Source.unfold examples (#28081)

akka-docs/src/test/java/jdocs/stream/operators/source/Unfold.java

@@ -0,0 +1,40 @@
+/*
+ * Copyright (C) 2009-2019 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package jdocs.stream.operators.source;
+
+import akka.NotUsed;
+import akka.japi.Pair;
+import akka.stream.javadsl.Source;
+
+import java.math.BigInteger;
+import java.util.Optional;
+
+interface Unfold {
+
+  // #countdown
+  public static Source<Integer, NotUsed> countDown(Integer from) {
+    return Source.unfold(
+        from,
+        current -> {
+          if (current == 0) return Optional.empty();
+          else return Optional.of(Pair.create(current - 1, current));
+        });
+  }
+  // #countdown
+
+  // #fibonacci
+  public static Source<BigInteger, NotUsed> fibonacci() {
+    return Source.unfold(
+        Pair.create(BigInteger.ZERO, BigInteger.ONE),
+        current -> {
+          BigInteger a = current.first();
+          BigInteger b = current.second();
+          Pair<BigInteger, BigInteger> next = Pair.create(b, a.add(b));
+          return Optional.of(Pair.create(next, a));
+        });
+  }
+  // #fibonacci
+
+}