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 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
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package java.lang;

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

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 ClassValueMap initializeMap(Class<?> type) {
 ClassValueMap map;
 synchronized (CRITICAL_SECTION) { // private object to avoid deadlocks
 // happens about once per type
 if ((map = type.classValueMap) == null)
 type.classValueMap = map = new ClassValueMap();
 }
 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.
 }
}