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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
	* 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.
	*/

	/*
	* This file is available under and governed by the GNU General Public
	* License version 2 only, as published by the Free Software Foundation.
	* However, the following notice accompanied the original version of this
	* file:
	*
	* 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/publicdomain/zero/1.0/
	*/

	package java.util.concurrent.atomic;
	import java.util.function.LongBinaryOperator;
	import java.util.function.DoubleBinaryOperator;
	import java.util.concurrent.ThreadLocalRandom;

	/**
	* A package-local class holding common representation and mechanics
	* for classes supporting dynamic striping on 64bit values. The class
	* extends Number so that concrete subclasses must publicly do so.
	*/
	@SuppressWarnings("serial")
	abstract class Striped64 extends Number {
	/*
	* This class maintains a lazily-initialized table of atomically
	* updated variables, plus an extra "base" field. The table size
	* is a power of two. Indexing uses masked per-thread hash codes.
	* Nearly all declarations in this class are package-private,
	* accessed directly by subclasses.
	*
	* Table entries are of class Cell; a variant of AtomicLong padded
	* (via @sun.misc.Contended) to reduce cache contention. Padding
	* is overkill for most Atomics because they are usually
	* irregularly scattered in memory and thus don't interfere much
	* with each other. But Atomic objects residing in arrays will
	* tend to be placed adjacent to each other, and so will most
	* often share cache lines (with a huge negative performance
	* impact) without this precaution.
	*
	* In part because Cells are relatively large, we avoid creating
	* them until they are needed. When there is no contention, all
	* updates are made to the base field. Upon first contention (a
	* failed CAS on base update), the table is initialized to size 2.
	* The table size is doubled upon further contention until
	* reaching the nearest power of two greater than or equal to the
	* number of CPUS. Table slots remain empty (null) until they are
	* needed.
	*
	* A single spinlock ("cellsBusy") is used for initializing and
	* resizing the table, as well as populating slots with new Cells.
	* There is no need for a blocking lock; when the lock is not
	* available, threads try other slots (or the base). During these
	* retries, there is increased contention and reduced locality,
	* which is still better than alternatives.
	*
	* The Thread probe fields maintained via ThreadLocalRandom serve
	* as per-thread hash codes. We let them remain uninitialized as
	* zero (if they come in this way) until they contend at slot
	* 0. They are then initialized to values that typically do not
	* often conflict with others. Contention and/or table collisions
	* are indicated by failed CASes when performing an update
	* operation. Upon a collision, if the table size is less than
	* the capacity, it is doubled in size unless some other thread
	* holds the lock. If a hashed slot is empty, and lock is
	* available, a new Cell is created. Otherwise, if the slot
	* exists, a CAS is tried. Retries proceed by "double hashing",
	* using a secondary hash (Marsaglia XorShift) to try to find a
	* free slot.
	*
	* The table size is capped because, when there are more threads
	* than CPUs, supposing that each thread were bound to a CPU,
	* there would exist a perfect hash function mapping threads to
	* slots that eliminates collisions. When we reach capacity, we
	* search for this mapping by randomly varying the hash codes of
	* colliding threads. Because search is random, and collisions
	* only become known via CAS failures, convergence can be slow,
	* and because threads are typically not bound to CPUS forever,
	* may not occur at all. However, despite these limitations,
	* observed contention rates are typically low in these cases.
	*
	* It is possible for a Cell to become unused when threads that
	* once hashed to it terminate, as well as in the case where
	* doubling the table causes no thread to hash to it under
	* expanded mask. We do not try to detect or remove such cells,
	* under the assumption that for long-running instances, observed
	* contention levels will recur, so the cells will eventually be
	* needed again; and for short-lived ones, it does not matter.
	*/

	/**
	* Padded variant of AtomicLong supporting only raw accesses plus CAS.
	*
	* JVM intrinsics note: It would be possible to use a release-only
	* form of CAS here, if it were provided.
	*/
	@sun.misc.Contended static final class Cell {
	volatile long value;
	Cell(long x) { value = x; }
	final boolean cas(long cmp, long val) {
	return UNSAFE.compareAndSwapLong(this, valueOffset, cmp, val);
	}

