From 69347251795c6971e2a8d0987c4b3511e6ef98d1 Mon Sep 17 00:00:00 2001
From: Fabio Scotto di Santolo <fabio.scottodisantolo@nttdata.com>
Date: Fri, 24 Jan 2020 15:38:05 +0100
Subject: [PATCH] Tail recursion abstraction

---
 .../src/main/java/org/gym/fp/fpjava/Main.java |  2 +
 .../org/gym/fp/fpjava/demo/RecursionDemo.java | 23 ++++++
 .../java/org/gym/fp/fpjava/type/TailCall.java | 71 +++++++++++++++++++
 3 files changed, 96 insertions(+)
 create mode 100644 functional-programming-java/src/main/java/org/gym/fp/fpjava/demo/RecursionDemo.java
 create mode 100644 functional-programming-java/src/main/java/org/gym/fp/fpjava/type/TailCall.java

diff --git a/functional-programming-java/src/main/java/org/gym/fp/fpjava/Main.java b/functional-programming-java/src/main/java/org/gym/fp/fpjava/Main.java
index 9ffe3b1..1f1f941 100644
--- a/functional-programming-java/src/main/java/org/gym/fp/fpjava/Main.java
+++ b/functional-programming-java/src/main/java/org/gym/fp/fpjava/Main.java
@@ -2,12 +2,14 @@ package org.gym.fp.fpjava;
 
 import org.gym.fp.fpjava.demo.AbstractControlStructureDemo;
 import org.gym.fp.fpjava.demo.FunctionDemo;
+import org.gym.fp.fpjava.demo.RecursionDemo;
 
 public class Main {
 
     public static void main(String[] args) {
         FunctionDemo.run();
         AbstractControlStructureDemo.run();
+        RecursionDemo.run();
     }
 
 }
diff --git a/functional-programming-java/src/main/java/org/gym/fp/fpjava/demo/RecursionDemo.java b/functional-programming-java/src/main/java/org/gym/fp/fpjava/demo/RecursionDemo.java
new file mode 100644
index 0000000..9cf9027
--- /dev/null
+++ b/functional-programming-java/src/main/java/org/gym/fp/fpjava/demo/RecursionDemo.java
@@ -0,0 +1,23 @@
+package org.gym.fp.fpjava.demo;
+
+import org.gym.fp.fpjava.type.TailCall;
+
+import static org.gym.fp.fpjava.type.TailCall.ret;
+import static org.gym.fp.fpjava.type.TailCall.suspend;
+
+public class RecursionDemo {
+
+    public static void run() {
+        System.out.println(add(10, 1000));
+    }
+
+    public static int add(int x, int y) {
+        return addRec(x, y).eval();
+    }
+
+    private static TailCall<Integer> addRec(int x, int y) {
+        return y == 0
+                ? ret(x)
+                : suspend(() -> addRec(x + 1, y - 1));
+    }
+}
diff --git a/functional-programming-java/src/main/java/org/gym/fp/fpjava/type/TailCall.java b/functional-programming-java/src/main/java/org/gym/fp/fpjava/type/TailCall.java
new file mode 100644
index 0000000..a84bb2e
--- /dev/null
+++ b/functional-programming-java/src/main/java/org/gym/fp/fpjava/type/TailCall.java
@@ -0,0 +1,71 @@
+package org.gym.fp.fpjava.type;
+
+public abstract class TailCall<T> {
+
+    private TailCall() {
+    }
+
+    public static <T> TailCall<T> ret(T t) {
+        return new Return<>(t);
+    }
+
+    public static <T> TailCall<T> suspend(Supplier<TailCall<T>> s) {
+        return new Suspend<>(s);
+    }
+
+    public abstract TailCall<T> resume();
+
+    public abstract T eval();
+
+    public abstract boolean isSuspend();
+
+    private static class Return<T> extends TailCall<T> {
+        private final T t;
+
+        public Return(T t) {
+            this.t = t;
+        }
+
+        @Override
+        public TailCall<T> resume() {
+            throw new IllegalStateException("Return has no resume");
+        }
+
+        @Override
+        public T eval() {
+            return t;
+        }
+
+        @Override
+        public boolean isSuspend() {
+            return false;
+        }
+    }
+
+    private static class Suspend<T> extends TailCall<T> {
+        private final Supplier<TailCall<T>> resume;
+
+        public Suspend(Supplier<TailCall<T>> resume) {
+            this.resume = resume;
+        }
+
+        @Override
+        public TailCall<T> resume() {
+            return resume.get();
+        }
+
+        @Override
+        public T eval() {
+            TailCall<T> tailRec = this;
+            while (tailRec.isSuspend()) {
+                tailRec = tailRec.resume();
+            }
+            return tailRec.eval();
+        }
+
+        @Override
+        public boolean isSuspend() {
+            return true;
+        }
+    }
+}