Merge pull request 'fix: add SubtractFunction (#3)' (#4) from ai/issue-3-subtract-function into main

Reviewed-on: #4

src/main/java/com/example/LoopTestApplication.java

@@ -2,6 +2,7 @@ package com.example;
 
 import com.example.util.AddFunction;
 import com.example.util.CalcFunction;
+import com.example.util.SubtractFunction;
 
 public class LoopTestApplication {
 
@@ -9,5 +10,7 @@ public class LoopTestApplication {
         System.out.println("============== Calculator =============");
         CalcFunction add = new AddFunction();
         System.out.println("ADDITION: " + add.calc(2, 5, 6));
+        CalcFunction subtract = new SubtractFunction();
+        System.out.println("SUBTRACTION: " + subtract.calc(2, 5, 6));
     }
 }

src/main/java/com/example/util/SubtractFunction.java

@@ -0,0 +1,15 @@
+package com.example.util;
+
+public class SubtractFunction implements CalcFunction {
+
+    public double calc(double... input) {
+        if (input.length == 0) {
+            return 0;
+        }
+        double result = input[0];
+        for (int i = 1; i < input.length; i++) {
+            result -= input[i];
+        }
+        return result;
+    }
+}

src/test/java/com/example/util/SubtractFunctionTest.java

@@ -0,0 +1,30 @@
+package com.example.util;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+class SubtractFunctionTest {
+
+    private final SubtractFunction subtractFunction = new SubtractFunction();
+
+    @Test
+    void subtractsMultipleValues() {
+        assertEquals(1.0, subtractFunction.calc(6.0, 2.0, 3.0));
+    }
+
+    @Test
+    void returnsSingleValue() {
+        assertEquals(5.0, subtractFunction.calc(5.0));
+    }
+
+    @Test
+    void returnsZeroForNoInput() {
+        assertEquals(0.0, subtractFunction.calc());
+    }
+
+    @Test
+    void subtractsNegativeValues() {
+        assertEquals(3.0, subtractFunction.calc(1.0, -2.0));
+    }
+}