	// Unsafe mechanics
	private static final sun.misc.Unsafe UNSAFE;
	private static final long valueOffset;
	static {
	try {
	UNSAFE = sun.misc.Unsafe.getUnsafe();
	Class<?> ak = Cell.class;
	valueOffset = UNSAFE.objectFieldOffset
	(ak.getDeclaredField("value"));
	} catch (Exception e) {
	throw new Error(e);
	}
	}
	}

	/** Number of CPUS, to place bound on table size */
	static final int NCPU = Runtime.getRuntime().availableProcessors();

	/**
	* Table of cells. When non-null, size is a power of 2.
	*/
	transient volatile Cell[] cells;

	/**
	* Base value, used mainly when there is no contention, but also as
	* a fallback during table initialization races. Updated via CAS.
	*/
	transient volatile long base;

	/**
	* Spinlock (locked via CAS) used when resizing and/or creating Cells.
	*/
	transient volatile int cellsBusy;

	/**
	* Package-private default constructor
	*/
	Striped64() {
	}

	/**
	* CASes the base field.
	*/
	final boolean casBase(long cmp, long val) {
	return UNSAFE.compareAndSwapLong(this, BASE, cmp, val);
	}

	/**
	* CASes the cellsBusy field from 0 to 1 to acquire lock.
	*/
	final boolean casCellsBusy() {
	return UNSAFE.compareAndSwapInt(this, CELLSBUSY, 0, 1);
	}

	/**
	* Returns the probe value for the current thread.
	* Duplicated from ThreadLocalRandom because of packaging restrictions.
	*/
	static final int getProbe() {
	return UNSAFE.getInt(Thread.currentThread(), PROBE);
	}

	/**
	* Pseudo-randomly advances and records the given probe value for the
	* given thread.
	* Duplicated from ThreadLocalRandom because of packaging restrictions.
	*/
	static final int advanceProbe(int probe) {
	probe ^= probe << 13; // xorshift
	probe ^= probe >>> 17;
	probe ^= probe << 5;
	UNSAFE.putInt(Thread.currentThread(), PROBE, probe);
	return probe;
	}

	/**
	* Handles cases of updates involving initialization, resizing,
	* creating new Cells, and/or contention. See above for
	* explanation. This method suffers the usual non-modularity
	* problems of optimistic retry code, relying on rechecked sets of
	* reads.
	*
	* @param x the value
	* @param fn the update function, or null for add (this convention
	* avoids the need for an extra field or function in LongAdder).
	* @param wasUncontended false if CAS failed before call
	*/
	final void longAccumulate(long x, LongBinaryOperator fn,
	boolean wasUncontended) {
	int h;
	if ((h = getProbe()) == 0) {
	ThreadLocalRandom.current(); // force initialization
	h = getProbe();
	wasUncontended = true;
	}
	boolean collide = false; // True if last slot nonempty
	for (;;) {
	Cell[] as; Cell a; int n; long v;
	if ((as = cells) != null && (n = as.length) > 0) {
	if ((a = as[(n - 1) & h]) == null) {
	if (cellsBusy == 0) { // Try to attach new Cell
	Cell r = new Cell(x); // Optimistically create
	if (cellsBusy == 0 && casCellsBusy()) {
	boolean created = false;
	try { // Recheck under lock
	Cell[] rs; int m, j;
	if ((rs = cells) != null &&
	(m = rs.length) > 0 &&
	rs[j = (m - 1) & h] == null) {
	rs[j] = r;
	created = true;
	}
	} finally {
	cellsBusy = 0;
	}
	if (created)
	break;
	continue; // Slot is now non-empty
	}
	}
	collide = false;
	}
	else if (!wasUncontended) // CAS already known to fail
	wasUncontended = true; // Continue after rehash
	else if (a.cas(v = a.value, ((fn == null) ? v + x :
	fn.applyAsLong(v, x))))
	break;
	else if (n >= NCPU \|\| cells != as)
	collide = false; // At max size or stale
	else if (!collide)
	collide = true;
	else if (cellsBusy == 0 && casCellsBusy()) {
	try {
	if (cells == as) { // Expand table unless stale
	Cell[] rs = new Cell[n << 1];
	for (int i = 0; i < n; ++i)
	rs[i] = as[i];
	cells = rs;
	}
	} finally {
	cellsBusy = 0;
	}
	collide = false;
	continue; // Retry with expanded table
	}
	h = advanceProbe(h);
	}
	else if (cellsBusy == 0 && cells == as && casCellsBusy()) {
	boolean init = false;
	try { // Initialize table
	if (cells == as) {
	Cell[] rs = new Cell[2];
	rs[h & 1] = new Cell(x);
	cells = rs;
	init = true;
	}
	} finally {
	cellsBusy = 0;
	}
	if (init)
	break;
	}
	else if (casBase(v = base, ((fn == null) ? v + x :
	fn.applyAsLong(v, x))))
	break; // Fall back on using base
	}
	}

