ODESolverRK4

package odeadapt;

import Jama.*;

/**
 * ODESolverRK4.java
 * solves a given ODE by giving the "next" value for x 
 * using Runge-Kutta method of order 4
 */
public class ODESolverRK4 extends ODESingleStepSolver {

  /**
   * construct solver for a given ODE
   */
  public ODESolverRK4(ODE ode) {
    super(ode);
    order = 4;
  }

  /*
   * integrate until t + h
   * using a fourth order Runge-Kutta scheme
   */
  public int nextStep(double h) {
    Matrix k1 = ode.f(x, t);
    double t1 = t + h / 2.0;
    
    // x1 = x + h*k1/2.0;
    Matrix x1 = x.plus(k1.times(h / 2.0));
    Matrix k2 = ode.f(x1, t1);
    double t2 = t1;
    
    // x2 = x + h*k2/2.0;
    Matrix x2 = x.plus(k2.times(h / 2.0));
    Matrix k3 = ode.f(x2, t2);
    double t3 = t + h;
    
    // x3 = x + k3*h;
    Matrix x3 = x.plus(k3.times(h));
    Matrix k4 = ode.f(x3, t3);
    
    // x += (k1 + 2.0*k2 + 2.0*k3 + k4)*h/6.0;
    x.plusEquals(k1.times(h / 6.0));
    x.plusEquals(k2.times(h / 3.0));
    x.plusEquals(k3.times(h / 3.0));
    x.plusEquals(k4.times(h / 6.0));
    
    t += h;
    return 1;
  }
}