// <applet code="GravitySim" width=800 height=600></applet>
// GravitySim.java (200810094) FB
import java.applet.Applet;
import java.awt.*;
import java.awt.image.*;
public class GravitySim extends Applet
{
Image img;
int wd,ht,pix[],alpha;
public void init()
{
wd=getSize().width;ht=getSize().height;alpha=255;
pix=new int[wd*ht];
gr();
img=createImage(new MemoryImageSource(wd,ht,pix,0,wd));
}
public void paint(Graphics g)
{
g.drawImage(img,0,0,this);
// Refresh Periodically
long t=System.currentTimeMillis();
while((System.currentTimeMillis()-t)<5000) {;}
// delete the old graph
for(int i=0;i<wd*ht;++i) pix[i]=0;
gr();
img=createImage(new MemoryImageSource(wd,ht,pix,0,wd));
repaint();
}
/*
public void update(Graphics g) // to stop refresh flickering
{
paint(g);
}
*/
public void gr()
{
// number of gravity sources
int n = (int)(Math.random() * 5)+2;
boolean fl;
double G = 1.0; // general gravity constant
double h = 0.0025; // time step (real solution; lim h->0)
double dmin = 40.0; // min allowed distance between objects
int mx, my, k, j;mx = wd-1;my = ht-1;
double ang, v, dx, dy, d, a, ax, ay;
double x[], y[], xnew[], ynew[], m[], vx[], vy[];
int rd[], gr[], bl[];
m=new double[n];x=new double[n];y=new double[n];
xnew=new double[n];ynew=new double[n];
vx=new double[n];vy=new double[n];
rd=new int[n];gr=new int[n];bl=new int[n];
double Sx, Sy, Svx, Svy, Sm;
Sx = Sy = Svx = Svy = Sm = 0;
for(k=0;k<n;++k)
{
x[k] = (double)((Math.random()*2-1) * mx/8+mx/2);
y[k] = (double)((Math.random()*2-1) * my/8+my/2);
m[k] = (double)(Math.random() * 30) + 10;
m[k] = (m[k]*m[k]*m[k])*10;
ang = (double)(Math.random() * 2 * Math.PI);
v = (double)(Math.random() * 200);
vx[k] = (double)(v * Math.cos(ang));
vy[k] = (double)(v * Math.sin(ang));
rd[k] = (int)(Math.random() * 256);
gr[k] = (int)(Math.random() * 256);
bl[k] = (int)(Math.random() * 256);
Sx = Sx + m[k] * x[k];
Sy = Sy + m[k] * y[k];
Svx = Svx + m[k] * vx[k];
Svy = Svy + m[k] * vy[k];
Sm = Sm + m[k];
}
for (k = 0; k < n; k++)
{
x[k] = x[k] - (Sx / Sm) + (mx / 2.0);
y[k] = y[k] - (Sy / Sm) + (my / 2.0);
vx[k] = vx[k] - (Svx / Sm);
vy[k] = vy[k] - (Svy / Sm);
}
//
do
{
fl=false;
for(k=0;k<n;++k)
{
ax = 0; ay = 0;
for(j=0;j<n;++j)
{
if(k!=j)
{
dx = x[j] - x[k]; dy = y[j] - y[k];
d = (double)Math.sqrt(dx * dx + dy * dy);
d = Math.max(dmin, d);
dx = dx / d; dy = dy / d;
a = G * m[j] / (d * d);
ax = ax + a * dx; ay = ay + a * dy;
}
}
xnew[k] = x[k] + vx[k] * h;
ynew[k] = y[k] + vy[k] * h;
vx[k] = vx[k] + ax * h; vy[k] = vy[k] + ay * h;
}
for(k=0;k<n;++k)
{
x[k] = xnew[k]; y[k] = ynew[k];
if(!(x[k]<0 || x[k]>mx || y[k]<0 || y[k]>my))
{
pix[wd*(int)y[k]+(int)x[k]]=(alpha<<24)|(rd[k]<<16)|(gr[k]<<8)|bl[k];
fl=true;
}
}
}
while(fl);
}
}
