Dual Numbers

package ch.luschmar.math.dualnumber;

import java.math.BigDecimal;
import java.util.function.UnaryOperator;

/**
 * @see <a href="https://www.youtube.com/watch?v=QwFLA5TrviI">Dual Number</a>
 */
public class DualNumber {
	BigDecimal real;
	BigDecimal dual;

	public DualNumber(int real) {
		this.real = new BigDecimal(real);
		this.dual = BigDecimal.ZERO;
	}
	public DualNumber(BigDecimal real) {
		this.real = real;
		this.dual = BigDecimal.ZERO;
	}

	public DualNumber(BigDecimal real, BigDecimal dual) {
		this.real = real;
		this.dual = dual;
	}

	public DualNumber add(DualNumber other) {
		return new DualNumber(
				this.real.add(other.real),
				this.dual.add(other.dual));
	}

	public DualNumber multiply(DualNumber other) {
		return new DualNumber(
				this.real.multiply(other.real),
				this.dual.multiply(other.real).add(this.real.multiply(other.dual)));
	}

	public static DualNumber diff(UnaryOperator<DualNumber> diff, int value) {
		return new DualNumber(
				diff.apply(
						new DualNumber(BigDecimal.valueOf(value),
						BigDecimal.ONE)).dual
		);
	}
}

package ch.luschmar.math.dualnumber;

import org.junit.jupiter.api.Test;

import java.util.function.UnaryOperator;

import static ch.luschmar.math.dualnumber.DualNumber.diff;
import static org.junit.jupiter.api.Assertions.assertEquals;

class DualNumberTest {
	@Test
	void test_dual_number() {
		// x^4 + x*3 + 5
		UnaryOperator<DualNumber> myFunction = x -> {
			var a = x.multiply(x.multiply(x.multiply(x)));
			var b = x.multiply(new DualNumber(3));
			var c = new DualNumber(5);
			return a.add(b).add(c);
		};

		// try to solve x = 12; diff(x^4 + x*3 + 5)
		var res = diff(myFunction, 12 );

		// diff -> 4*x^3+3
		// x = 12
		assertEquals(6915, res.real.intValue());
	}
}