ClassValue.get) are notified of such state changes
	* when this.version is bumped to a new token.
	* This variable must be volatile so that an unsynchronized reader
	* will receive the notification without delay.
	* <p>
	* If version were not volatile, one thread T1 could persistently hold onto
	* a stale value this.value == V1, while another thread T2 advances
	* (under a lock) to this.value == V2. This will typically be harmless,
	* but if T1 and T2 interact causally via some other channel, such that
	* T1's further actions are constrained (in the JMM) to happen after
	* the V2 event, then T1's observation of V1 will be an error.
	* <p>
	* The practical effect of making this.version be volatile is that it cannot
	* be hoisted out of a loop (by an optimizing JIT) or otherwise cached.
	* Some machines may also require a barrier instruction to execute
	* before this.version.
	*/
	private volatile Version<T> version = new Version<>(this);
	Version<T> version() { return version; }
	void bumpVersion() { version = new Version<>(this); }
	static class Version<T> {
	private final ClassValue<T> classValue;
	private final Entry<T> promise = new Entry<>(this);
	Version(ClassValue<T> classValue) { this.classValue = classValue; }
	ClassValue<T> classValue() { return classValue; }
	Entry<T> promise() { return promise; }
	boolean isLive() { return classValue.version() == this; }
	}

	/** One binding of a value to a class via a ClassValue.
	* States are:<ul>
	* <li> promise if value == Entry.this
	* <li> else dead if version == null
	* <li> else stale if version != classValue.version
	* <li> else live </ul>
	* Promises are never put into the cache; they only live in the
	* backing map while a computeValue call is in flight.
	* Once an entry goes stale, it can be reset at any time
	* into the dead state.
	*/
	static class Entry<T> extends WeakReference<Version<T>> {
	final Object value; // usually of type T, but sometimes (Entry)this
	Entry(Version<T> version, T value) {
	super(version);
	this.value = value; // for a regular entry, value is of type T
	}
	private void assertNotPromise() { assert(!isPromise()); }
	/** For creating a promise. */
	Entry(Version<T> version) {
	super(version);
	this.value = this; // for a promise, value is not of type T, but Entry!
	}
	/** Fetch the value. This entry must not be a promise. */
	@SuppressWarnings("unchecked") // if !isPromise, type is T
	T value() { assertNotPromise(); return (T) value; }
	boolean isPromise() { return value == this; }
	Version<T> version() { return get(); }
	ClassValue<T> classValueOrNull() {
	Version<T> v = version();
	return (v == null) ? null : v.classValue();
	}
	boolean isLive() {
	Version<T> v = version();
	if (v == null) return false;
	if (v.isLive()) return true;
	clear();
	return false;
	}
	Entry<T> refreshVersion(Version<T> v2) {
	assertNotPromise();
	@SuppressWarnings("unchecked") // if !isPromise, type is T
	Entry<T> e2 = new Entry<>(v2, (T) value);
	clear();
	// value = null -- caller must drop
	return e2;
	}
	static final Entry<?> DEAD_ENTRY = new Entry<>(null, null);
	}

	/** Return the backing map associated with this type. */
	private static ClassValueMap getMap(Class<?> type) {
	// racing type.classValueMap : null (blank) => unique ClassValueMap
	// if a null is observed, a map is created (lazily, synchronously, uniquely)
	// all further access to that map is synchronized
	ClassValueMap map = type.classValueMap;
	if (map != null) return map;
	return initializeMap(type);
	}

	private static final Object CRITICAL_SECTION = new Object();
	private static final Unsafe UNSAFE = Unsafe.getUnsafe();
	private static ClassValueMap initializeMap(Class<?> type) {
	ClassValueMap map;
	synchronized (CRITICAL_SECTION) { // private object to avoid deadlocks
	// happens about once per type
	if ((map = type.classValueMap) == null) {
	map = new ClassValueMap();
	// Place a Store fence after construction and before publishing to emulate
	// ClassValueMap containing final fields. This ensures it can be
	// published safely in the non-volatile field Class.classValueMap,
	// since stores to the fields of ClassValueMap will not be reordered
	// to occur after the store to the field type.classValueMap
	UNSAFE.storeFence();

	type.classValueMap = map;
	}
	}
	return map;
	}

	static <T> Entry<T> makeEntry(Version<T> explicitVersion, T value) {
	// Note that explicitVersion might be different from this.version.
	return new Entry<>(explicitVersion, value);

