Completed CalcSkin

src/main/java/rocks/zipcode/calcskin/CalcEngine.java

     }
 
     public double add(double v, double v1) {
-        return Double.NaN;
+        return (v + v1);
     }
 
     public double subtract(double v, double v1) {
-        return Double.NaN;
+        return (v - v1);
     }
 
     public double multiply(double v, double v1) {
-        return Double.NaN;
+        return (v * v1);
     }
 
     public double divide(double v, double v1) {
-        return Double.NaN;
+        return (v / v1);
     }
+
+    public double sqrt(double v) {
+        return Math.sqrt(v);
+    }
+
+    public double average(double v, double v1) {
+        return (v + v1) /2;
+    }
+
+    public double exponent(double v) {
+        return Math.exp(v);
+    }
+
+    public static double log(double V) {
+       return Math.pow(10, V);
+    }
+
 }

src/main/java/rocks/zipcode/calcskin/CalcSkin.java

 import javafx.scene.layout.*;
 import javafx.stage.Stage;
 import javafx.stage.StageStyle;
-import rocks.zipcode.calcskin.CalcEngine;
 
 import java.util.HashMap;
 import java.util.Map;
         launch(args);
     }
     private static final String[][] template = {
+            {"sqrt","exp","log","c"},
             { "7", "8", "9", "/" },
             { "4", "5", "6", "*" },
             { "1", "2", "3", "-" },
-            { "0", "c", "=", "+" }
+            {"0","avg", "=", "+" }
+
     };
 
     private final Map<String, Button> accelerators = new HashMap<>();
     private DoubleProperty currentValue = new SimpleDoubleProperty();
     private CalcEngine calcEngine = new CalcEngine();
 
-    private enum Op { NOOP, ADD, SUBTRACT, MULTIPLY, DIVIDE }
+    private enum Op { NOOP, ADD, SUBTRACT, MULTIPLY, DIVIDE, ROOT, AVERAGE, EXPONENT, LOG}
 
     private Op curOp   = Op.NOOP;
     private Op stackOp = Op.NOOP;
         final TextField screen  = createScreen();
         final TilePane  buttons = createButtons();
 
-        stage.setTitle("Calc");
+        stage.setTitle("<> Calculator <>");
         stage.initStyle(StageStyle.UTILITY);
         stage.setResizable(false);
         stage.setScene(new Scene(createLayout(screen, buttons)));
     }
 
     private VBox createLayout(TextField screen, TilePane buttons) {
-        final VBox layout = new VBox(20);
+        final VBox layout = new VBox(30);
         layout.setAlignment(Pos.CENTER);
         layout.setStyle("-fx-background-color: silver; -fx-padding: 20; -fx-font-size: 20;");
         layout.getChildren().setAll(screen, buttons);
     private TextField createScreen() {
         final TextField screen = new TextField();
         screen.setStyle("-fx-background-color: aquamarine;");
-        screen.setAlignment(Pos.CENTER_RIGHT);
+        screen.setAlignment(Pos.CENTER_LEFT);
         screen.setEditable(false);
         screen.textProperty().bind(Bindings.format("%.0f", currentValue));
         return screen;
             case "-": triggerOp.set(Op.SUBTRACT); break;
             case "*": triggerOp.set(Op.MULTIPLY); break;
             case "/": triggerOp.set(Op.DIVIDE);   break;
+            case "Sqrt": triggerOp.set(Op.ROOT); break;
+            case "log": triggerOp.set(Op.LOG); break;
+            case "exp": triggerOp.set(Op.EXPONENT); break;
+
         }
         return triggerOp;
     }
                     case SUBTRACT: currentValue.set(calcEngine.subtract(previousValue.get(), currentValue.get())); break;
                     case MULTIPLY: currentValue.set(calcEngine.multiply(previousValue.get(), currentValue.get())); break;
                     case DIVIDE:   currentValue.set(calcEngine.divide(previousValue.get(), currentValue.get())); break;
+                    case ROOT:   currentValue.set(calcEngine.sqrt(previousValue.get())); break;
+                    case AVERAGE:  currentValue.set(calcEngine.average(previousValue.get(), currentValue.get())); break;
+                    case EXPONENT:   currentValue.set(calcEngine.exponent(previousValue.get())); break;
+                    case LOG:   currentValue.set(calcEngine.log(previousValue.get())); break;
                 }
             }
         });

src/test/java/rocks/zipcode/calcskin/CalcEngineTest.java

     public void divide() {
         Assert.assertTrue("", (testCalc.divide(10.0, 2.0) == 5.0));
     }
+
+    @Test
+    public void sqrt() {
+        Assert.assertTrue("", (testCalc.sqrt(25.0 ) == 5.0));
+    }
+
+    @Test
+    public void average() {
+        Assert.assertTrue("", ((testCalc.average(25.0, 5.0)/2) != 5.0));
+    }
+
+    @Test
+    public void exponent() {
+        Assert.assertTrue("", (testCalc.sqrt(25.0 ) == 5.0));
+    }
+
+    @Test
+    public void log() {
+        Assert.assertTrue("", (testCalc.sqrt(25.0 ) == 5.0));
+    }
+
 }