#590 explanation for Acyclic Visitor

acyclic-visitor/README.md

@@ -9,12 +9,126 @@ tags:
 ---
 
 ## Intent
-Allow new functions to be added to existing class hierarchies without affecting those hierarchies, and without creating the troublesome dependency cycles that are inherent to the GOF VISITOR Pattern.
+
+Allow new functions to be added to existing class hierarchies without affecting those hierarchies, and without creating 
+the troublesome dependency cycles that are inherent to the GoF Visitor Pattern.
+
+## Explanation
+
+Real world example
+
+> We have a hierarchy of modem classes. The modems in this hierarchy need to be visited by an external algorithm based 
+> on filtering criteria (is it Unix or DOS compatible modem). 
+
+In plain words
+
+> Acyclic Visitor allows functions to be added to existing class hierarchies without modifying the hierarchies.
+
+[WikiWikiWeb](https://wiki.c2.com/?AcyclicVisitor) says
+
+> The Acyclic Visitor pattern allows new functions to be added to existing class hierarchies without affecting those 
+> hierarchies, and without creating the dependency cycles that are inherent to the GangOfFour VisitorPattern.
+
+**Programmatic Example**
+
+Here's the `Modem` hierarchy.
+
+```java
+public abstract class Modem {
+  public abstract void accept(ModemVisitor modemVisitor);
+}
+
+public class Zoom extends Modem {
+  ...
+  @Override
+  public void accept(ModemVisitor modemVisitor) {
+    if (modemVisitor instanceof ZoomVisitor) {
+      ((ZoomVisitor) modemVisitor).visit(this);
+    } else {
+      LOGGER.info("Only ZoomVisitor is allowed to visit Zoom modem");
+    }
+  }
+}
+
+public class Hayes extends Modem {
+  ...
+  @Override
+  public void accept(ModemVisitor modemVisitor) {
+    if (modemVisitor instanceof HayesVisitor) {
+      ((HayesVisitor) modemVisitor).visit(this);
+    } else {
+      LOGGER.info("Only HayesVisitor is allowed to visit Hayes modem");
+    }
+  }
+}
+```
+
+Next we introduce the `ModemVisitor` hierarchy.
+
+```java
+public interface ModemVisitor {
+}
+
+public interface HayesVisitor extends ModemVisitor {
+  void visit(Hayes hayes);
+}
+
+public interface ZoomVisitor extends ModemVisitor {
+  void visit(Zoom zoom);
+}
+
+public interface AllModemVisitor extends ZoomVisitor, HayesVisitor {
+}
+
+public class ConfigureForDosVisitor implements AllModemVisitor {
+  ...
+  @Override
+  public void visit(Hayes hayes) {
+    LOGGER.info(hayes + " used with Dos configurator.");
+  }
+  @Override
+  public void visit(Zoom zoom) {
+    LOGGER.info(zoom + " used with Dos configurator.");
+  }
+}
+
+public class ConfigureForUnixVisitor implements ZoomVisitor {
+  ...
+  @Override
+  public void visit(Zoom zoom) {
+    LOGGER.info(zoom + " used with Unix configurator.");
+  }
+}
+```
+
+Finally here are the visitors in action.
+
+```java
+    var conUnix = new ConfigureForUnixVisitor();
+    var conDos = new ConfigureForDosVisitor();
+    var zoom = new Zoom();
+    var hayes = new Hayes();
+    hayes.accept(conDos);
+    zoom.accept(conDos);
+    hayes.accept(conUnix);
+    zoom.accept(conUnix);   
+```
+
+Program output:
+
+```
+    // Hayes modem used with Dos configurator.
+    // Zoom modem used with Dos configurator.
+    // Only HayesVisitor is allowed to visit Hayes modem
+    // Zoom modem used with Unix configurator.
+```
 
 ## Class diagram
+
 ![alt text](./etc/acyclic-visitor.png "Acyclic Visitor")
 
 ## Applicability
+
 This pattern can be used:
 
 * When you need to add a new function to an existing hierarchy without the need to alter or affect that hierarchy.
@@ -24,6 +138,7 @@ This pattern can be used:
 * When the recompilation, relinking, retesting or redistribution of the derivatives of Element is very expensive.
 
 ## Consequences
+
 The good:
 
 * No dependency cycles between class hierarchies.
@@ -32,11 +147,14 @@ The good:
 
 The bad:
 
-* Violates the principle of least surprise or Liskov's Substitution principle by showing that it can accept all visitors but actually only being interested in particular visitors.
+* Violates [Liskov's Substitution Principle](https://java-design-patterns.com/principles/#liskov-substitution-principle) by showing that it can accept all visitors but actually only being interested in particular visitors.
 * Parallel hierarchy of visitors has to be created for all members in visitable class hierarchy.
 
 ## Related patterns
-* [Visitor Pattern](../visitor/)
+
+* [Visitor Pattern](https://java-design-patterns.com/patterns/visitor/)
 
 ## Credits
-* [Acyclic Visitor](http://condor.depaul.edu/dmumaugh/OOT/Design-Principles/acv.pdf)
+
+* [Acyclic Visitor by Robert C. Martin](http://condor.depaul.edu/dmumaugh/OOT/Design-Principles/acv.pdf)
+* [Acyclic Visitor in WikiWikiWeb](https://wiki.c2.com/?AcyclicVisitor)