move code to src/test

* so that it compiles and tests pass
* fix some additional snip references in getting started

akka-docs/src/test/scala/docs/stream/cookbook/RecipeReduceByKey.scala

@@ -0,0 +1,76 @@
+package docs.stream.cookbook
+
+import akka.NotUsed
+import akka.stream.{ Graph, FlowShape, Inlet, Outlet, Attributes, OverflowStrategy }
+import akka.stream.scaladsl._
+import scala.concurrent.{ Await, Future }
+import scala.concurrent.duration._
+import akka.stream.stage.{ GraphStage, GraphStageLogic }
+
+class RecipeReduceByKey extends RecipeSpec {
+
+  "Reduce by key recipe" must {
+
+    val MaximumDistinctWords = 1000
+
+    "work with simple word count" in {
+
+      def words = Source(List("hello", "world", "and", "hello", "universe", "akka") ++ List.fill(1000)("rocks!"))
+
+      //#word-count
+      val counts: Source[(String, Int), NotUsed] = words
+        // split the words into separate streams first
+        .groupBy(MaximumDistinctWords, identity)
+        //transform each element to pair with number of words in it
+        .map(_ -> 1)
+        // add counting logic to the streams
+        .reduce((l, r) => (l._1, l._2 + r._2))
+        // get a stream of word counts
+        .mergeSubstreams
+      //#word-count
+
+      Await.result(counts.limit(10).runWith(Sink.seq), 3.seconds).toSet should be(Set(
+        ("hello", 2),
+        ("world", 1),
+        ("and", 1),
+        ("universe", 1),
+        ("akka", 1),
+        ("rocks!", 1000)))
+    }
+
+    "work generalized" in {
+
+      def words = Source(List("hello", "world", "and", "hello", "universe", "akka") ++ List.fill(1000)("rocks!"))
+
+      //#reduce-by-key-general
+      def reduceByKey[In, K, Out](
+        maximumGroupSize: Int,
+        groupKey:         (In) => K,
+        map:              (In) => Out)(reduce: (Out, Out) => Out): Flow[In, (K, Out), NotUsed] = {
+
+        Flow[In]
+          .groupBy[K](maximumGroupSize, groupKey)
+          .map(e => groupKey(e) -> map(e))
+          .reduce((l, r) => l._1 -> reduce(l._2, r._2))
+          .mergeSubstreams
+      }
+
+      val wordCounts = words.via(
+        reduceByKey(
+          MaximumDistinctWords,
+          groupKey = (word: String) => word,
+          map = (word: String) => 1)((left: Int, right: Int) => left + right))
+      //#reduce-by-key-general
+
+      Await.result(wordCounts.limit(10).runWith(Sink.seq), 3.seconds).toSet should be(Set(
+        ("hello", 2),
+        ("world", 1),
+        ("and", 1),
+        ("universe", 1),
+        ("akka", 1),
+        ("rocks!", 1000)))
+
+    }
+  }
+
+}