	// As soon as the Entry is put into the cache, the value will be
	// reachable via a data race (as defined by the Java Memory Model).
	// This race is benign, assuming the value object itself can be
	// read safely by multiple threads. This is up to the user.
	//
	// The entry and version fields themselves can be safely read via
	// a race because they are either final or have controlled states.
	// If the pointer from the entry to the version is still null,
	// or if the version goes immediately dead and is nulled out,
	// the reader will take the slow path and retry under a lock.
	}

	// The following class could also be top level and non-public:

	/** A backing map for all ClassValues.
	* Gives a fully serialized "true state" for each pair (ClassValue cv, Class type).
	* Also manages an unserialized fast-path cache.
	*/
	static class ClassValueMap extends WeakHashMap<ClassValue.Identity, Entry<?>> {
	private Entry<?>[] cacheArray;
	private int cacheLoad, cacheLoadLimit;

	/** Number of entries initially allocated to each type when first used with any ClassValue.
	* It would be pointless to make this much smaller than the Class and ClassValueMap objects themselves.
	* Must be a power of 2.
	*/
	private static final int INITIAL_ENTRIES = 32;

	/** Build a backing map for ClassValues.
	* Also, create an empty cache array and install it on the class.
	*/
	ClassValueMap() {
	sizeCache(INITIAL_ENTRIES);
	}

	Entry<?>[] getCache() { return cacheArray; }

	/** Initiate a query. Store a promise (placeholder) if there is no value yet. */
	synchronized
	<T> Entry<T> startEntry(ClassValue<T> classValue) {
	@SuppressWarnings("unchecked") // one map has entries for all value types <T>
	Entry<T> e = (Entry<T>) get(classValue.identity);
	Version<T> v = classValue.version();
	if (e == null) {
	e = v.promise();
	// The presence of a promise means that a value is pending for v.
	// Eventually, finishEntry will overwrite the promise.
	put(classValue.identity, e);
	// Note that the promise is never entered into the cache!
	return e;
	} else if (e.isPromise()) {
	// Somebody else has asked the same question.
	// Let the races begin!
	if (e.version() != v) {
	e = v.promise();
	put(classValue.identity, e);
	}
	return e;
	} else {
	// there is already a completed entry here; report it
	if (e.version() != v) {
	// There is a stale but valid entry here; make it fresh again.
	// Once an entry is in the hash table, we don't care what its version is.
	e = e.refreshVersion(v);
	put(classValue.identity, e);
	}
	// Add to the cache, to enable the fast path, next time.
	checkCacheLoad();
	addToCache(classValue, e);
	return e;
	}
	}

	/** Finish a query. Overwrite a matching placeholder. Drop stale incoming values. */
	synchronized
	<T> Entry<T> finishEntry(ClassValue<T> classValue, Entry<T> e) {
	@SuppressWarnings("unchecked") // one map has entries for all value types <T>
	Entry<T> e0 = (Entry<T>) get(classValue.identity);
	if (e == e0) {
	// We can get here during exception processing, unwinding from computeValue.
	assert(e.isPromise());
	remove(classValue.identity);
	return null;
	} else if (e0 != null && e0.isPromise() && e0.version() == e.version()) {
	// If e0 matches the intended entry, there has not been a remove call
	// between the previous startEntry and now. So now overwrite e0.
	Version<T> v = classValue.version();
	if (e.version() != v)
	e = e.refreshVersion(v);
	put(classValue.identity, e);
	// Add to the cache, to enable the fast path, next time.
	checkCacheLoad();
	addToCache(classValue, e);
	return e;
	} else {
	// Some sort of mismatch; caller must try again.
	return null;
	}
	}

	/** Remove an entry. */
	synchronized
	void removeEntry(ClassValue<?> classValue) {
	Entry<?> e = remove(classValue.identity);
	if (e == null) {
	// Uninitialized, and no pending calls to computeValue. No change.
	} else if (e.isPromise()) {
	// State is uninitialized, with a pending call to finishEntry.
	// Since remove is a no-op in such a state, keep the promise
	// by putting it back into the map.
	put(classValue.identity, e);
	} else {
	// In an initialized state. Bump forward, and de-initialize.
	classValue.bumpVersion();
	// Make all cache elements for this guy go stale.
	removeStaleEntries(classValue);
	}
	}

	/** Change the value for an entry. */
	synchronized
	<T> void changeEntry(ClassValue<T> classValue, T value) {
	@SuppressWarnings("unchecked") // one map has entries for all value types <T>
	Entry<T> e0 = (Entry<T>) get(classValue.identity);
	Version<T> version = classValue.version();
	if (e0 != null) {
	if (e0.version() == version && e0.value() == value)
	// no value change => no version change needed
	return;
	classValue.bumpVersion();
	removeStaleEntries(classValue);
	}
	Entry<T> e = makeEntry(version, value);
	put(classValue.identity, e);
	// Add to the cache, to enable the fast path, next time.
	checkCacheLoad();
	addToCache(classValue, e);
	}

	/// --------
	/// Cache management.
	/// --------

	// Statics do not need synchronization.

