+doc #19429 initial merge of docs-dev and docs

akka-docs/rst/scala/code/docs/stream/cookbook/RecipeReduceByKey.scala

@@ -0,0 +1,81 @@
+package docs.stream.cookbook
+
+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), Unit] = words
+        // split the words into separate streams first
+        .groupBy(MaximumDistinctWords, identity)
+        // add counting logic to the streams
+        .fold(("", 0)) {
+          case ((_, count), word) => (word, count + 1)
+        }
+        // get a stream of word counts
+        .mergeSubstreams
+      //#word-count
+
+      Await.result(counts.grouped(10).runWith(Sink.head), 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,
+        foldZero: (K) => Out)(fold: (Out, In) => Out): Flow[In, (K, Out), Unit] = {
+
+        Flow[In]
+          .groupBy(maximumGroupSize, groupKey)
+          .fold(Option.empty[(K, Out)]) {
+            case (None, elem) =>
+              val key = groupKey(elem)
+              Some((key, fold(foldZero(key), elem)))
+            case (Some((key, out)), elem) =>
+              Some((key, fold(out, elem)))
+          }
+          .map(_.get)
+          .mergeSubstreams
+      }
+
+      val wordCounts = words.via(reduceByKey(
+        MaximumDistinctWords,
+        groupKey = (word: String) => word,
+        foldZero = (key: String) => 0)(fold = (count: Int, elem: String) => count + 1))
+
+      //#reduce-by-key-general
+
+      Await.result(wordCounts.grouped(10).runWith(Sink.head), 3.seconds).toSet should be(Set(
+        ("hello", 2),
+        ("world", 1),
+        ("and", 1),
+        ("universe", 1),
+        ("akka", 1),
+        ("rocks!", 1000)))
+
+    }
+  }
+
+}