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/*

 * Written by Doug Lea with assistance from members of JCP JSR-166

 * Expert Group and released to the public domain, as explained at

 * http://creativecommons.org/licenses/publicdomain

 */

package java.util.concurrent.locks;

import java.util.concurrent.*;

import java.util.concurrent.atomic.*;

import java.util.*;

/**

 * An implementation of {@link ReadWriteLock} supporting similar

 * semantics to {@link ReentrantLock}.

 * <p>This class has the following properties:

 *

 * <ul>

 * <li><b>Acquisition order</b>

 *

 * <p> This class does not impose a reader or writer preference

 * ordering for lock access.  However, it does support an optional

 * <em>fairness</em> policy.

 *

 * <dl>

 * <dt><b><i>Non-fair mode (default)</i></b>

 * <dd>When constructed as non-fair (the default), the order of entry

 * to the read and write lock is unspecified, subject to reentrancy

 * constraints.  A nonfair lock that is continously contended may

 * indefinitely postpone one or more reader or writer threads, but

 * will normally have higher throughput than a fair lock.

 * <p>

 *

 * <dt><b><i>Fair mode</i></b>

 * <dd> When constructed as fair, threads contend for entry using an

 * approximately arrival-order policy. When the currently held lock

 * is released either the longest-waiting single writer thread will

 * be assigned the write lock, or if there is a group of reader threads

 * waiting longer than all waiting writer threads, that group will be

 * assigned the read lock.

 *

 * <p>A thread that tries to acquire a fair read lock (non-reentrantly)

 * will block if either the write lock is held, or there is a waiting

 * writer thread. The thread will not acquire the read lock until

 * after the oldest currently waiting writer thread has acquired and

 * released the write lock. Of course, if a waiting writer abandons

 * its wait, leaving one or more reader threads as the longest waiters

 * in the queue with the write lock free, then those readers will be

 * assigned the read lock.

 *

 * <p>A thread that tries to acquire a fair write lock (non-reentrantly)

 * will block unless both the read lock and write lock are free (which

 * implies there are no waiting threads).  (Note that the non-blocking

 * {@link ReadLock#tryLock()} and {@link WriteLock#tryLock()} methods

 * do not honor this fair setting and will acquire the lock if it is

 * possible, regardless of waiting threads.)

 * <p>

 * </dl>

 *

 * <li><b>Reentrancy</b>

 *

 * <p>This lock allows both readers and writers to reacquire read or

 * write locks in the style of a {@link ReentrantLock}. Non-reentrant

 * readers are not allowed until all write locks held by the writing

 * thread have been released.

 *

 * <p>Additionally, a writer can acquire the read lock, but not

 * vice-versa.  Among other applications, reentrancy can be useful

 * when write locks are held during calls or callbacks to methods that

 * perform reads under read locks.  If a reader tries to acquire the

 * write lock it will never succeed.

 *

 * <li><b>Lock downgrading</b>

 * <p>Reentrancy also allows downgrading from the write lock to a read lock,

 * by acquiring the write lock, then the read lock and then releasing the

 * write lock. However, upgrading from a read lock to the write lock is

 * <b>not</b> possible.

 *

 * <li><b>Interruption of lock acquisition</b>

 * <p>The read lock and write lock both support interruption during lock

 * acquisition.

 *

 * <li><b>{@link Condition} support</b>

 * <p>The write lock provides a {@link Condition} implementation that

 * behaves in the same way, with respect to the write lock, as the

 * {@link Condition} implementation provided by

 * {@link ReentrantLock#newCondition} does for {@link ReentrantLock}.

 * This {@link Condition} can, of course, only be used with the write lock.

 *

 * <p>The read lock does not support a {@link Condition} and

 * {@code readLock().newCondition()} throws

 * {@code UnsupportedOperationException}.

 *

 * <li><b>Instrumentation</b>

 * <p>This class supports methods to determine whether locks

 * are held or contended. These methods are designed for monitoring

 * system state, not for synchronization control.

 * </ul>

 *

 * <p>Serialization of this class behaves in the same way as built-in

 * locks: a deserialized lock is in the unlocked state, regardless of

 * its state when serialized.