	/**
	* Same as longAccumulate, but injecting long/double conversions
	* in too many places to sensibly merge with long version, given
	* the low-overhead requirements of this class. So must instead be
	* maintained by copy/paste/adapt.
	*/
	final void doubleAccumulate(double x, DoubleBinaryOperator fn,
	boolean wasUncontended) {
	int h;
	if ((h = getProbe()) == 0) {
	ThreadLocalRandom.current(); // force initialization
	h = getProbe();
	wasUncontended = true;
	}
	boolean collide = false; // True if last slot nonempty
	for (;;) {
	Cell[] as; Cell a; int n; long v;
	if ((as = cells) != null && (n = as.length) > 0) {
	if ((a = as[(n - 1) & h]) == null) {
	if (cellsBusy == 0) { // Try to attach new Cell
	Cell r = new Cell(Double.doubleToRawLongBits(x));
	if (cellsBusy == 0 && casCellsBusy()) {
	boolean created = false;
	try { // Recheck under lock
	Cell[] rs; int m, j;
	if ((rs = cells) != null &&
	(m = rs.length) > 0 &&
	rs[j = (m - 1) & h] == null) {
	rs[j] = r;
	created = true;
	}
	} finally {
	cellsBusy = 0;
	}
	if (created)
	break;
	continue; // Slot is now non-empty
	}
	}
	collide = false;
	}
	else if (!wasUncontended) // CAS already known to fail
	wasUncontended = true; // Continue after rehash
	else if (a.cas(v = a.value,
	((fn == null) ?
	Double.doubleToRawLongBits
	(Double.longBitsToDouble(v) + x) :
	Double.doubleToRawLongBits
	(fn.applyAsDouble
	(Double.longBitsToDouble(v), x)))))
	break;
	else if (n >= NCPU \|\| cells != as)
	collide = false; // At max size or stale
	else if (!collide)
	collide = true;
	else if (cellsBusy == 0 && casCellsBusy()) {
	try {
	if (cells == as) { // Expand table unless stale
	Cell[] rs = new Cell[n << 1];
	for (int i = 0; i < n; ++i)
	rs[i] = as[i];
	cells = rs;
	}
	} finally {
	cellsBusy = 0;
	}
	collide = false;
	continue; // Retry with expanded table
	}
	h = advanceProbe(h);
	}
	else if (cellsBusy == 0 && cells == as && casCellsBusy()) {
	boolean init = false;
	try { // Initialize table
	if (cells == as) {
	Cell[] rs = new Cell[2];
	rs[h & 1] = new Cell(Double.doubleToRawLongBits(x));
	cells = rs;
	init = true;
	}
	} finally {
	cellsBusy = 0;
	}
	if (init)
	break;
	}
	else if (casBase(v = base,
	((fn == null) ?
	Double.doubleToRawLongBits
	(Double.longBitsToDouble(v) + x) :
	Double.doubleToRawLongBits
	(fn.applyAsDouble
	(Double.longBitsToDouble(v), x)))))
	break; // Fall back on using base
	}
	}

	// Unsafe mechanics
	private static final sun.misc.Unsafe UNSAFE;
	private static final long BASE;
	private static final long CELLSBUSY;
	private static final long PROBE;
	static {
	try {
	UNSAFE = sun.misc.Unsafe.getUnsafe();
	Class<?> sk = Striped64.class;
	BASE = UNSAFE.objectFieldOffset
	(sk.getDeclaredField("base"));
	CELLSBUSY = UNSAFE.objectFieldOffset
	(sk.getDeclaredField("cellsBusy"));
	Class<?> tk = Thread.class;
	PROBE = UNSAFE.objectFieldOffset
	(tk.getDeclaredField("threadLocalRandomProbe"));
	} catch (Exception e) {
	throw new Error(e);
	}
	}

	}

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