import java.util.function.DoubleFunction;

public class SimpsonIntegration {

	public static double integrate(DoubleFunction<Double> f, double a, double b) {
		double eps = 1e-10;
		double m = (a + b) / 2;
		double am = simpsonIntegration(f, a, m);
		double mb = simpsonIntegration(f, m, b);
		double ab = simpsonIntegration(f, a, b);
		if (Math.abs(am + mb - ab) < eps)
			return ab;
		return integrate(f, a, m) + integrate(f, m, b);
	}

	static double simpsonIntegration(DoubleFunction<Double> f, double a, double b) {
		return (f.apply(a) + 4 * f.apply((a + b) / 2) + f.apply(b)) * (b - a) / 6;
	}

	// Usage example
	public static void main(String[] args) {
		System.out.println(integrate(x -> Math.sin(x), 0, Math.PI / 2));
	}
}