1.x: request rebatch operator (#3971)

* 1.x: request rebatch operator

* Add argument test

* Fix javadoc, add experimental

Author: akarnokd

src/main/java/rx/Observable.java

@@ -6698,6 +6698,32 @@ public final <R> Observable<R> publish(Func1<? super Observable<T>, ? extends Ob
         return OperatorPublish.create(this, selector);
     }
 
+    /**
+     * Requests {@code n} initially from the upstream and then 75% of {@code n} subsequently
+     * after 75% of {@code n} values have been emitted to the downstream.
+     * 
+     * <p>This operator allows preventing the downstream to trigger unbounded mode via {@code request(Long.MAX_VALUE)}
+     * or compensate for the per-item overhead of small and frequent requests.
+     * 
+     * <dl>
+     *  <dt><b>Backpressure:</b></dt>
+     *  <dd>The operator expects backpressure from upstream and honors backpressure from downstream.</dd>
+     *  <dt><b>Scheduler:</b></dt>
+     *  <dd>{@code rebatchRequests} does not operate by default on a particular {@link Scheduler}.</dd>
+     * </dl>
+     *  
+     * @param n the initial request amount, further request will happen after 75% of this value
+     * @return the Observable that rebatches request amounts from downstream
+     * @since (if this graduates from Experimental/Beta to supported, replace this parenthetical with the release number)
+     */
+    @Experimental
+    public final Observable<T> rebatchRequests(int n) {
+        if (n <= 0) {
+            throw new IllegalArgumentException("n > 0 required but it was " + n);
+        }
+        return lift(OperatorObserveOn.<T>rebatch(n));
+    }
+    
     /**
      * Returns an Observable that applies a specified accumulator function to the first item emitted by a source
      * Observable, then feeds the result of that function along with the second item emitted by the source

src/main/java/rx/internal/operators/OperatorObserveOn.java

@@ -27,6 +27,7 @@
 import rx.internal.util.atomic.SpscAtomicArrayQueue;
 import rx.internal.util.unsafe.*;
 import rx.plugins.RxJavaPlugins;
+import rx.schedulers.Schedulers;
 
 /**
  * Delivers events on the specified {@code Scheduler} asynchronously via an unbounded buffer.
@@ -75,6 +76,17 @@ public Subscriber<? super T> call(Subscriber<? super T> child) {
             return parent;
         }
     }
+    
+    public static <T> Operator<T, T> rebatch(final int n) {
+        return new Operator<T, T>() {
+            @Override
+            public Subscriber<? super T> call(Subscriber<? super T> child) {
+                ObserveOnSubscriber<T> parent = new ObserveOnSubscriber<T>(Schedulers.immediate(), child, false, n);
+                parent.init();
+                return parent;
+            }
+        };
+    }
 
     /** Observe through individual queue per observer. */
     private static final class ObserveOnSubscriber<T> extends Subscriber<T> implements Action0 {

src/test/java/rx/internal/operators/OperatorObserveOnTest.java

@@ -933,4 +933,37 @@ public void bufferSizesWork() {
             ts.assertNoErrors();
         }
     }
+    
+    @Test
+    public void synchronousRebatching() {
+        final List<Long> requests = new ArrayList<Long>();
+        
+        TestSubscriber<Integer> ts = new TestSubscriber<Integer>();
+            
+        Observable.range(1, 50)
+        .doOnRequest(new Action1<Long>() {
+            @Override
+            public void call(Long r) {
+                requests.add(r);
+            }
+        })
+       .rebatchRequests(20)
+       .subscribe(ts);
+       
+       ts.assertValueCount(50);
+       ts.assertNoErrors();
+       ts.assertCompleted();
+       
+       assertEquals(Arrays.asList(20L, 15L, 15L, 15L), requests);
+    }
+    
+    @Test
+    public void rebatchRequestsArgumentCheck() {
+        try {
+            Observable.never().rebatchRequests(-99);
+            fail("Didn't throw IAE");
+        } catch (IllegalArgumentException ex) {
+            assertEquals("n > 0 required but it was -99", ex.getMessage());
+        }
+    }
 }