feat: add karatsuba multiplication (#5719)

* feat: add karatsuba multiplication

* fix: fallback size

* fix: big integer instances

---------

Co-authored-by: Alex Klymenko <alexanderklmn@gmail.com>

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

@@ -0,0 +1,58 @@
+package com.thealgorithms.maths;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.math.BigInteger;
+import java.util.stream.Stream;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.Arguments;
+import org.junit.jupiter.params.provider.MethodSource;
+
+/**
+ * Unit test class for {@link KaratsubaMultiplication} class.
+ *
+ * <p>
+ * This class tests various edge cases and normal cases for the
+ * Karatsuba multiplication algorithm implemented in the KaratsubaMultiplication class.
+ * It uses parameterized tests to handle multiple test cases.
+ * </p>
+ */
+class KaratsubaMultiplicationTest {
+
+    /**
+     * Provides test data for the parameterized test.
+     * Each entry in the stream contains three elements: x, y, and the expected result.
+     *
+     * @return a stream of arguments for the parameterized test
+     */
+    static Stream<Arguments> provideTestCases() {
+        return Stream.of(
+            // Test case 1: Two small numbers
+            Arguments.of(new BigInteger("1234"), new BigInteger("5678"), new BigInteger("7006652")),
+            // Test case 2: Two large numbers
+            Arguments.of(new BigInteger("342364"), new BigInteger("393958"), new BigInteger("134877036712")),
+            // Test case 3: One number is zero
+            Arguments.of(BigInteger.ZERO, new BigInteger("5678"), BigInteger.ZERO),
+            // Test case 4: Both numbers are zero
+            Arguments.of(BigInteger.ZERO, BigInteger.ZERO, BigInteger.ZERO),
+            // Test case 5: Single-digit numbers
+            Arguments.of(new BigInteger("9"), new BigInteger("8"), new BigInteger("72")));
+    }
+
+    /**
+     * Parameterized test for Karatsuba multiplication.
+     *
+     * <p>
+     * This method runs the Karatsuba multiplication algorithm for multiple test cases.
+     * </p>
+     *
+     * @param x the first number to multiply
+     * @param y the second number to multiply
+     * @param expected the expected result of x * y
+     */
+    @ParameterizedTest
+    @MethodSource("provideTestCases")
+    void testKaratsubaMultiplication(BigInteger x, BigInteger y, BigInteger expected) {
+        assertEquals(expected, KaratsubaMultiplication.karatsuba(x, y));
+    }
+}