refactor: SelectionSort like classes and their tests (#5265)

* Refactor: Adding test common approach, adding javadocs, renaming variables

* Refactor: Fix failed build, when generated test case for recursion is too big. To avoid stackoverflow

* Checkstyle: Adding newline to end of class

* refactor: simplify assign minIndex in recursiveSelectionSort, and improving SelectionSort

* checkstyle: adding newline to SelectionSort

---------

Co-authored-by: Alex Klymenko <alx@alx.com>
Co-authored-by: Piotr Idzik <65706193+vil02@users.noreply.github.com>

src/main/java/com/thealgorithms/sorts/SelectionSort.java

@@ -1,46 +1,31 @@
 package com.thealgorithms.sorts;
 
 public class SelectionSort implements SortAlgorithm {
-
     /**
-     * Generic selection sort algorithm in increasing order.
+     * Sorts an array of comparable elements in increasing order using the selection sort algorithm.
      *
-     * @param arr the array to be sorted.
-     * @param <T> the class of array.
-     * @return sorted array.
+     * @param array the array to be sorted
+     * @param <T> the class of array elements
+     * @return the sorted array
      */
     @Override
-    public <T extends Comparable<T>> T[] sort(T[] arr) {
-        int n = arr.length;
-        for (int i = 0; i < n - 1; i++) {
-            int minIndex = i;
-            for (int j = i + 1; j < n; j++) {
-                if (arr[minIndex].compareTo(arr[j]) > 0) {
-                    minIndex = j;
-                }
-            }
-            if (minIndex != i) {
-                SortUtils.swap(arr, i, minIndex);
-            }
+    public <T extends Comparable<T>> T[] sort(T[] array) {
+        // One by one move the boundary of the unsorted subarray
+        for (int i = 0; i < array.length - 1; i++) {
+
+            // Swap the remaining minimum element with the current element
+            SortUtils.swap(array, i, findIndexOfMin(array, i));
         }
-        return arr;
+        return array;
     }
 
-    /**
-     * Driver Code
-     */
-    public static void main(String[] args) {
-        Integer[] arr = {4, 23, 6, 78, 1, 54, 231, 9, 12};
-        SelectionSort selectionSort = new SelectionSort();
-        Integer[] sorted = selectionSort.sort(arr);
-        for (int i = 0; i < sorted.length - 1; ++i) {
-            assert sorted[i] <= sorted[i + 1];
-        }
-
-        String[] strings = {"c", "a", "e", "b", "d"};
-        String[] sortedStrings = selectionSort.sort(strings);
-        for (int i = 0; i < sortedStrings.length - 1; ++i) {
-            assert strings[i].compareTo(strings[i + 1]) <= 0;
+    private static <T extends Comparable<T>> int findIndexOfMin(T[] array, final int start) {
+        int minIndex = start;
+        for (int i = start + 1; i < array.length; i++) {
+            if (array[i].compareTo(array[minIndex]) < 0) {
+                minIndex = i;
+            }
         }
+        return minIndex;
     }
 }

src/main/java/com/thealgorithms/sorts/SelectionSortRecursive.java

@@ -32,13 +32,7 @@ public class SelectionSortRecursive implements SortAlgorithm {
             return;
         }
 
-        // Find the minimum element in the remaining unsorted array
-        final int minIndex = findMinIndex(array, index);
-
-        // Swap the found minimum element with the element at the current index
-        if (minIndex != index) {
-            SortUtils.swap(array, index, minIndex);
-        }
+        SortUtils.swap(array, index, findMinIndex(array, index));
 
         // Recursively call selection sort for the remaining array
         recursiveSelectionSort(array, index + 1);
@@ -47,20 +41,21 @@ public class SelectionSortRecursive implements SortAlgorithm {
     /**
      * Finds the index of the minimum element in the array starting from the given index.
      *
-     * @param array the array to search in.
-     * @param start the starting index.
-     * @param <T> the type of the elements in the array, which must be Comparable.
-     * @return the index of the minimum element starting from the given index.
+     * @param array the array to search
+     * @param start the starting index for the search
+     * @param <T>   the type of elements in the array
+     * @return the index of the minimum element
      */
     private static <T extends Comparable<T>> int findMinIndex(T[] array, int start) {
-        int currentMinIndex = start;
-
-        for (int currentIndex = start + 1; currentIndex < array.length; currentIndex++) {
-            if (array[currentIndex].compareTo(array[currentMinIndex]) < 0) {
-                currentMinIndex = currentIndex;
-            }
+        // Base case: if start is the last index, return start
+        if (start == array.length - 1) {
+            return start;
         }
 
