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package org.jgroups.util;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import java.util.NoSuchElementException;
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.concurrent.atomic.AtomicLong;
import java.util.concurrent.atomic.AtomicReferenceArray;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
/**
* Ring buffer, implemented with a circular array.
* Designed for multiple producers (add()) and a single consumer (remove()). Note that the remove() methods
* are not reentrant, so multiple consumers won't work correctly !
* The buffer has a fixed capacity, and a low (LOW), highest delivered (HD) and highest received (HR) seqno.
* An element with a sequence number (seqno) > low + capacity or < HD will get discarded.
*
* Elements are added after HD, but cannot wrap around beyond LOW. Addition doesn't need to be sequential, e.g.
* adding 5, 6, 8 is OK (as long as a seqno doesn't pass LOW). Addition may advance HR. Addition of elements that
* are already present is a no-op, and will not set the element again.
*
* Removal of elements starts at HD+1; any non-null element is removed and HD is advanced accordingly. If a remove
* method is called with nullify=true, then removed elements are nulled and LOW is advanced as well (LOW=HD). Note
* that all removals in a given RingBufferLockless must either have nullify=true, or all must be false. It is not
* permitted to do some removals with nullify=true, and others with nullify=false, in the same RingBufferLockless.
*
* The {@link #stable(long)} method is called periodically; it nulls all elements between LOW and HD and advances LOW
* to HD.
*
* @author Bela Ban
* @since 3.1
*/
public class RingBufferSeqnoLockless implements Iterable {
/** Atomic ref array so that elements can be checked for null and set atomically. Should always be sized to a power of 2. */
protected final AtomicReferenceArray buf;
/** The lowest seqno. Moved forward by stable() */
protected volatile long low;
/** The highest delivered seqno. Moved forward by a remove method. The next message to be removed is hd +1 */
protected volatile long hd;
/** The highest received seqno. Moved forward by add(). The next message to be added is hr +1 */
protected final AtomicLong hr=new AtomicLong(0);
protected final long offset;
/** Lock for adders to block on when the buffer is full */
protected final Lock lock=new ReentrantLock();
protected final Condition buffer_full=lock.newCondition();
protected volatile boolean running=true;
protected final AtomicBoolean processing=new AtomicBoolean(false);
/**
* Creates a RingBuffer
* @param capacity The number of elements the ring buffer's array should hold
* @param offset The offset. The first element to be added has to be offset +1.
*/
public RingBufferSeqnoLockless(int capacity, long offset) {
if(capacity < 1)
throw new IllegalArgumentException("incorrect capacity of " + capacity);
if(offset < 0)
throw new IllegalArgumentException("invalid offset of " + offset);
// Find a power of 2 >= buffer capacity
int cap = 1;
while (capacity > cap)
cap <<= 1;
this.buf=new AtomicReferenceArray<>(cap);
this.low=this.hd=this.offset=offset;
this.hr.set(offset);
}
public long getLow() {return low;}
public long getHighestDelivered() {return hd;}
public void setHighestDelivered(long hd) {this.hd=hd;}
public long getHighestReceived() {return hr.get();}
public long[] getDigest() {return new long[]{hd, hr.get()};}
public AtomicBoolean getProcessing() {return processing;}
public boolean add(long seqno, T element) {
return add(seqno, element, false);
}
/**
* Adds a new element to the buffer
* @param seqno The seqno of the element
* @param element The element
* @param block If true, add() will block when the buffer is full until there is space. Else, add() will
* return immediately, either successfully or unsuccessfully (if the buffer is full)
* @return True if the element was added, false otherwise.
*/
public boolean add(long seqno, T element, boolean block) {
validate(seqno);
if(seqno <= hd) // seqno already delivered, includes check seqno <= low
return false;
if(seqno - low > capacity() && (!block || !block(seqno))) // seqno too big
return false;
// now we can set any slow > hd and yet not overwriting low (check #1 above)
int index=index(seqno);
// Fix for correctness check #1 (see doc/design/RingBuffer.txt)
if(buf.get(index) != null || seqno <= hd)
return false;
if(!buf.compareAndSet(index, null, element)) // the element at buf[index] was already present
return false;
// now see if hr needs to moved forward, this can be concurrent as we may have multiple producers
for(;;) {
long current_hr=hr.get();
long new_hr=Math.max(seqno, current_hr);
if(new_hr <= current_hr || hr.compareAndSet(current_hr, new_hr))
break;
}
return true;
}
/**
* Removes the next element (at hd +1). Note that this method is not concurrent, as
* RingBuffer can only have 1 remover thread active at any time !
* @param nullify Nulls the element in the array if true
* @return T if there was a non-null element at position hd +1, or null if the element at hd+1 was null, or
* hd+1 > hr.
*/
public T remove(boolean nullify) {
long tmp=hd+1;
if(tmp > hr.get())
return null;
int index=index(tmp);
T element=buf.get(index);
if(element == null)
return null;
hd=tmp;
if(nullify) {
long tmp_low=low;
if(tmp == tmp_low +1)
buf.compareAndSet(index, element, null);
else {
int from=index(tmp_low+1), length=(int)(tmp - tmp_low), capacity=capacity();
for(int i=from; i < from+length; i++) {
index=i & (capacity - 1);
buf.set(index, null);
}
}
low=tmp;
lock.lock();
try {
buffer_full.signalAll();
}
finally {
lock.unlock();
}
}
return element;
}
/**
* Removes the next element (at hd +1). Note that this method is not concurrent, as
* RingBuffer can only have 1 remover thread active at any time !
