Improving BitonicSort (#5244)

* Improving BitonicSort

* Moving max method to SortingUtils

* Adding Javadoc to merge method

* Fix for test and code improvements

* Improving code readability

* Renaming method parameters

---------

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

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

@@ -1,79 +1,112 @@
 package com.thealgorithms.sorts;
 
-/* Java program for Bitonic Sort. Note that this program
-works only when size of input is a power of 2. */
-public class BitonicSort {
+import java.util.Arrays;
+import java.util.function.BiPredicate;
 
-    /* The parameter dir indicates the sorting direction,
-  ASCENDING or DESCENDING; if (a[i] > a[j]) agrees
-  with the direction, then a[i] and a[j] are
-  interchanged. */
-    void compAndSwap(int[] a, int i, int j, int dir) {
-        if ((a[i] > a[j] && dir == 1) || (a[i] < a[j] && dir == 0)) {
-            // Swapping elements
-            int temp = a[i];
-            a[i] = a[j];
-            a[j] = temp;
+/**
+ * BitonicSort class implements the SortAlgorithm interface using the bitonic sort technique.
+ */
+public class BitonicSort implements SortAlgorithm {
+    private enum Direction {
+        DESCENDING,
+        ASCENDING,
+    }
+
+    /**
+     * Sorts the given array using the Bitonic Sort algorithm.
+     *
+     * @param <T> the type of elements in the array, which must implement the Comparable interface
+     * @param array the array to be sorted
+     * @return the sorted array
+     */
+    @Override
+    public <T extends Comparable<T>> T[] sort(T[] array) {
+        if (array.length == 0) {
+            return array;
+        }
+
+        final int paddedSize = nextPowerOfTwo(array.length);
+        T[] paddedArray = Arrays.copyOf(array, paddedSize);
+
+        // Fill the padded part with a maximum value
+        final T maxValue = max(array);
+        Arrays.fill(paddedArray, array.length, paddedSize, maxValue);
+
+        bitonicSort(paddedArray, 0, paddedSize, Direction.ASCENDING);
+        return Arrays.copyOf(paddedArray, array.length);
+    }
+
+    private <T extends Comparable<T>> void bitonicSort(final T[] array, final int low, final int cnt, final Direction direction) {
+        if (cnt > 1) {
+            final int k = cnt / 2;
+
+            // Sort first half in ascending order
+            bitonicSort(array, low, k, Direction.ASCENDING);
+
+            // Sort second half in descending order
+            bitonicSort(array, low + k, cnt - k, Direction.DESCENDING);
+
+            // Merge the whole sequence in ascending order
+            bitonicMerge(array, low, cnt, direction);
         }
     }
 
-    /* It recursively sorts a bitonic sequence in ascending
-  order, if dir = 1, and in descending order otherwise
-  (means dir=0). The sequence to be sorted starts at
-  index position low, the parameter cnt is the number
-  of elements to be sorted.*/
-    void bitonicMerge(int[] a, int low, int cnt, int dir) {
+    /**
+     * Merges the bitonic sequence in the specified direction.
+     *
+     * @param <T> the type of elements in the array, which must be Comparable
+     * @param array the array containing the bitonic sequence to be merged
+     * @param low the starting index of the sequence to be merged
+     * @param cnt the number of elements in the sequence to be merged
+     * @param direction the direction of sorting
+     */
+    private <T extends Comparable<T>> void bitonicMerge(T[] array, int low, int cnt, Direction direction) {
         if (cnt > 1) {
-            int k = cnt / 2;
+            final int k = cnt / 2;
+
+            final BiPredicate<T, T> areSorted = (direction == Direction.ASCENDING) ? (a, b) -> a.compareTo(b) < 0 : (a, b) -> a.compareTo(b) > 0;
             for (int i = low; i < low + k; i++) {
-                compAndSwap(a, i, i + k, dir);
+                if (!areSorted.test(array[i], array[i + k])) {
+                    SortUtils.swap(array, i, i + k);
+                }
             }
-            bitonicMerge(a, low, k, dir);
-            bitonicMerge(a, low + k, k, dir);
+
+            bitonicMerge(array, low, k, direction);
+            bitonicMerge(array, low + k, cnt - k, direction);
         }
     }
 
-    /* This funcion first produces a bitonic sequence by
-  recursively sorting its two halves in opposite sorting
-  orders, and then calls bitonicMerge to make them in
-  the same order */
-    void bitonicSort(int[] a, int low, int cnt, int dir) {
-        if (cnt > 1) {
-            int k = cnt / 2;
+    /**
+     * Finds the next power of two greater than or equal to the given number.
+     *
+     * @param n the number
+     * @return the next power of two
+     */
+    private static int nextPowerOfTwo(int n) {
+        int count = 0;
 
-            // sort in ascending order since dir here is 1
-            bitonicSort(a, low, k, 1);
-
-            // sort in descending order since dir here is 0
-            bitonicSort(a, low + k, k, 0);
-
-            // Will merge whole sequence in ascending order
-            // since dir=1.
-            bitonicMerge(a, low, cnt, dir);
+        // First n in the below condition is for the case where n is 0
+        if ((n & (n - 1)) == 0) {
+            return n;
         }
-    }
 
-    /*Caller of bitonicSort for sorting the entire array
-  of length N in ASCENDING order */
-    void sort(int[] a, int n, int up) {
-        bitonicSort(a, 0, n, up);
-    }
-
-    /* A utility function to print array of size n */
-    static void printArray(int[] arr) {
-        int n = arr.length;
-        for (int i = 0; i < n; ++i) {
-            System.out.print(arr[i] + " ");
+        while (n != 0) {
+            n >>= 1;
+            count += 1;
         }
-        System.out.println();
+
+        return 1 << count;
     }
 
-    public static void main(String[] args) {
-        int[] a = {3, 7, 4, 8, 6, 2, 1, 5};
-        int up = 1;
-        BitonicSort ob = new BitonicSort();
-        ob.sort(a, a.length, up);
-        System.out.println("\nSorted array");
-        printArray(a);
+    /**
+     * Finds the maximum element in the given array.
+     *
+     * @param <T> the type of elements in the array, which must implement the Comparable interface
+     * @param array the array to be searched
+     * @return the maximum element in the array
+     * @throws IllegalArgumentException if the array is null or empty
+     */
+    private static <T extends Comparable<T>> T max(final T[] array) {
+        return Arrays.stream(array).max(Comparable::compareTo).orElseThrow();
     }
 }