Ricorsione doppia esempio serie di fibonacci

functional-programming-java/src/main/java/org/gym/fp/fpjava/demo/RecursionDemo.java

@@ -1,14 +1,20 @@
 package org.gym.fp.fpjava.demo;
 
+import lombok.AccessLevel;
+import lombok.NoArgsConstructor;
 import org.gym.fp.fpjava.type.TailCall;
 
+import java.math.BigInteger;
+
 import static org.gym.fp.fpjava.type.TailCall.ret;
 import static org.gym.fp.fpjava.type.TailCall.suspend;
 
+@NoArgsConstructor(access = AccessLevel.PRIVATE)
 public class RecursionDemo {
 
     public static void run() {
         System.out.println(add(10, 1000));
+        System.out.print(fibonacci(10000));
     }
 
     public static int add(int x, int y) {
@@ -20,4 +26,20 @@ public class RecursionDemo {
                 ? ret(x)
                 : suspend(() -> addRec(x + 1, y - 1));
     }
+
+    // ES. 4.1, 4.2
+    public static BigInteger fibonacci(int number) {
+        class Aux {
+            public TailCall<BigInteger> fib(BigInteger acc1, BigInteger acc2, BigInteger x) {
+                if (x.equals(BigInteger.ZERO)) {
+                    return ret(BigInteger.ZERO);
+                } else if (x.equals(BigInteger.ONE)) {
+                    return ret(acc1.add(acc2));
+                } else {
+                    return suspend(() -> fib(acc2, acc1.add(acc2), x.subtract(BigInteger.ONE)));
+                }
+            }
+        }
+        return (new Aux()).fib(BigInteger.ONE, BigInteger.ZERO, BigInteger.valueOf(number)).eval();
+    }
 }