Enhance docs, add tests in FibonacciHeap (#5979)

src/test/java/com/thealgorithms/datastructures/heaps/FibonacciHeapTest.java

@@ -6,7 +6,7 @@ import org.junit.jupiter.api.Test;
 public class FibonacciHeapTest {
 
     @Test
-    void testHeap() {
+    void testHeapInsertionAndMinimum() {
         FibonacciHeap fibonacciHeap = new FibonacciHeap();
         fibonacciHeap.insert(5);
         fibonacciHeap.insert(3);
@@ -14,8 +14,95 @@ public class FibonacciHeapTest {
         fibonacciHeap.insert(18);
         fibonacciHeap.insert(33);
 
-        Assertions.assertEquals(fibonacciHeap.findMin().getKey(), 1);
+        Assertions.assertEquals(1, fibonacciHeap.findMin().getKey());
         fibonacciHeap.deleteMin();
-        Assertions.assertEquals(fibonacciHeap.findMin().getKey(), 3);
+        Assertions.assertEquals(3, fibonacciHeap.findMin().getKey());
+    }
+
+    @Test
+    void testDeleteMinOnSingleElementHeap() {
+        FibonacciHeap fibonacciHeap = new FibonacciHeap(10);
+        Assertions.assertEquals(10, fibonacciHeap.findMin().getKey());
+        fibonacciHeap.deleteMin();
+        Assertions.assertTrue(fibonacciHeap.empty());
+    }
+
+    @Test
+    void testHeapMeld() {
+        FibonacciHeap heap1 = new FibonacciHeap();
+        FibonacciHeap heap2 = new FibonacciHeap();
+        heap1.insert(1);
+        heap1.insert(2);
+        heap2.insert(3);
+        heap2.insert(4);
+
+        heap1.meld(heap2);
+        Assertions.assertEquals(1, heap1.findMin().getKey());
+    }
+
+    @Test
+    void testHeapSize() {
+        FibonacciHeap fibonacciHeap = new FibonacciHeap();
+        Assertions.assertEquals(0, fibonacciHeap.size());
+        fibonacciHeap.insert(5);
+        Assertions.assertEquals(1, fibonacciHeap.size());
+        fibonacciHeap.insert(3);
+        Assertions.assertEquals(2, fibonacciHeap.size());
+        fibonacciHeap.deleteMin();
+        Assertions.assertEquals(1, fibonacciHeap.size());
+    }
+
+    @Test
+    void testCountersRep() {
+        FibonacciHeap fibonacciHeap = new FibonacciHeap();
+        fibonacciHeap.insert(5);
+        fibonacciHeap.insert(3);
+        fibonacciHeap.insert(8);
+        fibonacciHeap.insert(1);
+
+        int[] counters = fibonacciHeap.countersRep();
+        Assertions.assertEquals(4, counters[0]);
+        Assertions.assertEquals(0, counters[1]);
+    }
+
+    @Test
+    void testDeleteMinMultipleElements() {
+        FibonacciHeap fibonacciHeap = new FibonacciHeap();
+        fibonacciHeap.insert(5);
+        fibonacciHeap.insert(2);
+        fibonacciHeap.insert(8);
+        fibonacciHeap.insert(1);
+
+        Assertions.assertEquals(1, fibonacciHeap.findMin().getKey());
+        fibonacciHeap.deleteMin();
+        Assertions.assertEquals(2, fibonacciHeap.findMin().getKey());
+    }
+
+    @Test
+    void testInsertNegativeKeys() {
+        FibonacciHeap fibonacciHeap = new FibonacciHeap();
+        fibonacciHeap.insert(-10);
+        fibonacciHeap.insert(-5);
+        fibonacciHeap.insert(-20);
+
+        Assertions.assertEquals(-20, fibonacciHeap.findMin().getKey());
+    }
+
+    @Test
+    void testDeleteOnEmptyHeap() {
+        FibonacciHeap fibonacciHeap = new FibonacciHeap();
+        Assertions.assertThrows(NullPointerException.class, () -> { fibonacciHeap.delete(fibonacciHeap.findMin()); });
+    }
+
+    @Test
+    void testPotentialCalculation() {
+        FibonacciHeap fibonacciHeap = new FibonacciHeap();
+        fibonacciHeap.insert(10);
+        fibonacciHeap.insert(20);
+
+        Assertions.assertEquals(2, fibonacciHeap.potential()); // 2 trees, no marked nodes
+        var node = fibonacciHeap.findMin();
+        fibonacciHeap.delete(node);
+        Assertions.assertEquals(1, fibonacciHeap.potential());
     }
 }