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Written by Gil Tene of Azul Systems, and released to the public domain, as explained at http://creativecommons.org/publicdomain/zero/1.0/
  
  
  package org.HdrHistogram;
  
WriterReaderPhaser instances provide an asymmetric means for synchronizing the execution of wait-free "writer" critical sections against a "reader phase flip" that needs to make sure no writer critical sections that were active at the beginning of the flip are still active after the flip is done. Multiple writers and multiple readers are supported.

While a WriterReaderPhaser can be useful in multiple scenarios, a specific and common use case is that of safely managing "double buffered" data stream access in which writers can proceed without being blocked, while readers gain access to stable and unchanging buffer samples

NOTE: WriterReaderPhaser writers are wait-free on architectures that support wait-free atomic increment operations. They remain lock-free (but not wait-free) on architectures that do not support wait-free atomic increment operations.
WriterReaderPhaser "writers" are wait free, "readers" block for other "readers", and "readers" are only blocked by "writers" whose critical was entered before the reader's flipPhase() attempt.

When used to protect an actively recording data structure, the assumptions on how readers and writers act are:

  1. There are two sets of data structures ("active" and "inactive")
  2. Writing is done to the perceived active version (as perceived by the writer), and only within critical sections delineated by writerCriticalSectionEnter() and writerCriticalSectionExit(long)).
  3. Only readers switch the perceived roles of the active and inactive data structures. They do so only while under readerLock(), and only before calling flipPhase().
When the above assumptions are met, WriterReaderPhaser guarantees that the inactive data structures are not being modified by any writers while being read while under readerLock() protection after a flipPhase() operation.
 
 public class WriterReaderPhaser {
     private volatile long startEpoch = 0;
     private volatile long evenEndEpoch = 0;
     private volatile long oddEndEpoch = .;
 
     private final ReentrantLock readerLock = new ReentrantLock();
 
     private static final AtomicLongFieldUpdater<WriterReaderPhaserstartEpochUpdater =
             AtomicLongFieldUpdater.newUpdater(WriterReaderPhaser.class"startEpoch");
     private static final AtomicLongFieldUpdater<WriterReaderPhaserevenEndEpochUpdater =
             AtomicLongFieldUpdater.newUpdater(WriterReaderPhaser.class"evenEndEpoch");
     private static final AtomicLongFieldUpdater<WriterReaderPhaseroddEndEpochUpdater =
             AtomicLongFieldUpdater.newUpdater(WriterReaderPhaser.class"oddEndEpoch");

    
Indicate entry to a critical section containing a write operation.

This call is wait-free on architectures that support wait free atomic increment operations, and is lock-free on architectures that do not.

writerCriticalSectionEnter() must be matched with a subsequent writerCriticalSectionExit(long) in order for CriticalSectionPhaser synchronization to function properly.

Returns:
an (opaque) value associated with the critical section entry, which MUST be provided to the matching writerCriticalSectionEnter() call.
 
     public long writerCriticalSectionEnter() {
         return .getAndIncrement(this);
     }

    
Indicate exit from a critical section containing a write operation.

This call is wait-free on architectures that support wait free atomic increment operations, and is lock-free on architectures that do not.

writerCriticalSectionExit(long) must be matched with a preceding writerCriticalSectionEnter() call, and must be provided with the matching writerCriticalSectionEnter() call's return value, in order for CriticalSectionPhaser synchronization to function properly.

Parameters:
criticalValueAtEnter the (opaque) value returned from the matching writerCriticalSectionEnter() call.
 
     public void writerCriticalSectionExit(long criticalValueAtEnter) {
         if (criticalValueAtEnter < 0) {
             .getAndIncrement(this);
         } else {
             .getAndIncrement(this);
         }
     }

    
Enter to a critical section containing a read operation (mutually excludes against other readerLock calls).

readerLock DOES NOT provide synchronization against writerCriticalSectionEnter() calls. Use flipPhase() to synchronize reads against writers.

    public void readerLock() {
        .lock();
    }

    
Exit from a critical section containing a read operation (relinquishes mutual exclusion against other readerLock calls).
    public void readerUnlock() {
        .unlock();
    }

    
Flip a phase in the WriterReaderPhaser instance, flipPhase() can only be called while holding the readerLock(). flipPhase() will return only after all writer critical sections (protected by writerCriticalSectionEnter() ()} and writerCriticalSectionExit(long) ()}) that may have been in flight when the flipPhase() call were made had completed.

No actual writer critical section activity is required for flipPhase() to succeed.

However, flipPhase() is lock-free with respect to calls to writerCriticalSectionEnter() and writerCriticalSectionExit(long). It may spin-wait for for active writer critical section code to complete.

Parameters:
yieldTimeNsec The amount of time (in nanoseconds) to sleep in each yield if yield loop is needed.
    public void flipPhase(long yieldTimeNsec) {
        if (!.isHeldByCurrentThread()) {
            throw new IllegalStateException("flipPhase() can only be called while holding the readerLock()");
        }
        boolean nextPhaseIsEven = ( < 0); // Current phase is odd...
        long initialStartValue;
        // First, clear currently unused [next] phase end epoch (to proper initial value for phase):
        if (nextPhaseIsEven) {
             = initialStartValue = 0;
        } else {
             = initialStartValue = .;
        }
        // Next, reset start value, indicating new phase, and retain value at flip:
        long startValueAtFlip = .getAndSet(thisinitialStartValue);
        // Now, spin until previous phase end value catches up with start value at flip:
        boolean caughtUp = false;
        do {
            if (nextPhaseIsEven) {
                caughtUp = ( == startValueAtFlip);
            } else {
                caughtUp = ( == startValueAtFlip);
            }
            if (!caughtUp) {
                if (yieldTimeNsec == 0) {
                    Thread.yield();
                } else {
                    try {
                        ..sleep(yieldTimeNsec);
                    } catch (InterruptedException ex) {
                    }
                }
            }
        } while (!caughtUp);
    }


    
Flip a phase in the WriterReaderPhaser instance, flipPhase() can only be called while holding the readerLock(). flipPhase() will return only after all writer critical sections (protected by writerCriticalSectionEnter() ()} and writerCriticalSectionExit(long) ()}) that may have been in flight when the flipPhase() call were made had completed.

No actual writer critical section activity is required for flipPhase() to succeed.

However, flipPhase() is lock-free with respect to calls to writerCriticalSectionEnter() and writerCriticalSectionExit(long). It may spin-wait for for active writer critical section code to complete.

    public void flipPhase() {
        flipPhase(0);
    }
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