#590 explanation for Trampoline

trampoline/README.md

@@ -9,14 +9,111 @@ tags:
 ---
 
 ## Intent
-Trampoline pattern is used for implementing algorithms recursively in Java without blowing the stack 
+
-and to interleave the execution of functions without hard coding them together
+Trampoline pattern is used for implementing algorithms recursively in Java without blowing the stack and to interleave 
-It is possible by representing a computation in one of 2 states : done | more 
+the execution of functions without hard coding them together.
-(completed with result, or a reference to the reminder of the computation, 
-something like the way a java.util.Supplier does).
 
 ## Explanation
-Trampoline pattern allows to define recursive algorithms by iterative loop.
+
+Recursion is a frequently adopted technique for solving algorithmic problems in a divide and conquer
+style. For example calculating fibonacci accumulating sum and factorials. In these kinds of problems recursion is 
+more straightforward than their loop counterpart. Furthermore recursion may need less code and looks more concise. 
+There is a saying that every recursion problem can be solved using a loop with the cost of writing code that is more
+difficult to understand.
+
+However recursion type solutions have one big caveat. For each recursive call it typically needs an intermediate value 
+stored and there is a limited amount of stack memory available. Running out of stack memory creates a stack overflow
+error and halts the program execution.
+
+Trampoline pattern is a trick that allows us define recursive algorithms in Java without blowing the stack. 
+
+Real world example
+
+> A recursive Fibonacci calculation without the stack overflow problem using the Trampoline pattern.       
+
+In plain words
+
+> Trampoline pattern allows recursion without running out of stack memory. 
+
+Wikipedia says
+
+> In Java, trampoline refers to using reflection to avoid using inner classes, for example in event listeners. The time overhead of a reflection call is traded for the space overhead of an inner class. Trampolines in Java usually involve the creation of a GenericListener to pass events to an outer class.
+
+**Programmatic Example**
+
+Here's the `Trampoline` implementation in Java.
+
+When `get` is called on the returned Trampoline, internally it will iterate calling `jump` on the returned `Trampoline` 
+as long as the concrete instance returned is `Trampoline`, stopping once the returned instance is `done`.
+
+```java
+public interface Trampoline<T> {
+
+  T get();
+
+  default Trampoline<T> jump() {
+    return this;
+  }
+
+  default T result() {
+    return get();
+  }
+
+  default boolean complete() {
+    return true;
+  }
+
+  static <T> Trampoline<T> done(final T result) {
+    return () -> result;
+  }
+
+  static <T> Trampoline<T> more(final Trampoline<Trampoline<T>> trampoline) {
+    return new Trampoline<T>() {
+      @Override
+      public boolean complete() {
+        return false;
+      }
+
+      @Override
+      public Trampoline<T> jump() {
+        return trampoline.result();
+      }
+
+      @Override
+      public T get() {
+        return trampoline(this);
+      }
+
+      T trampoline(final Trampoline<T> trampoline) {
+        return Stream.iterate(trampoline, Trampoline::jump)
+            .filter(Trampoline::complete)
+            .findFirst()
+            .map(Trampoline::result)
+            .orElseThrow();
+      }
+    };
+  }
+}
+```
+
+Using the `Trampoline` to get Fibonacci values.
+
+```java
+  public static Trampoline<Integer> loop(int times, int prod) {
+    if (times == 0) {
+      return Trampoline.done(prod);
+    } else {
+      return Trampoline.more(() -> loop(times - 1, prod * times));
+    }
+  }
+  
+  log.info("start pattern");
+  var result = loop(10, 1).result();
+  log.info("result {}", result);
+  
+  // start pattern
+  // result 3628800
+```
 
 ## Class diagram
 ![alt text](./etc/trampoline.urm.png "Trampoline pattern class diagram")
@@ -27,18 +124,16 @@ Use the Trampoline pattern when
 * For implementing tail recursive function. This pattern allows to switch on a stackless operation.
 * For interleaving the execution of two or more functions on the same thread.
 
-## Known uses(real world examples)
+## Known uses
 
-* Trampoline refers to using reflection to avoid using inner classes, for example in event listeners. 
+* [cyclops-react](https://github.com/aol/cyclops-react)
-The time overhead of a reflection call is traded for the space overhead of an inner class. 
-Trampolines in Java usually involve the creation of a GenericListener to pass events to an outer class.
-
-
-## Tutorials 
-
-* [Trampolining: a practical guide for awesome Java Developers](https://medium.com/@johnmcclean/trampolining-a-practical-guide-for-awesome-java-developers-4b657d9c3076)
-* [Trampoline in java ](http://mindprod.com/jgloss/trampoline.html)
 
 ## Credits
 
-* [library 'cyclops-react' uses the pattern](https://github.com/aol/cyclops-react)
+* [Trampolining: a practical guide for awesome Java Developers](https://medium.com/@johnmcclean/trampolining-a-practical-guide-for-awesome-java-developers-4b657d9c3076)
+* [Trampoline in java ](http://mindprod.com/jgloss/trampoline.html)
+* [Laziness, trampolines, monoids and other functional amenities: this is not your father's Java](https://www.slideshare.net/mariofusco/lazine)
+* [Trampoline implementation](https://github.com/bodar/totallylazy/blob/master/src/com/googlecode/totallylazy/Trampoline.java)
+* [What is a trampoline function?](https://stackoverflow.com/questions/189725/what-is-a-trampoline-function)
+* [Modern Java in Action: Lambdas, streams, functional and reactive programming](https://www.amazon.com/gp/product/1617293563/ref=as_li_qf_asin_il_tl?ie=UTF8&tag=javadesignpat-20&creative=9325&linkCode=as2&creativeASIN=1617293563&linkId=ad53ae6f9f7c0982e759c3527bd2595c)
+* [Java 8 in Action: Lambdas, Streams, and functional-style programming](https://www.amazon.com/gp/product/1617291994/ref=as_li_qf_asin_il_tl?ie=UTF8&tag=javadesignpat-20&creative=9325&linkCode=as2&creativeASIN=1617291994&linkId=e3e5665b0732c59c9d884896ffe54f4f)