 *

 * <p><b>Sample usages</b>. Here is a code sketch showing how to exploit

 * reentrancy to perform lock downgrading after updating a cache (exception

 * handling is elided for simplicity):

 * <pre>

 * class CachedData {

 *   Object data;

 *   volatile boolean cacheValid;

 *   ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();

 *

 *   void processCachedData() {

 *     rwl.readLock().lock();

 *     if (!cacheValid) {

 *        // Must release read lock before acquiring write lock

 *        rwl.readLock().unlock();

 *        rwl.writeLock().lock();

 *        // Recheck state because another thread might have acquired

 *        //   write lock and changed state before we did.

 *        if (!cacheValid) {

 *          data = ...

 *          cacheValid = true;

 *        }

 *        // Downgrade by acquiring read lock before releasing write lock

 *        rwl.readLock().lock();

 *        rwl.writeLock().unlock(); // Unlock write, still hold read

 *     }

 *

 *     use(data);

 *     rwl.readLock().unlock();

 *   }

 * }

 * </pre>

 *

 * ReentrantReadWriteLocks can be used to improve concurrency in some

 * uses of some kinds of Collections. This is typically worthwhile

 * only when the collections are expected to be large, accessed by

 * more reader threads than writer threads, and entail operations with

 * overhead that outweighs synchronization overhead. For example, here

 * is a class using a TreeMap that is expected to be large and

 * concurrently accessed.

 *

 * <pre>{@code

 * class RWDictionary {

 *    private final Map<String, Data> m = new TreeMap<String, Data>();

 *    private final ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();

 *    private final Lock r = rwl.readLock();

 *    private final Lock w = rwl.writeLock();

 *

 *    public Data get(String key) {

 *        r.lock();

 *        try { return m.get(key); }

 *        finally { r.unlock(); }

 *    }

 *    public String[] allKeys() {

 *        r.lock();

 *        try { return m.keySet().toArray(); }

 *        finally { r.unlock(); }

 *    }

 *    public Data put(String key, Data value) {

 *        w.lock();

 *        try { return m.put(key, value); }

 *        finally { w.unlock(); }

 *    }

 *    public void clear() {

 *        w.lock();

 *        try { m.clear(); }

 *        finally { w.unlock(); }

 *    }

 * }}</pre>

 *

 * <h3>Implementation Notes</h3>

 *

 * <p>This lock supports a maximum of 65535 recursive write locks

 * and 65535 read locks. Attempts to exceed these limits result in

 * {@link Error} throws from locking methods.

 *

 * @since 1.5

 * @author Doug Lea

 *

 */

public class ReentrantReadWriteLock implements ReadWriteLock, java.io.Serializable  {

    private static final long serialVersionUID = -6992448646407690164L;

    /** Inner class providing readlock */

    private final ReentrantReadWriteLock.ReadLock readerLock;

    /** Inner class providing writelock */

    private final ReentrantReadWriteLock.WriteLock writerLock;

    /** Performs all synchronization mechanics */

    private final Sync sync;

    /**

     * Creates a new {@code ReentrantReadWriteLock} with

     * default (nonfair) ordering properties.

     */

    public ReentrantReadWriteLock() {

        this(false);

    }

    /**

     * Creates a new {@code ReentrantReadWriteLock} with

     * the given fairness policy.

     *

     * @param fair {@code true} if this lock should use a fair ordering policy

     */

    public ReentrantReadWriteLock(boolean fair) {

        sync = (fair)? new FairSync() : new NonfairSync();

        readerLock = new ReadLock(this);

        writerLock = new WriteLock(this);

    }

    public ReentrantReadWriteLock.WriteLock writeLock() { return writerLock; }

    public ReentrantReadWriteLock.ReadLock  readLock()  { return readerLock; }

    /**

     * Synchronization implementation for ReentrantReadWriteLock.

     * Subclassed into fair and nonfair versions.

     */

    static abstract class Sync extends AbstractQueuedSynchronizer {

        private static final long serialVersionUID = 6317671515068378041L;

        /*

         * Read vs write count extraction constants and functions.

         * Lock state is logically divided into two shorts: The lower

         * one representing the exclusive (writer) lock hold count,

         * and the upper the shared (reader) hold count.

