refactor: cleanup RadixSort (#5280)

* refactor: refactoring RadixSort, adding test, update DIRECTORY.md

* checkstyle: fix formatting for test

* refactor: adding possibility to sort negative numbers. Improve tests. Improving code readability

* checkstyle: fix formatting

* refactor: resolve conflicts with master branch

* refactor: remove negative integers support

* checkstyle: fix formatting

* checkstyle: fix formatting, revert test

* refactor: adding return array to countDigits and buildOutput method, adding more specific description to javadocs

---------

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/RadixSort.java

@@ -1,62 +1,98 @@
 package com.thealgorithms.sorts;
 
+import com.thealgorithms.maths.NumberOfDigits;
 import java.util.Arrays;
 
-final class RadixSort {
+/**
+ * This class provides an implementation of the radix sort algorithm.
+ * It sorts an array of nonnegative integers in increasing order.
+ */
+public final class RadixSort {
+    private static final int BASE = 10;
+
     private RadixSort() {
     }
 
-    private static int getMax(int[] arr, int n) {
-        int mx = arr[0];
-        for (int i = 1; i < n; i++) {
-            if (arr[i] > mx) {
-                mx = arr[i];
+    /**
+     * Sorts an array of nonnegative integers using the radix sort algorithm.
+     *
+     * @param array the array to be sorted
+     * @return the sorted array
+     * @throws IllegalArgumentException if any negative integers are found
+     */
+    public static int[] sort(int[] array) {
+        if (array.length == 0) {
+            return array;
+        }
+
+        checkForNegativeInput(array);
+        radixSort(array);
+        return array;
+    }
+
+    /**
+     * Checks if the array contains any negative integers.
+     *
+     * @param array the array to be checked
+     * @throws IllegalArgumentException if any negative integers are found
+     */
+    private static void checkForNegativeInput(int[] array) {
+        for (int number : array) {
+            if (number < 0) {
+                throw new IllegalArgumentException("Array contains non-positive integers.");
             }
         }
-        return mx;
     }
 
-    private static void countSort(int[] arr, int n, int exp) {
-        int[] output = new int[n];
-        int i;
-        int[] count = new int[10];
-        Arrays.fill(count, 0);
-
-        for (i = 0; i < n; i++) {
-            count[(arr[i] / exp) % 10]++;
+    private static void radixSort(int[] array) {
+        final int max = Arrays.stream(array).max().getAsInt();
+        for (int i = 0, exp = 1; i < NumberOfDigits.numberOfDigits(max); i++, exp *= BASE) {
+            countingSortByDigit(array, exp);
         }
+    }
 
-        for (i = 1; i < 10; i++) {
+    /**
+     * A utility method to perform counting sort of array[] according to the digit represented by exp.
+     *
+     * @param array the array to be sorted
+     * @param exp   the exponent representing the current digit position
+     */
+    private static void countingSortByDigit(int[] array, int exp) {
+        int[] count = countDigits(array, exp);
+        accumulateCounts(count);
+        int[] output = buildOutput(array, exp, count);
+        copyOutput(array, output);
+    }
+
+    private static int[] countDigits(int[] array, int exp) {
+        int[] count = new int[BASE];
+        for (int i = 0; i < array.length; i++) {
+            count[getDigit(array[i], exp)]++;
+        }
+        return count;
+    }
+
+    private static int getDigit(int number, int position) {
+        return (number / position) % BASE;
+    }
+
+    private static void accumulateCounts(int[] count) {
+        for (int i = 1; i < BASE; i++) {
             count[i] += count[i - 1];
         }
-
-        for (i = n - 1; i >= 0; i--) {
-            output[count[(arr[i] / exp) % 10] - 1] = arr[i];
-            count[(arr[i] / exp) % 10]--;
-        }
-
-        System.arraycopy(output, 0, arr, 0, n);
     }
 
-    private static void radixsort(int[] arr, int n) {
-        int m = getMax(arr, n);
-
-        for (int exp = 1; m / exp > 0; exp *= 10) {
-            countSort(arr, n, exp);
+    private static int[] buildOutput(int[] array, int exp, int[] count) {
+        int[] output = new int[array.length];
+        for (int i = array.length - 1; i >= 0; i--) {
+            int digit = getDigit(array[i], exp);
+            output[count[digit] - 1] = array[i];
+            count[digit]--;
         }
+        return output;
     }
 
-    static void print(int[] arr, int n) {
-        for (int i = 0; i < n; i++) {
-            System.out.print(arr[i] + " ");
-        }
-    }
-
-    public static void main(String[] args) {
-        int[] arr = {170, 45, 75, 90, 802, 24, 2, 66};
-        int n = arr.length;
-        radixsort(arr, n);
-        print(arr, n);
+    private static void copyOutput(int[] array, int[] output) {
+        System.arraycopy(output, 0, array, 0, array.length);
     }
 }
-// Written by James Mc Dermott(theycallmemac)