/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.apache.commons.math.ode;

import org.apache.commons.math.ode.DerivativeException;
import org.apache.commons.math.ode.FirstOrderConverter;
import org.apache.commons.math.ode.IntegratorException;
import org.apache.commons.math.ode.SecondOrderDifferentialEquations;
import org.apache.commons.math.ode.nonstiff.ClassicalRungeKuttaIntegrator;

import junit.framework.*;

public class FirstOrderConverterTest extends TestCase {

	public FirstOrderConverterTest(String name) {
		super(name);
	}

	public void testDoubleDimension() {
		for (int i = 1; i < 10; ++i) {
			SecondOrderDifferentialEquations eqn2 = new Equations(i, 0.2);
			FirstOrderConverter eqn1 = new FirstOrderConverter(eqn2);
			assertTrue(eqn1.getDimension() == (2 * eqn2.getDimension()));
		}
	}

	public void testDecreasingSteps() throws DerivativeException, IntegratorException {

		double previousError = Double.NaN;
		for (int i = 0; i < 10; ++i) {

			double step = Math.pow(2.0, -(i + 1));
			double error = integrateWithSpecifiedStep(4.0, 0.0, 1.0, step) - Math.sin(4.0);
			if (i > 0) {
				assertTrue(Math.abs(error) < Math.abs(previousError));
			}
			previousError = error;

		}
	}

	public void testSmallStep() throws DerivativeException, IntegratorException {
		double error = integrateWithSpecifiedStep(4.0, 0.0, 1.0, 1.0e-4) - Math.sin(4.0);
		assertTrue(Math.abs(error) < 1.0e-10);
	}

	public void testBigStep() throws DerivativeException, IntegratorException {
		double error = integrateWithSpecifiedStep(4.0, 0.0, 1.0, 0.5) - Math.sin(4.0);
		assertTrue(Math.abs(error) > 0.1);
	}

	public static Test suite() {
		return new TestSuite(FirstOrderConverterTest.class);
	}

	private static class Equations implements SecondOrderDifferentialEquations {

		private int n;

		private double omega2;

		public Equations(int n, double omega) {
			this.n = n;
			omega2 = omega * omega;
		}

		public int getDimension() {
			return n;
		}

		public void computeSecondDerivatives(double t, double[] y, double[] yDot, double[] yDDot) {
			for (int i = 0; i < n; ++i) {
				yDDot[i] = -omega2 * y[i];
			}
		}

	}

	private double integrateWithSpecifiedStep(double omega, double t0, double t, double step)
			throws DerivativeException, IntegratorException {
		double[] y0 = new double[2];
		y0[0] = Math.sin(omega * t0);
		y0[1] = omega * Math.cos(omega * t0);
		ClassicalRungeKuttaIntegrator i = new ClassicalRungeKuttaIntegrator(step);
		double[] y = new double[2];
		i.integrate(new FirstOrderConverter(new Equations(1, omega)), t0, y0, t, y);
		return y[0];
	}

}