         */

        static final int SHARED_SHIFT   = 16;

        static final int SHARED_UNIT    = (1 << SHARED_SHIFT);

        static final int MAX_COUNT      = (1 << SHARED_SHIFT) - 1;

        static final int EXCLUSIVE_MASK = (1 << SHARED_SHIFT) - 1;

        /** Returns the number of shared holds represented in count  */

        static int sharedCount(int c)    { return c >>> SHARED_SHIFT; }

        /** Returns the number of exclusive holds represented in count  */

        static int exclusiveCount(int c) { return c & EXCLUSIVE_MASK; }

        /**

         * A counter for per-thread read hold counts.

         * Maintained as a ThreadLocal; cached in cachedHoldCounter

         */

        static final class HoldCounter {

            int count;

            // Use id, not reference, to avoid garbage retention

            final long tid = Thread.currentThread().getId();

            /** Decrement if positive; return previous value */

            int tryDecrement() {

                int c = count;

                if (c > 0)

                    count = c - 1;

                return c;

            }

        }

        /**

         * ThreadLocal subclass. Easiest to explicitly define for sake

         * of deserialization mechanics.

         */

        static final class ThreadLocalHoldCounter

            extends ThreadLocal<HoldCounter> {

            public HoldCounter initialValue() {

                return new HoldCounter();

            }

        }

        /**

         * The number of read locks held by current thread.

         * Initialized only in constructor and readObject.

         */

        transient ThreadLocalHoldCounter readHolds;

        /**

         * The hold count of the last thread to successfully acquire

         * readLock. This saves ThreadLocal lookup in the common case

         * where the next thread to release is the last one to

         * acquire. This is non-volatile since it is just used

         * as a heuristic, and would be great for threads to cache.

         */

        transient HoldCounter cachedHoldCounter;

        Sync() {

            readHolds = new ThreadLocalHoldCounter();

            setState(getState()); // ensures visibility of readHolds

        }

        /*

         * Acquires and releases use the same code for fair and

         * nonfair locks, but differ in whether/how they allow barging

         * when queues are non-empty.

         */

        /**

         * Return true if a reader thread that is otherwise

         * eligible for lock should block because of policy

         * for overtaking other waiting threads.

         */

        abstract boolean readerShouldBlock(Thread current);

        /**

         * Return true if a writer thread that is otherwise

         * eligible for lock should block because of policy

         * for overtaking other waiting threads.

         */

        abstract boolean writerShouldBlock(Thread current);

        /*

         * Note that tryRelease and tryAcquire can be called by

         * Conditions. So it is possible that their arguments contain

         * both read and write holds that are all released during a

         * condition wait and re-established in tryAcquire.

         */

        protected final boolean tryRelease(int releases) {

            int nextc = getState() - releases;

            if (Thread.currentThread() != getExclusiveOwnerThread())

                throw new IllegalMonitorStateException();

            if (exclusiveCount(nextc) == 0) {

                setExclusiveOwnerThread(null);

                setState(nextc);

                return true;

            } else {

                setState(nextc);

                return false;

            }

        }

        protected final boolean tryAcquire(int acquires) {

            /*

             * Walkthrough:

             * 1. if read count nonzero or write count nonzero

             *     and owner is a different thread, fail.

             * 2. If count would saturate, fail. (This can only

             *    happen if count is already nonzero.)

             * 3. Otherwise, this thread is eligible for lock if

             *    it is either a reentrant acquire or

             *    queue policy allows it. If so, update state

             *    and set owner.

             */

            Thread current = Thread.currentThread();

            int c = getState();

            int w = exclusiveCount(c);

            if (c != 0) {

                // (Note: if c != 0 and w == 0 then shared count != 0)

                if (w == 0 || current != getExclusiveOwnerThread())

                    return false;

                if (w + exclusiveCount(acquires) > MAX_COUNT)

                    throw new Error("Maximum lock count exceeded");

            }

            if ((w == 0 && writerShouldBlock(current)) ||

                !compareAndSetState(c, c + acquires))

                return false;

            setExclusiveOwnerThread(current);

            return true;

        }

        protected final boolean tryReleaseShared(int unused) {

            HoldCounter rh = cachedHoldCounter;