-        return currentMinIndex;
+        // Recursive call to find the minimum index in the rest of the array
+        final int minIndexInRest = findMinIndex(array, start + 1);
+
+        // Return the index of the smaller element between array[start] and the minimum element in the rest of the array
+        return array[start].compareTo(array[minIndexInRest]) < 0 ? start : minIndexInRest;
     }
 }

src/test/java/com/thealgorithms/sorts/SelectionSortRecursiveTest.java

@@ -5,4 +5,8 @@ public class SelectionSortRecursiveTest extends SortingAlgorithmTest {
     SortAlgorithm getSortAlgorithm() {
         return new SelectionSortRecursive();
     }
+
+    protected int getGeneratedArraySize() {
+        return 5000;
+    }
 }

src/test/java/com/thealgorithms/sorts/SelectionSortTest.java

@@ -1,35 +1,8 @@
 package com.thealgorithms.sorts;
 
-import static org.junit.jupiter.api.Assertions.assertArrayEquals;
-
-import org.junit.jupiter.api.Test;
-
-class SelectionSortTest {
-
-    @Test
-    // valid test case
-    void integerArrTest() {
-        Integer[] arr = {4, 23, 6, 78, 1, 54, 231, 9, 12};
-        SelectionSort selectionSort = new SelectionSort();
-
-        assertArrayEquals(new Integer[] {1, 4, 6, 9, 12, 23, 54, 78, 231}, selectionSort.sort(arr));
-    }
-
-    @Test
-    // valid test case
-    void stringArrTest() {
-        String[] arr = {"c", "a", "e", "b", "d"};
-        SelectionSort selectionSort = new SelectionSort();
-
-        assertArrayEquals(new String[] {"a", "b", "c", "d", "e"}, selectionSort.sort(arr));
-    }
-
-    @Test
-    // invalid test case
-    void emptyArrTest() {
-        Integer[] arr = {};
-        SelectionSort selectionSort = new SelectionSort();
-
-        assertArrayEquals(new Integer[] {}, selectionSort.sort(arr));
+class SelectionSortTest extends SortingAlgorithmTest {
+    @Override
+    SortAlgorithm getSortAlgorithm() {
+        return new SelectionSort();
     }
 }

src/test/java/com/thealgorithms/sorts/SortingAlgorithmTest.java

@@ -11,6 +11,10 @@ import org.junit.jupiter.api.Test;
 public abstract class SortingAlgorithmTest {
     abstract SortAlgorithm getSortAlgorithm();
 
+    protected int getGeneratedArraySize() {
+        return 10_000;
+    }
+
     @Test
     void shouldAcceptWhenEmptyArrayIsPassed() {
         Integer[] array = new Integer[] {};
@@ -153,7 +157,7 @@ public abstract class SortingAlgorithmTest {
 
     @Test
     void shouldAcceptWhenRandomArrayIsPassed() {
-        int randomSize = SortUtilsRandomGenerator.generateInt(10_000);
+        int randomSize = SortUtilsRandomGenerator.generateInt(getGeneratedArraySize());
         Double[] array = SortUtilsRandomGenerator.generateArray(randomSize);
         Double[] sorted = getSortAlgorithm().sort(array);
         assertTrue(SortUtils.isSorted(sorted));
@@ -161,7 +165,7 @@ public abstract class SortingAlgorithmTest {
 
     @Test
     void shouldAcceptWhenRandomListIsPassed() {
-        int randomSize = SortUtilsRandomGenerator.generateInt(10_000);
+        int randomSize = SortUtilsRandomGenerator.generateInt(getGeneratedArraySize());
         Double[] array = SortUtilsRandomGenerator.generateArray(randomSize);
         List<Double> list = List.of(array);
         List<Double> sorted = getSortAlgorithm().sort(list);