quietsamurai98 - 1 month ago 5
Java Question

# Optimizing an n-body gravitational attraction algorithm

I'm writing a simulation of a 2d protoplanetary disk, and right now, the most time consuming bit of code is calculating the gravitational attraction. This is the code I'm currently using.

``````for(int i=0; i<particleCount; i++){
if(boolArray[i]){    //boolArray is linked with particleArray, false means the linked particle has collided with another particle and no longer exists
double iX = particleArray[i].getXPosition();
double iY = particleArray[i].getYPosition();
double iM = particleArray[i].getMass();
for(int j=0; j<particleCount; j++){
if(i!=j&&boolArray[j]){
double rX = iX-particleArray[j].getXPosition();
double rY = iY-particleArray[j].getYPosition();
double rT = Math.sqrt(rX*rX+rY*rY);
double rF = rT*rT*rT;
double fT = -constantGravity*iM*particleArray[j].getMass()/rF;
particleArray[i].updateForce(rX*fT, rY*fT);
}
}
}
}
``````

Does anybody have any ideas on how to speed it up? I think the sqrt in

``````double rT = Math.sqrt(rX*rX+rY*rY);
``````

is the biggest culprit, but I'm not sure if I could even get rid of it.

The compile-ready code can be found at https://github.com/quietsamurai98/2D-Accretion-Simulation/tree/Trails-png

You are calculating twice for each pair of point. Try this.

``````for (int i = 0; i < particleCount; i++) {
if (boolArray[i]) { // boolArray is linked with particleArray, false
// means the linked particle has collided with
// another particle and no longer exists
double iX = particleArray[i].getXPosition();
double iY = particleArray[i].getYPosition();
double iM = particleArray[i].getMass();
for (int j = i + 1; j < particleCount; j++) {
if (boolArray[j]) {
double rX = iX - particleArray[j].getXPosition();
double rY = iY - particleArray[j].getYPosition();
double rT = Math.sqrt(rX * rX + rY * rY);
double rF = rT * rT * rT;
double fT = -constantGravity * iM * particleArray[j].getMass() / rF;
particleArray[i].updateForce(rX * fT, rY * fT);
particleArray[j].updateForce(-rX * fT, -rY * fT);
}
}
}
}
``````