            Thread current = Thread.currentThread();

            if (rh == null || rh.tid != current.getId())

                rh = readHolds.get();

            if (rh.tryDecrement() <= 0)

                throw new IllegalMonitorStateException();

            for (;;) {

                int c = getState();

                int nextc = c - SHARED_UNIT;

                if (compareAndSetState(c, nextc))

                    return nextc == 0;

            }

        }

        protected final int tryAcquireShared(int unused) {

            /*

             * Walkthrough:

             * 1. If write lock held by another thread, fail

             * 2. If count saturated, throw error

             * 3. Otherwise, this thread is eligible for

             *    lock wrt state, so ask if it should block

             *    because of queue policy. If not, try

             *    to grant by CASing state and updating count.

             *    Note that step does not check for reentrant

             *    acquires, which is postponed to full version

             *    to avoid having to check hold count in

             *    the more typical non-reentrant case.

             * 4. If step 3 fails either because thread

             *    apparently not eligible or CAS fails,

             *    chain to version with full retry loop.

             */

            Thread current = Thread.currentThread();

            int c = getState();

            if (exclusiveCount(c) != 0 &&

                getExclusiveOwnerThread() != current)

                return -1;

            if (sharedCount(c) == MAX_COUNT)

                throw new Error("Maximum lock count exceeded");

            if (!readerShouldBlock(current) &&

                compareAndSetState(c, c + SHARED_UNIT)) {

                HoldCounter rh = cachedHoldCounter;

                if (rh == null || rh.tid != current.getId())

                    cachedHoldCounter = rh = readHolds.get();

                rh.count++;

                return 1;

            }

            return fullTryAcquireShared(current);

        }

        /**

         * Full version of acquire for reads, that handles CAS misses

         * and reentrant reads not dealt with in tryAcquireShared.

         */

        final int fullTryAcquireShared(Thread current) {

            /*

             * This code is in part redundant with that in

             * tryAcquireShared but is simpler overall by not

             * complicating tryAcquireShared with interactions between

             * retries and lazily reading hold counts.

             */

            HoldCounter rh = cachedHoldCounter;

            if (rh == null || rh.tid != current.getId())

                rh = readHolds.get();

            for (;;) {

                int c = getState();

                int w = exclusiveCount(c);

                if ((w != 0 && getExclusiveOwnerThread() != current) ||

                    ((rh.count | w) == 0 && readerShouldBlock(current)))

                    return -1;

                if (sharedCount(c) == MAX_COUNT)

                    throw new Error("Maximum lock count exceeded");

                if (compareAndSetState(c, c + SHARED_UNIT)) {

                    cachedHoldCounter = rh; // cache for release

                    rh.count++;

                    return 1;

                }

            }

        }

        /**

         * Performs tryLock for write, enabling barging in both modes.

         * This is identical in effect to tryAcquire except for lack

         * of calls to writerShouldBlock

         */

        final boolean tryWriteLock() {

            Thread current = Thread.currentThread();

            int c = getState();

            if (c != 0) {

                int w = exclusiveCount(c);

                if (w == 0 ||current != getExclusiveOwnerThread())

                    return false;

                if (w == MAX_COUNT)

                    throw new Error("Maximum lock count exceeded");

            }

            if (!compareAndSetState(c, c + 1))

                return false;

            setExclusiveOwnerThread(current);

            return true;

        }

        /**

         * Performs tryLock for read, enabling barging in both modes.

         * This is identical in effect to tryAcquireShared except for

         * lack of calls to readerShouldBlock

         */

        final boolean tryReadLock() {

            Thread current = Thread.currentThread();

            for (;;) {

                int c = getState();

                if (exclusiveCount(c) != 0 &&

                    getExclusiveOwnerThread() != current)

                    return false;

                if (sharedCount(c) == MAX_COUNT)

                    throw new Error("Maximum lock count exceeded");

                if (compareAndSetState(c, c + SHARED_UNIT)) {

                    HoldCounter rh = cachedHoldCounter;

                    if (rh == null || rh.tid != current.getId())

                        cachedHoldCounter = rh = readHolds.get();

                    rh.count++;

                    return true;

                }

            }

        }

        protected final boolean isHeldExclusively() {

            // While we must in general read state before owner,

            // we don't need to do so to check if current thread is owner

            return getExclusiveOwnerThread() == Thread.currentThread();