	/** Load the cache entry at the given (hashed) location. */
	static Entry<?> loadFromCache(Entry<?>[] cache, int i) {
	// non-racing cache.length : constant
	// racing cache[i & (mask)] : null <=> Entry
	return cache[i & (cache.length-1)];
	// invariant: returned value is null or well-constructed (ready to match)
	}

	/** Look in the cache, at the home location for the given ClassValue. */
	static <T> Entry<T> probeHomeLocation(Entry<?>[] cache, ClassValue<T> classValue) {
	return classValue.castEntry(loadFromCache(cache, classValue.hashCodeForCache));
	}

	/** Given that first probe was a collision, retry at nearby locations. */
	static <T> Entry<T> probeBackupLocations(Entry<?>[] cache, ClassValue<T> classValue) {
	if (PROBE_LIMIT <= 0) return null;
	// Probe the cache carefully, in a range of slots.
	int mask = (cache.length-1);
	int home = (classValue.hashCodeForCache & mask);
	Entry<?> e2 = cache[home]; // victim, if we find the real guy
	if (e2 == null) {
	return null; // if nobody is at home, no need to search nearby
	}
	// assume !classValue.match(e2), but do not assert, because of races
	int pos2 = -1;
	for (int i = home + 1; i < home + PROBE_LIMIT; i++) {
	Entry<?> e = cache[i & mask];
	if (e == null) {
	break; // only search within non-null runs
	}
	if (classValue.match(e)) {
	// relocate colliding entry e2 (from cache[home]) to first empty slot
	cache[home] = e;
	if (pos2 >= 0) {
	cache[i & mask] = Entry.DEAD_ENTRY;
	} else {
	pos2 = i;
	}
	cache[pos2 & mask] = ((entryDislocation(cache, pos2, e2) < PROBE_LIMIT)
	? e2 // put e2 here if it fits
	: Entry.DEAD_ENTRY);
	return classValue.castEntry(e);
	}
	// Remember first empty slot, if any:
	if (!e.isLive() && pos2 < 0) pos2 = i;
	}
	return null;
	}

	/** How far out of place is e? */
	private static int entryDislocation(Entry<?>[] cache, int pos, Entry<?> e) {
	ClassValue<?> cv = e.classValueOrNull();
	if (cv == null) return 0; // entry is not live!
	int mask = (cache.length-1);
	return (pos - cv.hashCodeForCache) & mask;
	}

	/// --------
	/// Below this line all functions are private, and assume synchronized access.
	/// --------

	private void sizeCache(int length) {
	assert((length & (length-1)) == 0); // must be power of 2
	cacheLoad = 0;
	cacheLoadLimit = (int) ((double) length * CACHE_LOAD_LIMIT / 100);
	cacheArray = new Entry<?>[length];
	}

	/** Make sure the cache load stays below its limit, if possible. */
	private void checkCacheLoad() {
	if (cacheLoad >= cacheLoadLimit) {
	reduceCacheLoad();
	}
	}
	private void reduceCacheLoad() {
	removeStaleEntries();
	if (cacheLoad < cacheLoadLimit)
	return; // win
	Entry<?>[] oldCache = getCache();
	if (oldCache.length > HASH_MASK)
	return; // lose
	sizeCache(oldCache.length * 2);
	for (Entry<?> e : oldCache) {
	if (e != null && e.isLive()) {
	addToCache(e);
	}
	}
	}

	/** Remove stale entries in the given range.
	* Should be executed under a Map lock.
	*/
	private void removeStaleEntries(Entry<?>[] cache, int begin, int count) {
	if (PROBE_LIMIT <= 0) return;
	int mask = (cache.length-1);
	int removed = 0;
	for (int i = begin; i < begin + count; i++) {
	Entry<?> e = cache[i & mask];
	if (e == null \|\| e.isLive())
	continue; // skip null and live entries
	Entry<?> replacement = null;
	if (PROBE_LIMIT > 1) {
	// avoid breaking up a non-null run
	replacement = findReplacement(cache, i);
	}
	cache[i & mask] = replacement;
	if (replacement == null) removed += 1;
	}
	cacheLoad = Math.max(0, cacheLoad - removed);
	}

