#include "TrueBASIC.h"
void EulerRichardson(double pos[], double vel[], double *t, double *GM, 
                     double *dt, char *kick_flag, double *xsave, double *ysave);
void acceleration(double pos[], double accel[], double *GM);

void EulerRichardson(double pos[], double vel[], double *t, double *GM, 
                     double *dt, char *kick_flag, double *xsave, double *ysave)
{
	int s, i, Itmp1_;
	float x, y;
	double diffx, velmid[3], diffy, v, v2, accel[3], posmid[3], impulse;
	
	if(GWmouse(&Itmp1_, &x, &x)) s = 1;
	else if(Itmp1_) s = 2;
	else s = 3;
	if(s == 2)
	{
		(*xsave) = x;                      // save position where mouse clicked
		(*ysave) = y;
		strcpy(kick_flag, "on");
	}
	if((strcmp(kick_flag, "on") == 0))
	{
		diffx = fabs(pos[1] - (*xsave));
		diffy = fabs(pos[2] - (*ysave));
		// wait until orbit reaches point where mouse clicked
		if((diffx < 0.02) && (diffy < 0.02))
		{
			v2 = vel[1]*vel[1] + vel[2]*vel[2];
			v = sqrt(v2);
			impulse = 0.1*v;               // magnitude of impulse/mass
			vel[2] = vel[2] + impulse;     // vertical kick
			strcpy(kick_flag, "off");
		}
	}
	// compute acceleration at beginning of interval
	acceleration(pos, accel, GM);
	for(i = 1; i <= 2; ++i)
	{
		velmid[i] = vel[i] + 0.5*accel[i]*(*dt);
		posmid[i] = pos[i] + 0.5*vel[i]*(*dt);
	}
	// compute acceleration at middle of interval
	acceleration(posmid, accel, GM);
	// position and velocity at end of interval
	for(i = 1; i <= 2; ++i)
	{
		vel[i] += accel[i]*(*dt);
		pos[i] += velmid[i]*(*dt);
	}
	(*t) += (*dt);
}