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/**
 * Copyright 2015 Netflix, Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not
 * use this file except in compliance with the License. You may obtain a copy of
 * the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 * License for the specific language governing permissions and limitations under
 * the License.
 */
package rx.internal.operators;

import java.util.Queue;
import java.util.concurrent.atomic.*;

import rx.Subscriber;
import rx.functions.Func1;
import rx.internal.util.UtilityFunctions;

/**
 * Utility functions for use with backpressure.
 *
 */
public final class BackpressureUtils {
    /**
     * Masks the most significant bit, i.e., 0x8000_0000_0000_0000L.
     */
    static final long COMPLETED_MASK = Long.MIN_VALUE;
    /**
     * Masks the request amount bits, i.e., 0x7FFF_FFFF_FFFF_FFFF.
     */
    static final long REQUESTED_MASK = Long.MAX_VALUE;

    /** Utility class, no instances. */
    private BackpressureUtils() {
        throw new IllegalStateException("No instances!");
    }
    /**
     * Adds {@code n} to {@code requested} field and returns the value prior to
     * addition once the addition is successful (uses CAS semantics). If
     * overflows then sets {@code requested} field to {@code Long.MAX_VALUE}.
     * 
     * @param  the type of the target object on which the field updater operates
     * 
     * @param requested
     *            atomic field updater for a request count
     * @param object
     *            contains the field updated by the updater
     * @param n
     *            the number of requests to add to the requested count
     * @return requested value just prior to successful addition
     * @deprecated Android has issues with reflection-based atomics
     */
    @Deprecated
    public static  long getAndAddRequest(AtomicLongFieldUpdater requested, T object, long n) {
        // add n to field but check for overflow
        while (true) {
            long current = requested.get(object);
            long next = addCap(current, n);
            if (requested.compareAndSet(object, current, next)) {
                return current;
            }
        }
    }

    /**
     * Adds {@code n} (not validated) to {@code requested} and returns the value prior to addition once the
     * addition is successful (uses CAS semantics). If overflows then sets
     * {@code requested} field to {@code Long.MAX_VALUE}.
     * 
     * @param requested
     *            atomic long that should be updated
     * @param n
     *            the number of requests to add to the requested count, positive (not validated)
     * @return requested value just prior to successful addition
     */
    public static long getAndAddRequest(AtomicLong requested, long n) {
        // add n to field but check for overflow
        while (true) {
            long current = requested.get();
            long next = addCap(current, n);
            if (requested.compareAndSet(current, next)) {
                return current;
            }
        }
    }
    
    /**
     * Multiplies two positive longs and caps the result at Long.MAX_VALUE.
     * @param a the first value
     * @param b the second value
     * @return the capped product of a and b
     */
    public static long multiplyCap(long a, long b) {
        long u = a * b;
        if (((a | b) >>> 31) != 0) {
            if (b != 0L && (u / b != a)) {
                u = Long.MAX_VALUE;
            }
        }
        return u;
    }
    
    /**
     * Adds two positive longs and caps the result at Long.MAX_VALUE.
     * @param a the first value
     * @param b the second value
     * @return the capped sum of a and b
     */
    public static long addCap(long a, long b) {
        long u = a + b;
        if (u < 0L) {
            u = Long.MAX_VALUE;
        }
        return u;
    }
    
    /**
     * Signals the completion of the main sequence and switches to post-completion replay mode.
     * 
     * 

* Don't modify the queue after calling this method! * *

* Post-completion backpressure handles the case when a source produces values based on * requests when it is active but more values are available even after its completion. * In this case, the onCompleted() can't just emit the contents of the queue but has to * coordinate with the requested amounts. This requires two distinct modes: active and * completed. In active mode, requests flow through and the queue is not accessed but * in completed mode, requests no-longer reach the upstream but help in draining the queue. *