	/** Clearing a cache slot risks disconnecting following entries
	* from the head of a non-null run, which would allow them
	* to be found via reprobes. Find an entry after cache[begin]
	* to plug into the hole, or return null if none is needed.
	*/
	private Entry<?> findReplacement(Entry<?>[] cache, int home1) {
	Entry<?> replacement = null;
	int haveReplacement = -1, replacementPos = 0;
	int mask = (cache.length-1);
	for (int i2 = home1 + 1; i2 < home1 + PROBE_LIMIT; i2++) {
	Entry<?> e2 = cache[i2 & mask];
	if (e2 == null) break; // End of non-null run.
	if (!e2.isLive()) continue; // Doomed anyway.
	int dis2 = entryDislocation(cache, i2, e2);
	if (dis2 == 0) continue; // e2 already optimally placed
	int home2 = i2 - dis2;
	if (home2 <= home1) {
	// e2 can replace entry at cache[home1]
	if (home2 == home1) {
	// Put e2 exactly where he belongs.
	haveReplacement = 1;
	replacementPos = i2;
	replacement = e2;
	} else if (haveReplacement <= 0) {
	haveReplacement = 0;
	replacementPos = i2;
	replacement = e2;
	}
	// And keep going, so we can favor larger dislocations.
	}
	}
	if (haveReplacement >= 0) {
	if (cache[(replacementPos+1) & mask] != null) {
	// Be conservative, to avoid breaking up a non-null run.
	cache[replacementPos & mask] = (Entry<?>) Entry.DEAD_ENTRY;
	} else {
	cache[replacementPos & mask] = null;
	cacheLoad -= 1;
	}
	}
	return replacement;
	}

	/** Remove stale entries in the range near classValue. */
	private void removeStaleEntries(ClassValue<?> classValue) {
	removeStaleEntries(getCache(), classValue.hashCodeForCache, PROBE_LIMIT);
	}

	/** Remove all stale entries, everywhere. */
	private void removeStaleEntries() {
	Entry<?>[] cache = getCache();
	removeStaleEntries(cache, 0, cache.length + PROBE_LIMIT - 1);
	}

	/** Add the given entry to the cache, in its home location, unless it is out of date. */
	private <T> void addToCache(Entry<T> e) {
	ClassValue<T> classValue = e.classValueOrNull();
	if (classValue != null)
	addToCache(classValue, e);
	}

	/** Add the given entry to the cache, in its home location. */
	private <T> void addToCache(ClassValue<T> classValue, Entry<T> e) {
	if (PROBE_LIMIT <= 0) return; // do not fill cache
	// Add e to the cache.
	Entry<?>[] cache = getCache();
	int mask = (cache.length-1);
	int home = classValue.hashCodeForCache & mask;
	Entry<?> e2 = placeInCache(cache, home, e, false);
	if (e2 == null) return; // done
	if (PROBE_LIMIT > 1) {
	// try to move e2 somewhere else in his probe range
	int dis2 = entryDislocation(cache, home, e2);
	int home2 = home - dis2;
	for (int i2 = home2; i2 < home2 + PROBE_LIMIT; i2++) {
	if (placeInCache(cache, i2 & mask, e2, true) == null) {
	return;
	}
	}
	}
	// Note: At this point, e2 is just dropped from the cache.
	}

	/** Store the given entry. Update cacheLoad, and return any live victim.
	* 'Gently' means return self rather than dislocating a live victim.
	*/
	private Entry<?> placeInCache(Entry<?>[] cache, int pos, Entry<?> e, boolean gently) {
	Entry<?> e2 = overwrittenEntry(cache[pos]);
	if (gently && e2 != null) {
	// do not overwrite a live entry
	return e;
	} else {
	cache[pos] = e;
	return e2;
	}
	}

	/** Note an entry that is about to be overwritten.
	* If it is not live, quietly replace it by null.
	* If it is an actual null, increment cacheLoad,
	* because the caller is going to store something
	* in its place.
	*/
	private <T> Entry<T> overwrittenEntry(Entry<T> e2) {
	if (e2 == null) cacheLoad += 1;
	else if (e2.isLive()) return e2;
	return null;
	}

	/** Percent loading of cache before resize. */
	private static final int CACHE_LOAD_LIMIT = 67; // 0..100
	/** Maximum number of probes to attempt. */
	private static final int PROBE_LIMIT = 6; // 1..
	// N.B. Set PROBE_LIMIT=0 to disable all fast paths.
	}
	}

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