        }

        // Methods relayed to outer class

        final ConditionObject newCondition() {

            return new ConditionObject();

        }

        final Thread getOwner() {

            // Must read state before owner to ensure memory consistency

            return ((exclusiveCount(getState()) == 0)?

                    null :

                    getExclusiveOwnerThread());

        }

        final int getReadLockCount() {

            return sharedCount(getState());

        }

        final boolean isWriteLocked() {

            return exclusiveCount(getState()) != 0;

        }

        final int getWriteHoldCount() {

            return isHeldExclusively() ? exclusiveCount(getState()) : 0;

        }

        final int getReadHoldCount() {

            return getReadLockCount() == 0? 0 : readHolds.get().count;

        }

        /**

         * Reconstitute this lock instance from a stream

         * @param s the stream

         */

        private void readObject(java.io.ObjectInputStream s)

            throws java.io.IOException, ClassNotFoundException {

            s.defaultReadObject();

            readHolds = new ThreadLocalHoldCounter();

            setState(0); // reset to unlocked state

        }

        final int getCount() { return getState(); }

    }

    /**

     * Nonfair version of Sync

     */

    final static class NonfairSync extends Sync {

        private static final long serialVersionUID = -8159625535654395037L;

        final boolean writerShouldBlock(Thread current) {

            return false; // writers can always barge

        }

        final boolean readerShouldBlock(Thread current) {

            /* As a heuristic to avoid indefinite writer starvation,

             * block if the thread that momentarily appears to be head

             * of queue, if one exists, is a waiting writer. This is

             * only a probablistic effect since a new reader will not

             * block if there is a waiting writer behind other enabled

             * readers that have not yet drained from the queue.

             */

            return apparentlyFirstQueuedIsExclusive();

        }

    }

    /**

     * Fair version of Sync

     */

    final static class FairSync extends Sync {

        private static final long serialVersionUID = -2274990926593161451L;

        final boolean writerShouldBlock(Thread current) {

            // only proceed if queue is empty or current thread at head

            return !isFirst(current);

        }

        final boolean readerShouldBlock(Thread current) {

            // only proceed if queue is empty or current thread at head

            return !isFirst(current);

        }

    }

    /**

     * The lock returned by method {@link ReentrantReadWriteLock#readLock}.

     */

    public static class ReadLock implements Lock, java.io.Serializable  {

        private static final long serialVersionUID = -5992448646407690164L;

        private final Sync sync;

        /**

         * Constructor for use by subclasses

         *

         * @param lock the outer lock object

         * @throws NullPointerException if the lock is null

         */

        protected ReadLock(ReentrantReadWriteLock lock) {

            sync = lock.sync;

        }

        /**

         * Acquires the read lock.

         *

         * <p>Acquires the read lock if the write lock is not held by

         * another thread and returns immediately.

         *

         * <p>If the write lock is held by another thread then

         * the current thread becomes disabled for thread scheduling

         * purposes and lies dormant until the read lock has been acquired.

         */

        public void lock() {

            sync.acquireShared(1);

        }

        /**

         * Acquires the read lock unless the current thread is

         * {@linkplain Thread#interrupt interrupted}.

         *

         * <p>Acquires the read lock if the write lock is not held

         * by another thread and returns immediately.

         *

         * <p>If the write lock is held by another thread then the

         * current thread becomes disabled for thread scheduling

         * purposes and lies dormant until one of two things happens:

         *

         * <ul>

         *

         * <li>The read lock is acquired by the current thread; or

         *

         * <li>Some other thread {@linkplain Thread#interrupt interrupts}

         * the current thread.

         *

         * </ul>

         *

         * <p>If the current thread:

         *

         * <ul>

         *

         * <li>has its interrupted status set on entry to this method; or

         *

         * <li>is {@linkplain Thread#interrupt interrupted} while

         * acquiring the read lock,

         *

         * </ul>

         *

         * then {@link InterruptedException} is thrown and the current

         * thread's interrupted status is cleared.

         *

         * <p>In this implementation, as this method is an explicit

         * interruption point, preference is given to responding to

         * the interrupt over normal or reentrant acquisition of the

         * lock.

