Scan/Reduce with Seed Factory

src/main/java/rx/Observable.java

@@ -5429,6 +5429,41 @@ public final Observable<T> reduce(Func2<T, T, T> accumulator) {
     public final <R> Observable<R> reduce(R initialValue, Func2<R, ? super T, R> accumulator) {
         return scan(initialValue, accumulator).takeLast(1);
     }
+
+    /**
+     * Returns an Observable that applies a function of your choosing to the first item emitted by a source
+     * Observable and a specified seed value, then feeds the result of that function along with the second item
+     * emitted by an Observable into the same function, and so on until all items have been emitted by the
+     * source Observable, emitting the final result from the final call to your function as its sole item.
+     * <p>
+     * <img width="640" height="325" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/reduceSeed.png" alt="">
+     * <p>
+     * This technique, which is called "reduce" here, is sometimec called "aggregate," "fold," "accumulate,"
+     * "compress," or "inject" in other programming contexts. Groovy, for instance, has an {@code inject} method
+     * that does a similar operation on lists.
+     * <dl>
+     *  <dt><b>Backpressure Support:</b></dt>
+     *  <dd>This operator does not support backpressure because by intent it will receive all values and reduce
+     *      them to a single {@code onNext}.</dd>
+     *  <dt><b>Scheduler:</b></dt>
+     *  <dd>{@code reduce} does not operate by default on a particular {@link Scheduler}.</dd>
+     * </dl>
+     * 
+     * @param initialValueFactory
+    *            factory to produce the initial (seed) accumulator item each time the Observable is subscribed to
+     * @param accumulator
+     *            an accumulator function to be invoked on each item emitted by the source Observable, the
+     *            result of which will be used in the next accumulator call
+     * @return an Observable that emits a single item that is the result of accumulating the output from the
+     *         items emitted by the source Observable
+     * @see <a href="https://github.com/ReactiveX/RxJava/wiki/Mathematical-and-Aggregate-Operators#reduce">RxJava wiki: reduce</a>
+     * @see <a href="http://msdn.microsoft.com/en-us/library/hh229154.aspx">MSDN: Observable.Aggregate</a>
+     * @see <a href="http://en.wikipedia.org/wiki/Fold_(higher-order_function)">Wikipedia: Fold (higher-order function)</a>
+     */
+    public final <R> Observable<R> reduce(Func0<R> initialValueFactory, Func2<R, ? super T, R> accumulator) {
+        return scan(initialValueFactory, accumulator).takeLast(1);
+    }
+
 
     /**
      * Returns an Observable that repeats the sequence of items emitted by the source Observable indefinitely.
@@ -6491,6 +6526,38 @@ public final Observable<T> scan(Func2<T, T, T> accumulator) {
     public final <R> Observable<R> scan(R initialValue, Func2<R, ? super T, R> accumulator) {
         return lift(new OperatorScan<R, T>(initialValue, accumulator));
     }
+
+    /**
+     * Returns an Observable that applies a function of your choosing to the first item emitted by a source
+     * Observable and a seed value, then feeds the result of that function along with the second item emitted by
+     * the source Observable into the same function, and so on until all items have been emitted by the source
+     * Observable, emitting the result of each of these iterations.
+     * <p>
+     * <img width="640" height="320" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/scanSeed.png" alt="">
+     * <p>
+     * This sort of function is sometimes called an accumulator.
+     * <p>
+     * Note that the Observable that results from this method will emit {@code initialValue} as its first
+     * emitted item.
+     * <dl>
+     *  <dt><b>Scheduler:</b></dt>
+     *  <dd>{@code scan} does not operate by default on a particular {@link Scheduler}.</dd>
+     * </dl>
+     * 
+     * @param initialValueFactory
+     *            factory to produce the initial (seed) accumulator item each time the Observable is subscribed to
+     * @param accumulator
+     *            an accumulator function to be invoked on each item emitted by the source Observable, whose
+     *            result will be emitted to {@link Observer}s via {@link Observer#onNext onNext} and used in the
+     *            next accumulator call
+     * @return an Observable that emits {@code initialValue} followed by the results of each call to the
+     *         accumulator function
+     * @see <a href="https://github.com/ReactiveX/RxJava/wiki/Transforming-Observables#scan">RxJava wiki: scan</a>
+     * @see <a href="http://msdn.microsoft.com/en-us/library/hh211665.aspx">MSDN: Observable.Scan</a>
+     */
+    public final <R> Observable<R> scan(Func0<R> initialValueFactory, Func2<R, ? super T, R> accumulator) {
+        return lift(new OperatorScan<R, T>(initialValueFactory, accumulator));
+    }
 