* The algorithm utilizes the most significant bit (bit 63) of a long value (AtomicLong) since * request amount only goes up to Long.MAX_VALUE (bits 0-62) and negative values aren't * allowed. * * @param the value type to emit * @param requested the holder of current requested amount * @param queue the queue holding values to be emitted after completion * @param actual the subscriber to receive the values */ public static void postCompleteDone(AtomicLong requested, Queue queue, Subscriber actual) { postCompleteDone(requested, queue, actual, UtilityFunctions.identity()); } /** * Accumulates requests (validated) and handles the completed mode draining of the queue based on the requests. * *

* Post-completion backpressure handles the case when a source produces values based on * requests when it is active but more values are available even after its completion. * In this case, the onCompleted() can't just emit the contents of the queue but has to * coordinate with the requested amounts. This requires two distinct modes: active and * completed. In active mode, requests flow through and the queue is not accessed but * in completed mode, requests no-longer reach the upstream but help in draining the queue. * * @param the value type to emit * @param requested the holder of current requested amount * @param n the value requested; * @param queue the queue holding values to be emitted after completion * @param actual the subscriber to receive the values * @return true if in the active mode and the request amount of n can be relayed to upstream, false if * in the post-completed mode and the queue is draining. */ public static boolean postCompleteRequest(AtomicLong requested, long n, Queue queue, Subscriber actual) { return postCompleteRequest(requested, n, queue, actual, UtilityFunctions.identity()); } /** * Signals the completion of the main sequence and switches to post-completion replay mode * and allows exit transformation on the queued values. * *

* Don't modify the queue after calling this method! * *

* Post-completion backpressure handles the case when a source produces values based on * requests when it is active but more values are available even after its completion. * In this case, the onCompleted() can't just emit the contents of the queue but has to * coordinate with the requested amounts. This requires two distinct modes: active and * completed. In active mode, requests flow through and the queue is not accessed but * in completed mode, requests no-longer reach the upstream but help in draining the queue. *

* The algorithm utilizes the most significant bit (bit 63) of a long value (AtomicLong) since * request amount only goes up to Long.MAX_VALUE (bits 0-62) and negative values aren't * allowed. * * @param the value type in the queue * @param the value type to emit * @param requested the holder of current requested amount * @param queue the queue holding values to be emitted after completion * @param actual the subscriber to receive the values * @param exitTransform the transformation to apply on the dequeued value to get the value to be emitted */ public static void postCompleteDone(AtomicLong requested, Queue queue, Subscriber actual, Func1 exitTransform) { for (;;) { long r = requested.get(); // switch to completed mode only once if ((r & COMPLETED_MASK) != 0L) { return; } // long u = r | COMPLETED_MASK; if (requested.compareAndSet(r, u)) { // if we successfully switched to post-complete mode and there // are requests available start draining the queue if (r != 0L) { // if the switch happened when there was outstanding requests, start draining postCompleteDrain(requested, queue, actual, exitTransform); } return; } } } /** * Accumulates requests (validated) and handles the completed mode draining of the queue based on the requests * and allows exit transformation on the queued values. * *

