Java/src/test/java/com/thealgorithms/maths/FastExponentiationTest.java

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.Test;

/**
 * Unit tests for the {@link FastExponentiation} class.
 *
 * <p>This class contains various test cases to verify the correctness of the fastExponentiation method.
 * It covers basic functionality, edge cases, and exceptional cases.
 */
class FastExponentiationTest {

    /**
     * Tests fast exponentiation with small numbers.
     */
    @Test
    void testSmallNumbers() {
        assertEquals(1024, FastExponentiation.fastExponentiation(2, 10, 10000), "2^10 mod 10000 should be 1024");
        assertEquals(81, FastExponentiation.fastExponentiation(3, 4, 1000), "3^4 mod 1000 should be 81");
    }

    /**
     * Tests the behavior of the fast exponentiation method when using a modulus.
     */
    @Test
    void testWithModulo() {
        assertEquals(24, FastExponentiation.fastExponentiation(2, 10, 1000), "2^10 mod 1000 should be 24");
        assertEquals(0, FastExponentiation.fastExponentiation(10, 5, 10), "10^5 mod 10 should be 0");
    }

    /**
     * Tests the edge cases where base or exponent is 0.
     */
    @Test
    void testBaseCases() {
        assertEquals(1, FastExponentiation.fastExponentiation(2, 0, 1000), "Any number raised to the power 0 mod anything should be 1");
        assertEquals(0, FastExponentiation.fastExponentiation(0, 10, 1000), "0 raised to any power should be 0");
        assertEquals(1, FastExponentiation.fastExponentiation(0, 0, 1000), "0^0 is considered 0 in modular arithmetic.");
    }

    /**
     * Tests fast exponentiation with a negative base to ensure correctness under modular arithmetic.
     */
    @Test
    void testNegativeBase() {
        assertEquals(9765625, FastExponentiation.fastExponentiation(-5, 10, 1000000007), "-5^10 mod 1000000007 should be 9765625");
    }

    /**
     * Tests that a negative exponent throws an ArithmeticException.
     */
    @Test
    void testNegativeExponent() {
        assertThrows(ArithmeticException.class, () -> { FastExponentiation.fastExponentiation(2, -5, 1000); });
    }

    /**
     * Tests that the method throws an IllegalArgumentException for invalid modulus values.
     */
    @Test
    void testInvalidModulus() {
        assertThrows(IllegalArgumentException.class, () -> { FastExponentiation.fastExponentiation(2, 5, 0); });
    }
}