         *

         * @throws InterruptedException if the current thread is interrupted

         */

        public void lockInterruptibly() throws InterruptedException {

            sync.acquireSharedInterruptibly(1);

        }

        /**

         * Acquires the read lock only if the write lock is not held by

         * another thread at the time of invocation.

         *

         * <p>Acquires the read lock if the write lock is not held by

         * another thread and returns immediately with the value

         * {@code true}. Even when this lock has been set to use a

         * fair ordering policy, a call to {@code tryLock()}

         * <em>will</em> immediately acquire the read lock if it is

         * available, whether or not other threads are currently

         * waiting for the read lock.  This "barging" behavior

         * can be useful in certain circumstances, even though it

         * breaks fairness. If you want to honor the fairness setting

         * for this lock, then use {@link #tryLock(long, TimeUnit)

         * tryLock(0, TimeUnit.SECONDS) } which is almost equivalent

         * (it also detects interruption).

         *

         * <p>If the write lock is held by another thread then

         * this method will return immediately with the value

         * {@code false}.

         *

         * @return {@code true} if the read lock was acquired

         */

        public  boolean tryLock() {

            return sync.tryReadLock();

        }

        /**

         * Acquires the read lock if the write lock is not held by

         * another thread within the given waiting time and the

         * current thread has not been {@linkplain Thread#interrupt

         * interrupted}.

         *

         * <p>Acquires the read lock if the write lock is not held by

         * another thread and returns immediately with the value

         * {@code true}. If this lock has been set to use a fair

         * ordering policy then an available lock <em>will not</em> be

         * acquired if any other threads are waiting for the

         * lock. This is in contrast to the {@link #tryLock()}

         * method. If you want a timed {@code tryLock} that does

         * permit barging on a fair lock then combine the timed and

         * un-timed forms together:

         *

         * <pre>if (lock.tryLock() || lock.tryLock(timeout, unit) ) { ... }

         * </pre>

         *

         * <p>If the write lock is held by another thread then the

         * current thread becomes disabled for thread scheduling

         * purposes and lies dormant until one of three things happens:

         *

         * <ul>

         *

         * <li>The read lock is acquired by the current thread; or

         *

         * <li>Some other thread {@linkplain Thread#interrupt interrupts}

         * the current thread; or

         *

         * <li>The specified waiting time elapses.

         *

         * </ul>

         *

         * <p>If the read lock is acquired then the value {@code true} is

         * returned.

         *

         * <p>If the current thread:

         *

         * <ul>

         *

         * <li>has its interrupted status set on entry to this method; or

         *

         * <li>is {@linkplain Thread#interrupt interrupted} while

         * acquiring the read lock,

         *

         * </ul> then {@link InterruptedException} is thrown and the

         * current thread's interrupted status is cleared.

         *

         * <p>If the specified waiting time elapses then the value

         * {@code false} is returned.  If the time is less than or

         * equal to zero, the method will not wait at all.

         *

         * <p>In this implementation, as this method is an explicit

         * interruption point, preference is given to responding to

         * the interrupt over normal or reentrant acquisition of the

         * lock, and over reporting the elapse of the waiting time.

         *

         * @param timeout the time to wait for the read lock

         * @param unit the time unit of the timeout argument

         * @return {@code true} if the read lock was acquired

         * @throws InterruptedException if the current thread is interrupted

         * @throws NullPointerException if the time unit is null

         *

         */

        public boolean tryLock(long timeout, TimeUnit unit) throws InterruptedException {

            return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));

        }

        /**

         * Attempts to release this lock.

         *

         * <p> If the number of readers is now zero then the lock

         * is made available for write lock attempts.

         */

        public  void unlock() {

            sync.releaseShared(1);

        }

        /**

         * Throws {@code UnsupportedOperationException} because

         * {@code ReadLocks} do not support conditions.

         *

         * @throws UnsupportedOperationException always

         */

        public Condition newCondition() {

            throw new UnsupportedOperationException();

        }

        /**

         * Returns a string identifying this lock, as well as its lock state.

         * The state, in brackets, includes the String {@code "Read locks ="}

         * followed by the number of held read locks.

         *

         * @return a string identifying this lock, as well as its lock state

         */

        public String toString() {

            int r = sync.getReadLockCount();

            return super.toString() +