/*

 * 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 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.