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  * 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

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  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

  *

  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

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 package java.lang;

 import java.lang.ClassValue.ClassValueMap;

 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 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 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(type);

         }

             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, relative a single given type.

      *  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 final Class<?> type;

         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, relative the given type.

          *  Also, create an empty cache array and install it on the class.

          */

         ClassValueMap(Class<?> type) {

             this.type = type;

             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.

     }

 }