test(sorting): add even-only merge sort tests

app/src/test/java/ru/project/sorting/EvenOnlyMergeSortStrategyTest.java

@@ -0,0 +1,149 @@
+package ru.project.sorting;
+
+import org.junit.jupiter.api.Test;
+import ru.project.collection.MyLinkedList;
+import ru.project.collection.MyList;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+class EvenOnlyMergeSortStrategyTest {
+
+    @Test
+    void shouldSortOnlyEvenElements() {
+        MyList<Integer> list = new MyLinkedList<>();
+
+        list.add(7);
+        list.add(8);
+        list.add(3);
+        list.add(2);
+        list.add(5);
+        list.add(4);
+
+        EvenOnlyMergeSortStrategy<Integer> strategy =
+                new EvenOnlyMergeSortStrategy<>(value -> value);
+
+        strategy.sort(list, Integer::compare);
+
+        assertEquals(7, list.get(0));
+        assertEquals(2, list.get(1));
+        assertEquals(3, list.get(2));
+        assertEquals(4, list.get(3));
+        assertEquals(5, list.get(4));
+        assertEquals(8, list.get(5));
+
+        assertEquals(6, list.size());
+    }
+
+    @Test
+    void shouldKeepAllOddElementsUnchanged() {
+        MyList<Integer> list = new MyLinkedList<>();
+
+        list.add(7);
+        list.add(3);
+        list.add(5);
+        list.add(1);
+
+        EvenOnlyMergeSortStrategy<Integer> strategy =
+                new EvenOnlyMergeSortStrategy<>(value -> value);
+
+        strategy.sort(list, Integer::compare);
+
+        assertEquals(7, list.get(0));
+        assertEquals(3, list.get(1));
+        assertEquals(5, list.get(2));
+        assertEquals(1, list.get(3));
+    }
+
+    @Test
+    void shouldSortAllEvenElements() {
+        MyList<Integer> list = new MyLinkedList<>();
+
+        list.add(8);
+        list.add(2);
+        list.add(6);
+        list.add(4);
+
+        EvenOnlyMergeSortStrategy<Integer> strategy =
+                new EvenOnlyMergeSortStrategy<>(value -> value);
+
+        strategy.sort(list, Integer::compare);
+
+        assertEquals(2, list.get(0));
+        assertEquals(4, list.get(1));
+        assertEquals(6, list.get(2));
+        assertEquals(8, list.get(3));
+    }
+
+    @Test
+    void shouldKeepSingleElementUnchanged() {
+        MyList<Integer> list = new MyLinkedList<>();
+
+        list.add(4);
+
+        EvenOnlyMergeSortStrategy<Integer> strategy =
+                new EvenOnlyMergeSortStrategy<>(value -> value);
+
+        strategy.sort(list, Integer::compare);
+
+        assertEquals(4, list.get(0));
+        assertEquals(1, list.size());
+    }
+
+    @Test
+    void shouldHandleEmptyList() {
+        MyList<Integer> list = new MyLinkedList<>();
+
+        EvenOnlyMergeSortStrategy<Integer> strategy =
+                new EvenOnlyMergeSortStrategy<>(value -> value);
+
+        strategy.sort(list, Integer::compare);
+
+        assertEquals(0, list.size());
+    }
+
+    @Test
+    void shouldUseValueExtractorToDetermineEvenElements() {
+        MyList<Integer> list = new MyLinkedList<>();
+
+        list.add(4);
+        list.add(3);
+        list.add(2);
+        list.add(1);
+
+        EvenOnlyMergeSortStrategy<Integer> strategy =
+                new EvenOnlyMergeSortStrategy<>(value -> value + 1);
+
+        strategy.sort(list, Integer::compare);
+
+        assertEquals(4, list.get(0));
+        assertEquals(1, list.get(1));
+        assertEquals(2, list.get(2));
+        assertEquals(3, list.get(3));
+    }
+
+    @Test
+    void shouldUseProvidedComparator() {
+        MyList<Integer> list = new MyLinkedList<>();
+
+        list.add(7);
+        list.add(2);
+        list.add(3);
+        list.add(8);
+        list.add(5);
+        list.add(4);
+
+        EvenOnlyMergeSortStrategy<Integer> strategy =
+                new EvenOnlyMergeSortStrategy<>(value -> value);
+
+        strategy.sort(list, (first, second) -> Integer.compare(second, first));
+
+        assertEquals(7, list.get(0));
+        assertEquals(8, list.get(1));
+        assertEquals(3, list.get(2));
+        assertEquals(4, list.get(3));
+        assertEquals(5, list.get(4));
+        assertEquals(2, list.get(5));
+    }
+}
+
+