Dev: Added NumberPersistence utility class to find multiplicative and additive persistance (#6612)

* Add NumberPersistence utility class with multiplicative and additive persistence methods

* Fixed identified style issues

Author: BanulaKumarage

src/main/java/com/thealgorithms/maths/NumberPersistence.java

@@ -0,0 +1,79 @@
+package com.thealgorithms.maths;
+
+/**
+ * A utility class for calculating the persistence of a number.
+ *
+ * <p>This class provides methods to calculate:
+ * <ul>
+ *   <li>Multiplicative persistence: The number of steps required to reduce a number to a single digit by multiplying its digits.</li>
+ *   <li>Additive persistence: The number of steps required to reduce a number to a single digit by summing its digits.</li>
+ * </ul>
+ *
+ * <p>This class is final and cannot be instantiated.
+ *
+ * @see <a href="https://en.wikipedia.org/wiki/Persistence_of_a_number">Wikipedia: Persistence of a number</a>
+ */
+public final class NumberPersistence {
+
+    // Private constructor to prevent instantiation
+    private NumberPersistence() {
+    }
+
+    /**
+     * Calculates the multiplicative persistence of a given number.
+     *
+     * <p>Multiplicative persistence is the number of steps required to reduce a number to a single digit
+     * by multiplying its digits repeatedly.
+     *
+     * @param num the number to calculate persistence for; must be non-negative
+     * @return the multiplicative persistence of the number
+     * @throws IllegalArgumentException if the input number is negative
+     */
+    public static int multiplicativePersistence(int num) {
+        if (num < 0) {
+            throw new IllegalArgumentException("multiplicativePersistence() does not accept negative values");
+        }
+
+        int steps = 0;
+        while (num >= 10) {
+            int product = 1;
+            int temp = num;
+            while (temp > 0) {
+                product *= temp % 10;
+                temp /= 10;
+            }
+            num = product;
+            steps++;
+        }
+        return steps;
+    }
+
+    /**
+     * Calculates the additive persistence of a given number.
+     *
+     * <p>Additive persistence is the number of steps required to reduce a number to a single digit
+     * by summing its digits repeatedly.
+     *
+     * @param num the number to calculate persistence for; must be non-negative
+     * @return the additive persistence of the number
+     * @throws IllegalArgumentException if the input number is negative
+     */
+    public static int additivePersistence(int num) {
+        if (num < 0) {
+            throw new IllegalArgumentException("additivePersistence() does not accept negative values");
+        }
+
+        int steps = 0;
+        while (num >= 10) {
+            int sum = 0;
+            int temp = num;
+            while (temp > 0) {
+                sum += temp % 10;
+                temp /= 10;
+            }
+            num = sum;
+            steps++;
+        }
+        return steps;
+    }
+}

src/test/java/com/thealgorithms/maths/NumberPersistenceTest.java

@@ -0,0 +1,42 @@
+package com.thealgorithms.maths;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.CsvSource;
+import org.junit.jupiter.params.provider.ValueSource;
+
+class NumberPersistenceTest {
+
+    @ParameterizedTest(name = "multiplicativePersistence({0}) = {1}")
+    @CsvSource({"0, 0", "7, 0", "217, 2", "39, 3", "999, 4"})
+    @DisplayName("Test multiplicative persistence with valid inputs")
+    void testMultiplicativePersistenceValid(int input, int expected) {
+        assertEquals(expected, NumberPersistence.multiplicativePersistence(input));
+    }
+
+    @ParameterizedTest(name = "multiplicativePersistence({0}) throws IllegalArgumentException")
+    @ValueSource(ints = {-1, -100, -9999})
+    @DisplayName("Test multiplicative persistence with negative numbers")
+    void testMultiplicativePersistenceNegative(int input) {
+        Exception exception = assertThrows(IllegalArgumentException.class, () -> NumberPersistence.multiplicativePersistence(input));
+        assertEquals("multiplicativePersistence() does not accept negative values", exception.getMessage());
+    }
+
+    @ParameterizedTest(name = "additivePersistence({0}) = {1}")
+    @CsvSource({"0, 0", "5, 0", "199, 3", "999, 2", "1234, 2"})
+    @DisplayName("Test additive persistence with valid inputs")
+    void testAdditivePersistenceValid(int input, int expected) {
+        assertEquals(expected, NumberPersistence.additivePersistence(input));
+    }
+
+    @ParameterizedTest(name = "additivePersistence({0}) throws IllegalArgumentException")
+    @ValueSource(ints = {-1, -100, -9999})
+    @DisplayName("Test additive persistence with negative numbers")
+    void testAdditivePersistenceNegative(int input) {
+        Exception exception = assertThrows(IllegalArgumentException.class, () -> NumberPersistence.additivePersistence(input));
+        assertEquals("additivePersistence() does not accept negative values", exception.getMessage());
+    }
+}