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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:

 * <blockquote><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></blockquote>

 * <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:

 * <blockquote><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></blockquote>

 * @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);

    }

}