#include "TrueBASIC.h"
void Euler_Richardson(double *x, double *y, double *vx, double *vy, double *t, 
                      double C, double g, double dt, double dt_2);

void Euler_Richardson(double *x, double *y, double *vx, double *vy, double *t, 
                      double C, double g, double dt, double dt_2)
{
    double ax, axmid, ay, aymid, v, v2, vmid, vmid2, vxmid, vymid, xmid, ymid;

    v2 = (*vx)*(*vx) + (*vy)*(*vy);
    v = sqrt(v2);
    ax = -C*v*(*vx);
    ay = -g - C*v*(*vy);
    vxmid = (*vx) + ax*dt_2;                // 中点での速度
    vymid = (*vy) + ay*dt_2;
    xmid = (*x) + (*vx)*dt_2;               // 中点での位置
    ymid = (*y) + (*vy)*dt_2;
    vmid2 = vxmid*vxmid + vymid*vymid;
    vmid = sqrt(vmid2);
    axmid = -C*vmid*vxmid;                  // 中点での加速度
    aymid = -g - C*vmid*vymid;
    (*vx) += axmid*dt;
    (*vy) += aymid*dt;
    (*x) += vxmid*dt;
    (*y) += vymid*dt;
    (*t) += dt;
}