Added tests for NumberOfDigits (#5107)

Co-authored-by: Maria Paszkiewicz SCC <maria.paszkiewicz@kit.edu>
Co-authored-by: Piotr Idzik <65706193+vil02@users.noreply.github.com>

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

@@ -0,0 +1,40 @@
+package com.thealgorithms.maths;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.util.function.IntFunction;
+import java.util.stream.Stream;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.Arguments;
+import org.junit.jupiter.params.provider.MethodSource;
+
+public class NumberOfDigitsTest {
+
+    @ParameterizedTest
+    @MethodSource("testCases")
+    void testNumberOfDigits(final int expected, final int number, final IntFunction<Integer> methodUnderTest) {
+        assertEquals(expected, methodUnderTest.apply(number));
+        assertEquals(expected, methodUnderTest.apply(-number));
+    }
+
+    private static Stream<Arguments> testCases() {
+        final Integer[][] inputs = new Integer[][] {
+            {3, 100},
+            {1, 0},
+            {2, 12},
+            {3, 123},
+            {4, 1234},
+            {5, 12345},
+            {6, 123456},
+            {7, 1234567},
+            {8, 12345678},
+            {9, 123456789},
+            {9, 987654321},
+        };
+
+        final IntFunction<Integer>[] methods = new IntFunction[] {NumberOfDigits::numberOfDigits, NumberOfDigits::numberOfDigitsFast, NumberOfDigits::numberOfDigitsFaster, NumberOfDigits::numberOfDigitsRecursion};
+
+        return Stream.of(inputs).flatMap(input -> Stream.of(methods).map(method -> Arguments.of(input[0], input[1], method)));
+    }
+}