nbody.java

/* The Computer Language Shootout
   http://shootout.alioth.debian.org/

   contributed by Mark C. Lewis
*/

import java.text.*;

public final class nbody {
    private static final NumberFormat nf = new DecimalFormat("#0.000000000");

    public static void main(String[] args) {
        int n = Integer.parseInt(args[0]);

        NBodySystem bodies = new NBodySystem();

        System.out.println(nf.format(bodies.energy()) );
        for (int i=0; i<n; ++i) {
            bodies.advance(0.01);
        }
        System.out.println(nf.format(bodies.energy()) );
    }
}


final class NBodySystem {
    private Body[] bodies;

    public NBodySystem(){
        bodies = new Body[]{
              Body.sun(),
              Body.jupiter(),
              Body.saturn(),
              Body.uranus(),
              Body.neptune()
        };

        double px = 0.0;
        double py = 0.0;
        double pz = 0.0;
        for(int i=0; i < bodies.length; ++i) {
            px += bodies[i].vx * bodies[i].mass;
            py += bodies[i].vy * bodies[i].mass;
            pz += bodies[i].vz * bodies[i].mass;
        }
        bodies[0].offsetMomentum(px,py,pz);
    }

    public void advance(double dt) {
       double dx, dy, dz, distance, mag;

        for(int i=0; i < bodies.length; ++i) {
            for(int j=i+1; j < bodies.length; ++j) {
                dx = bodies[i].x - bodies[j].x;
                dy = bodies[i].y - bodies[j].y;
                dz = bodies[i].z - bodies[j].z;

                distance = Math.sqrt(dx*dx + dy*dy + dz*dz);
                mag = dt / (distance * distance * distance);

                bodies[i].vx -= dx * bodies[j].mass * mag;
                bodies[i].vy -= dy * bodies[j].mass * mag;
                bodies[i].vz -= dz * bodies[j].mass * mag;

                bodies[j].vx += dx * bodies[i].mass * mag;
                bodies[j].vy += dy * bodies[i].mass * mag;
                bodies[j].vz += dz * bodies[i].mass * mag;
            }
        }

        for(int i=0; i < bodies.length; ++i) {
            bodies[i].x += dt * bodies[i].vx;
            bodies[i].y += dt * bodies[i].vy;
            bodies[i].z += dt * bodies[i].vz;
        }
    }

    public double energy(){
        double dx, dy, dz, distance;
        double e = 0.0;

        for (int i=0; i < bodies.length; ++i) {
            e += 0.5 * bodies[i].mass *
               ( bodies[i].vx * bodies[i].vx
               + bodies[i].vy * bodies[i].vy
               + bodies[i].vz * bodies[i].vz );

            for (int j=i+1; j < bodies.length; ++j) {
                dx = bodies[i].x - bodies[j].x;
                dy = bodies[i].y - bodies[j].y;
                dz = bodies[i].z - bodies[j].z;

                distance = Math.sqrt(dx*dx + dy*dy + dz*dz);
                e -= (bodies[i].mass * bodies[j].mass) / distance;
            }
        }
        return e;
    }
}


final class Body {
    static final double PI = 3.141592653589793;
    static final double SOLAR_MASS = 4 * PI * PI;
    static final double DAYS_PER_YEAR = 365.24;

    public double x, y, z, vx, vy, vz, mass;

    public Body(){}

    static Body jupiter(){
       Body p = new Body();
       p.x = 4.84143144246472090e+00;
       p.y = -1.16032004402742839e+00;
       p.z = -1.03622044471123109e-01;
       p.vx = 1.66007664274403694e-03 * DAYS_PER_YEAR;
       p.vy = 7.69901118419740425e-03 * DAYS_PER_YEAR;
       p.vz = -6.90460016972063023e-05 * DAYS_PER_YEAR;
       p.mass = 9.54791938424326609e-04 * SOLAR_MASS;
       return p;
    }

    static Body saturn(){
       Body p = new Body();
       p.x = 8.34336671824457987e+00;
       p.y = 4.12479856412430479e+00;
       p.z = -4.03523417114321381e-01;
       p.vx = -2.76742510726862411e-03 * DAYS_PER_YEAR;
       p.vy = 4.99852801234917238e-03 * DAYS_PER_YEAR;
       p.vz = 2.30417297573763929e-05 * DAYS_PER_YEAR;
       p.mass = 2.85885980666130812e-04 * SOLAR_MASS;
       return p;
    }

    static Body uranus(){
       Body p = new Body();
       p.x = 1.28943695621391310e+01;
       p.y = -1.51111514016986312e+01;
       p.z = -2.23307578892655734e-01;
       p.vx = 2.96460137564761618e-03 * DAYS_PER_YEAR;
       p.vy = 2.37847173959480950e-03 * DAYS_PER_YEAR;
       p.vz = -2.96589568540237556e-05 * DAYS_PER_YEAR;
       p.mass = 4.36624404335156298e-05 * SOLAR_MASS;
       return p;
    }

    static Body neptune(){
       Body p = new Body();
       p.x = 1.53796971148509165e+01;
       p.y = -2.59193146099879641e+01;
       p.z = 1.79258772950371181e-01;
       p.vx = 2.68067772490389322e-03 * DAYS_PER_YEAR;
       p.vy = 1.62824170038242295e-03 * DAYS_PER_YEAR;
       p.vz = -9.51592254519715870e-05 * DAYS_PER_YEAR;
       p.mass = 5.15138902046611451e-05 * SOLAR_MASS;
       return p;
    }

    static Body sun(){
       Body p = new Body();
       p.mass = SOLAR_MASS;
       return p;
    }

    Body offsetMomentum(double px, double py, double pz){
       vx = -px / SOLAR_MASS;
       vy = -py / SOLAR_MASS;
       vz = -pz / SOLAR_MASS;
       return this;
    }
}