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	/*
	* 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
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	* version 2 for more details (a copy is included in the LICENSE file that
	* accompanied this code).
	*
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	*
	* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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	package java.lang.invoke;

	/**
	* <p>
	* A {@code SwitchPoint} is an object which can publish state transitions to other threads.
	* A switch point is initially in the <em>valid</em> state, but may at any time be
	* changed to the <em>invalid</em> state. Invalidation cannot be reversed.
	* A switch point can combine a <em>guarded pair</em> of method handles into a
	* <em>guarded delegator</em>.
	* The guarded delegator is a method handle which delegates to one of the old method handles.
	* The state of the switch point determines which of the two gets the delegation.
	* <p>
	* A single switch point may be used to control any number of method handles.
	* (Indirectly, therefore, it can control any number of call sites.)
	* This is done by using the single switch point as a factory for combining
	* any number of guarded method handle pairs into guarded delegators.
	* <p>
	* When a guarded delegator is created from a guarded pair, the pair
	* is wrapped in a new method handle {@code M},
	* which is permanently associated with the switch point that created it.
	* Each pair consists of a target {@code T} and a fallback {@code F}.
	* While the switch point is valid, invocations to {@code M} are delegated to {@code T}.
	* After it is invalidated, invocations are delegated to {@code F}.
	* <p>
	* Invalidation is global and immediate, as if the switch point contained a
	* volatile boolean variable consulted on every call to {@code M}.
	* The invalidation is also permanent, which means the switch point
	* can change state only once.
	* The switch point will always delegate to {@code F} after being invalidated.
	* At that point {@code guardWithTest} may ignore {@code T} and return {@code F}.
	* <p>
	* Here is an example of a switch point in action:
	* <pre>{@code
	* MethodHandle MH_strcat = MethodHandles.lookup()
	* .findVirtual(String.class, "concat", MethodType.methodType(String.class, String.class));
	* SwitchPoint spt = new SwitchPoint();
	* assert(!spt.hasBeenInvalidated());
	* // the following steps may be repeated to re-use the same switch point:
	* MethodHandle worker1 = MH_strcat;
	* MethodHandle worker2 = MethodHandles.permuteArguments(MH_strcat, MH_strcat.type(), 1, 0);
	* MethodHandle worker = spt.guardWithTest(worker1, worker2);
	* assertEquals("method", (String) worker.invokeExact("met", "hod"));
	* SwitchPoint.invalidateAll(new SwitchPoint[]{ spt });
	* assert(spt.hasBeenInvalidated());
	* assertEquals("hodmet", (String) worker.invokeExact("met", "hod"));
	* }</pre>
	* <p style="font-size:smaller;">
	* <em>Discussion:</em>
	* Switch points are useful without subclassing. They may also be subclassed.
	* This may be useful in order to associate application-specific invalidation logic
	* with the switch point.
	* Notice that there is no permanent association between a switch point and
	* the method handles it produces and consumes.
	* The garbage collector may collect method handles produced or consumed
	* by a switch point independently of the lifetime of the switch point itself.
	* <p style="font-size:smaller;">
	* <em>Implementation Note:</em>
	* A switch point behaves as if implemented on top of {@link MutableCallSite},
	* approximately as follows:
	* <pre>{@code
	* public class SwitchPoint {
	* private static final MethodHandle
	* K_true = MethodHandles.constant(boolean.class, true),
	* K_false = MethodHandles.constant(boolean.class, false);
	* private final MutableCallSite mcs;
	* private final MethodHandle mcsInvoker;
	* public SwitchPoint() {
	* this.mcs = new MutableCallSite(K_true);
	* this.mcsInvoker = mcs.dynamicInvoker();
	* }
	* public MethodHandle guardWithTest(
	* MethodHandle target, MethodHandle fallback) {
	* // Note: mcsInvoker is of type ()boolean.
	* // Target and fallback may take any arguments, but must have the same type.
	* return MethodHandles.guardWithTest(this.mcsInvoker, target, fallback);
	* }
	* public static void invalidateAll(SwitchPoint[] spts) {
	* List<MutableCallSite> mcss = new ArrayList<>();
	* for (SwitchPoint spt : spts) mcss.add(spt.mcs);
	* for (MutableCallSite mcs : mcss) mcs.setTarget(K_false);
	* MutableCallSite.syncAll(mcss.toArray(new MutableCallSite[0]));
	* }
	* }
	* }</pre>
	* @author Remi Forax, JSR 292 EG
	*/
	public class SwitchPoint {
	private static final MethodHandle
	K_true = MethodHandles.constant(boolean.class, true),
	K_false = MethodHandles.constant(boolean.class, false);

	private final MutableCallSite mcs;
	private final MethodHandle mcsInvoker;