* @return T if there was a non-null element at position hd +1, or null if the element at hd+1 was null.
*/
public T remove() {
return remove(false);
}
public List removeMany(boolean nullify, int max_results) {
return removeMany(null, nullify, max_results);
}
public List removeMany(final AtomicBoolean processing, boolean nullify, int max_results) {
List list=null;
int num_results=0;
long original_hd=hd, start=original_hd, end=hr.get();
T element;
while(start+1 <= end && (element=buf.get(index(start+1))) != null) {
if(list == null)
list=new ArrayList<>(max_results > 0? max_results : 20);
list.add(element);
start++;
if(max_results > 0 && ++num_results >= max_results)
break;
}
if(start > original_hd) { // do we need to move HD forward ?
hd=start;
if(nullify) {
long tmp_low=low;
int from=index(tmp_low+1), length=(int)(start - tmp_low), capacity=capacity();
for(int i=from; i < from+length; i++) {
int index=i & (capacity - 1);
buf.set(index, null);
}
// Releases some of the blocked adders
if(start > low) {
low=start;
lock.lock();
try {
buffer_full.signalAll();
}
finally {
lock.unlock();
}
}
}
}
if((list == null || list.isEmpty()) && processing != null)
processing.set(false);
return list;
}
public T get(long seqno) {
validate(seqno);
if(seqno <= low || seqno > hr.get())
return null;
int index=index(seqno);
return buf.get(index);
}
/** Only used for testing !! */
public T _get(long seqno) {
int index=index(seqno);
return index < 0? null : buf.get(index);
}
/**
* Returns a list of messages in the range [from .. to], including from and to
* @param from
* @param to
* @return A list of messages, or null if none in range [from .. to] was found
*/
public List get(long from, long to) {
if(from > to)
throw new IllegalArgumentException("from (" + from + ") has to be <= to (" + to + ")");
validate(from);
List retval=null;
for(long i=from; i <= to; i++) {
T element=get(i);
if(element != null) {
if(retval == null)
retval=new ArrayList<>();
retval.add(element);
}
}
return retval;
}
/** Nulls elements between low and seqno and forwards low */
public void stable(long seqno) {
validate(seqno);
if(seqno <= low)
return;
if(seqno > hd)
throw new IllegalArgumentException("seqno " + seqno + " cannot be bigger than hd (" + hd + ")");
int from=index(low+1), length=(int)(seqno - low), capacity=capacity();
for(int i=from; i < from+length; i++) {
int index=i & (capacity - 1);
buf.set(index, null);
}
// Releases some of the blocked adders
if(seqno > low) {
low=seqno;
lock.lock();
try {
buffer_full.signalAll();
}
finally {
lock.unlock();
}
}
}
public void destroy() {
lock.lock();
try {
running=false;
buffer_full.signalAll();
}
finally {
lock.unlock();
}
}
public final int capacity() {return buf.length();}
public int size() {return count(false);}
public int missing() {return count(true);}
public int spaceUsed() {return (int)(hr.get() - low);}
public double saturation() {
int space=spaceUsed();
return space == 0? 0.0 : space / (double)capacity();
}
public SeqnoList getMissing() {
SeqnoList missing=null;
long tmp_hd=hd, tmp_hr=hr.get();
for(long i=tmp_hd+1; i <= tmp_hr; i++) {
if(buf.get(index(i)) == null) {
if(missing == null)
missing=new SeqnoList((int)(tmp_hr-tmp_hd), hd);
long end=i;
while(buf.get(index(end+1)) == null && end <= tmp_hr)
end++;
if(end == i)
missing.add(i);
else {
missing.add(i, end);
i=end;
}
}
}
return missing;
}
/**
* Returns an iterator over the elements of the ring buffer in the range [HD+1 .. HR]
* @return RingBufferIterator
* @throws NoSuchElementException is HD is moved forward during the iteration
*/
public Iterator iterator() {
return new RingBufferIterator(buf);
}
public String toString() {
StringBuilder sb=new StringBuilder();
sb.append("[" + low + " | " + hd + " | " + hr + "] (" + size() + " elements, " + missing() + " missing)");
return sb.toString();
}
protected static final void validate(long seqno) {
if(seqno < 0)
throw new IllegalArgumentException("seqno " + seqno + " cannot be negative");
}
protected int index(long seqno) {
return (int)((seqno - offset -1) & (capacity() - 1));
}
protected boolean block(long seqno) {
lock.lock();
try {
while(running && seqno - low > capacity()) {
try {
buffer_full.await();
}
catch(InterruptedException e) {
}
}
return running;
}
finally {
lock.unlock();
}
}
protected int count(boolean missing) {
int retval=0;
long tmp_hd=hd, tmp_hr=hr.get();
for(long i=tmp_hd+1; i <= tmp_hr; i++) {
int index=index(i);
T element=buf.get(index);
if(missing && element == null)
retval++;
if(!missing && element != null)
retval++;
}
return retval;
}
protected class RingBufferIterator implements Iterator {
protected final AtomicReferenceArray buffer;
protected long current=hd+1;
public RingBufferIterator(AtomicReferenceArray buffer) {
this.buffer=buffer;
}
public boolean hasNext() {
return current <= hr.get();
}
public T next() {
if(!hasNext()){
throw new NoSuchElementException();
}
if(current <= hd)
current=hd+1;
return buffer.get(index(current++));
}
public void remove() {}
}
}
