#include "TrueBASIC.h"
void update(double *x, double *v, double w2, double gamma, double omega, 
            double *t, double dt, double dt2);
double f(double x, double v, double t, double w2, double gamma, double omega);

void update(double *x, double *v, double w2, double gamma, double omega, 
            double *t, double dt, double dt2)
{
    double a, amid, tmid, vmid, xmid;

    a = f(*x, *v, *t, w2, gamma, omega);    // �O���֐�
    vmid = (*v) + a*dt2;
    xmid = (*x) + (*v)*dt2;
    tmid = (*t) + dt2;
    amid = f(xmid, vmid, tmid, w2, gamma, omega);
    (*v) += amid*dt;
    (*x) += vmid*dt;
    (*t) = tmid + dt2;
}

double f(double x, double v, double t, double w2, double gamma, double omega)
{
    double f_;

    f_ = -w2*x - gamma*v + cos(omega*t);
    return f_;
}