	/**
	* Creates a new switch point.
	*/
	public SwitchPoint() {
	this.mcs = new MutableCallSite(K_true);
	this.mcsInvoker = mcs.dynamicInvoker();
	}

	/**
	* Determines if this switch point has been invalidated yet.
	*
	* <p style="font-size:smaller;">
	* <em>Discussion:</em>
	* Because of the one-way nature of invalidation, once a switch point begins
	* to return true for {@code hasBeenInvalidated},
	* it will always do so in the future.
	* On the other hand, a valid switch point visible to other threads may
	* be invalidated at any moment, due to a request by another thread.
	* <p style="font-size:smaller;">
	* Since invalidation is a global and immediate operation,
	* the execution of this query, on a valid switchpoint,
	* must be internally sequenced with any
	* other threads that could cause invalidation.
	* This query may therefore be expensive.
	* The recommended way to build a boolean-valued method handle
	* which queries the invalidation state of a switch point {@code s} is
	* to call {@code s.guardWithTest} on
	* {@link MethodHandles#constant constant} true and false method handles.
	*
	* @return true if this switch point has been invalidated
	*/
	public boolean hasBeenInvalidated() {
	return (mcs.getTarget() != K_true);
	}

	/**
	* Returns a method handle which always delegates either to the target or the fallback.
	* The method handle will delegate to the target exactly as long as the switch point is valid.
	* After that, it will permanently delegate to the fallback.
	* <p>
	* The target and fallback must be of exactly the same method type,
	* and the resulting combined method handle will also be of this type.
	*
	* @param target the method handle selected by the switch point as long as it is valid
	* @param fallback the method handle selected by the switch point after it is invalidated
	* @return a combined method handle which always calls either the target or fallback
	* @throws NullPointerException if either argument is null
	* @throws IllegalArgumentException if the two method types do not match
	* @see MethodHandles#guardWithTest
	*/
	public MethodHandle guardWithTest(MethodHandle target, MethodHandle fallback) {
	if (mcs.getTarget() == K_false)
	return fallback; // already invalid
	return MethodHandles.guardWithTest(mcsInvoker, target, fallback);
	}

	/**
	* Sets all of the given switch points into the invalid state.
	* After this call executes, no thread will observe any of the
	* switch points to be in a valid state.
	* <p>
	* This operation is likely to be expensive and should be used sparingly.
	* If possible, it should be buffered for batch processing on sets of switch points.
	* <p>
	* If {@code switchPoints} contains a null element,
	* a {@code NullPointerException} will be raised.
	* In this case, some non-null elements in the array may be
	* processed before the method returns abnormally.
	* Which elements these are (if any) is implementation-dependent.
	*
	* <p style="font-size:smaller;">
	* <em>Discussion:</em>
	* For performance reasons, {@code invalidateAll} is not a virtual method
	* on a single switch point, but rather applies to a set of switch points.
	* Some implementations may incur a large fixed overhead cost
	* for processing one or more invalidation operations,
	* but a small incremental cost for each additional invalidation.
	* In any case, this operation is likely to be costly, since
	* other threads may have to be somehow interrupted
	* in order to make them notice the updated switch point state.
	* However, it may be observed that a single call to invalidate
	* several switch points has the same formal effect as many calls,
	* each on just one of the switch points.
	*
	* <p style="font-size:smaller;">
	* <em>Implementation Note:</em>
	* Simple implementations of {@code SwitchPoint} may use
	* a private {@link MutableCallSite} to publish the state of a switch point.
	* In such an implementation, the {@code invalidateAll} method can
	* simply change the call site's target, and issue one call to
	* {@linkplain MutableCallSite#syncAll synchronize} all the
	* private call sites.
	*
	* @param switchPoints an array of call sites to be synchronized
	* @throws NullPointerException if the {@code switchPoints} array reference is null
	* or the array contains a null
	*/
	public static void invalidateAll(SwitchPoint[] switchPoints) {
	if (switchPoints.length == 0) return;
	MutableCallSite[] sites = new MutableCallSite[switchPoints.length];
	for (int i = 0; i < switchPoints.length; i++) {
	SwitchPoint spt = switchPoints[i];
	if (spt == null) break; // MSC.syncAll will trigger a NPE
	sites[i] = spt.mcs;
	spt.mcs.setTarget(K_false);
	}
	MutableCallSite.syncAll(sites);
	}
	}

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