     /**
      * Forces an Observable's emissions and notifications to be serialized and for it to obey the Rx contract

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

@@ -21,7 +21,9 @@
 import rx.Producer;
 import rx.Subscriber;
 import rx.exceptions.OnErrorThrowable;
+import rx.functions.Func0;
 import rx.functions.Func2;
+import rx.internal.util.UtilityFunctions;
 
 /**
  * Returns an Observable that applies a function to the first item emitted by a source Observable, then feeds
@@ -38,7 +40,7 @@
  */
 public final class OperatorScan<R, T> implements Operator<R, T> {
 
-    private final R initialValue;
+    private final Func0<R> initialValueFactory;
     private final Func2<R, ? super T, R> accumulator;
     // sentinel if we don't receive an initial value
     private static final Object NO_INITIAL_VALUE = new Object();
@@ -54,8 +56,19 @@ public final class OperatorScan<R, T> implements Operator<R, T> {
      * @see <a href="http://msdn.microsoft.com/en-us/library/hh212007.aspx">Observable.Scan(TSource, TAccumulate) Method (IObservable(TSource), TAccumulate, Func(TAccumulate, TSource,
      *      TAccumulate))</a>
      */
-    public OperatorScan(R initialValue, Func2<R, ? super T, R> accumulator) {
-        this.initialValue = initialValue;
+    public OperatorScan(final R initialValue, Func2<R, ? super T, R> accumulator) {
+        this(new Func0<R>() {
+
+            @Override
+            public R call() {
+                return initialValue;
+            }
+
+        }, accumulator);
+    }
+
+    public OperatorScan(Func0<R> initialValueFactory, Func2<R, ? super T, R> accumulator) {
+        this.initialValueFactory = initialValueFactory;
         this.accumulator = accumulator;
     }
 
@@ -75,6 +88,7 @@ public OperatorScan(final Func2<R, ? super T, R> accumulator) {
     @Override
     public Subscriber<? super T> call(final Subscriber<? super R> child) {
         return new Subscriber<T>(child) {
+            private final R initialValue = initialValueFactory.call();
             private R value = initialValue;
             boolean initialized = false;
 

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

@@ -15,7 +15,8 @@
  */
 package rx.internal.operators;
 
-import static org.junit.Assert.*;
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.fail;
 import static org.mockito.Matchers.any;
 import static org.mockito.Matchers.anyInt;
 import static org.mockito.Matchers.anyString;
@@ -24,6 +25,9 @@
 import static org.mockito.Mockito.times;
 import static org.mockito.Mockito.verify;
 
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
 import java.util.concurrent.atomic.AtomicInteger;
 
 import org.junit.Before;
@@ -33,6 +37,7 @@
 import rx.Observable;
 import rx.Observer;
 import rx.Subscriber;
+import rx.functions.Func0;
 import rx.functions.Func1;
 import rx.functions.Func2;
 import rx.observers.TestSubscriber;
@@ -263,4 +268,28 @@ public void onNext(Integer t) {
         // we only expect to receive 101 as we'll receive all 100 + the initial value
         assertEquals(101, count.get());
     }
+
+    @Test
+    public void testSeedFactory() {
+        Observable<List<Integer>> o = Observable.range(1, 10)
+                .scan(new Func0<List<Integer>>() {
+
+                    @Override
+                    public List<Integer> call() {
+                        return new ArrayList<Integer>();
+                    }
+
+                }, new Func2<List<Integer>, Integer, List<Integer>>() {
+
+                    @Override
+                    public List<Integer> call(List<Integer> list, Integer t2) {
+                        list.add(t2);
+                        return list;
+                    }
+
+                }).takeLast(1);
+
+        assertEquals(Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10), o.toBlocking().single());
+        assertEquals(Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10), o.toBlocking().single());
+    }
 }