* Post-completion backpressure handles the case when a source produces values based on * requests when it is active but more values are available even after its completion. * In this case, the onCompleted() can't just emit the contents of the queue but has to * coordinate with the requested amounts. This requires two distinct modes: active and * completed. In active mode, requests flow through and the queue is not accessed but * in completed mode, requests no-longer reach the upstream but help in draining the queue. * * @param the value type in the queue * @param the value type to emit * @param requested the holder of current requested amount * @param n the value requested; * @param queue the queue holding values to be emitted after completion * @param actual the subscriber to receive the values * @param exitTransform the transformation to apply on the dequeued value to get the value to be emitted * @return true if in the active mode and the request amount of n can be relayed to upstream, false if * in the post-completed mode and the queue is draining. */ public static boolean postCompleteRequest(AtomicLong requested, long n, Queue queue, Subscriber actual, Func1 exitTransform) { if (n < 0L) { throw new IllegalArgumentException("n >= 0 required but it was " + n); } if (n == 0) { return (requested.get() & COMPLETED_MASK) == 0; } for (;;) { long r = requested.get(); // mask of the completed flag long c = r & COMPLETED_MASK; // mask of the requested amount long u = r & REQUESTED_MASK; // add the current requested amount and the new requested amount // cap at Long.MAX_VALUE; long v = addCap(u, n); // restore the completed flag v |= c; if (requested.compareAndSet(r, v)) { // if there was no outstanding request before and in // the post-completed state, start draining if (r == COMPLETED_MASK) { postCompleteDrain(requested, queue, actual, exitTransform); return false; } // returns true for active mode and false if the completed flag was set return c == 0L; } } } /** * Drains the queue based on the outstanding requests in post-completed mode (only!) * and allows exit transformation on the queued values. * * @param the value type in the queue * @param the value type to emit * @param requested the holder of current requested amount * @param queue the queue holding values to be emitted after completion * @param subscriber the subscriber to receive the values * @param exitTransform the transformation to apply on the dequeued value to get the value to be emitted */ static void postCompleteDrain(AtomicLong requested, Queue queue, Subscriber subscriber, Func1 exitTransform) { long r = requested.get(); // Run on a fast-path if the downstream is unbounded if (r == Long.MAX_VALUE) { for (;;) { if (subscriber.isUnsubscribed()) { return; } T v = queue.poll(); if (v == null) { subscriber.onCompleted(); return; } subscriber.onNext(exitTransform.call(v)); } } /* * Since we are supposed to be in the post-complete state, * requested will have its top bit set. * To allow direct comparison, we start with an emission value which has also * this flag set, then increment it as usual. * Since COMPLETED_MASK is essentially Long.MIN_VALUE, * there won't be any overflow or sign flip. */ long e = COMPLETED_MASK; for (;;) { /* * This is an improved queue-drain algorithm with a specialization * in which we know the queue won't change anymore (i.e., done is always true * when looking at the classical algorithm and there is no error). * * Note that we don't check for cancellation or emptiness upfront for two reasons: * 1) if e != r, the loop will do this and we quit appropriately * 2) if e == r, then either there was no outstanding requests or we emitted the requested amount * and the execution simply falls to the e == r check below which checks for emptiness anyway. */ while (e != r) { if (subscriber.isUnsubscribed()) { return; } T v = queue.poll(); if (v == null) { subscriber.onCompleted(); return; } subscriber.onNext(exitTransform.call(v)); e++; } /* * If the emission count reaches the requested amount the same time the queue becomes empty * this will make sure the subscriber is completed immediately instead of on the next request. * This is also true if there are no outstanding requests (this the while loop doesn't run) * and the queue is empty from the start. */ if (e == r) { if (subscriber.isUnsubscribed()) { return; } if (queue.isEmpty()) { subscriber.onCompleted(); return; } } /* * Fast flow: see if more requests have arrived in the meantime. * This avoids an atomic add (~40 cycles) and resumes the emission immediately. */ r = requested.get(); if (r == e) { /* * Atomically decrement the requested amount by the emission amount. * We can't use the full emission value because of the completed flag, * however, due to two's complement representation, the flag on requested * is preserved. */ r = requested.addAndGet(-(e & REQUESTED_MASK)); // The requested amount actually reached zero, quit if (r == COMPLETED_MASK) { return; } // reset the emission count e = COMPLETED_MASK; } } } /** * Atomically subtracts a value from the requested amount unless it's at Long.MAX_VALUE. * @param requested the requested amount holder * @param n the value to subtract from the requested amount, has to be positive (not verified) * @return the new requested amount * @throws IllegalStateException if n is greater than the current requested amount, which * indicates a bug in the request accounting logic */ public static long produced(AtomicLong requested, long n) { for (;;) { long current = requested.get(); if (current == Long.MAX_VALUE) { return Long.MAX_VALUE; } long next = current - n; if (next < 0L) { throw new IllegalStateException("More produced than requested: " + next); } if (requested.compareAndSet(current, next)) { return next; } } } /** * Validates the requested amount and returns true if it is positive. * @param n the requested amount * @return true if n is positive * @throws IllegalArgumentException if n is negative */ public static boolean validate(long n) { if (n < 0) { throw new IllegalArgumentException("n >= 0 required but it was " + n); } return n != 0L; } }





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