nbody.java

package shootout.nbody;


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

   contributed by Mark C. Lewis
   modified slightly by Chad Whipkey
   steady state approximation by Isaac Gouy
*/

public final class nbody {

	public static void main(String[] args) {
		long start = System.currentTimeMillis();
		for (int i = 0; i < 65; ++i) nbody.program_main(args, false);
		nbody.program_main(args, true);
		long total = System.currentTimeMillis() - start;
		System.out.println("[NBody-Java Benchmark Result: " + total + "]");
	}

	public static void program_main(String[] args, boolean isWarm) {
		int n = 10000;


		try {
			n = Integer.parseInt(args[0]);
		} catch (Exception e) {
		}

		NBodySystem bodies = new NBodySystem();

		double e = bodies.energy();
		if (isWarm) System.out.printf("%.9f\n", e);
		for (int i = 0; i < n; ++i)
			bodies.advance(0.01);
		e = bodies.energy();
		if (isWarm) System.out.printf("%.9f\n", bodies.energy());


	}

	static 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) {

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

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

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

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

			for (Body body : bodies) {
				body.x += dt * body.vx;
				body.y += dt * body.vy;
				body.z += dt * body.vz;
			}
		}

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

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

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

					distance = Math.sqrt(dx * dx + dy * dy + dz * dz);
					e -= (iBody.mass * jBody.mass) / distance;
				}
			}
			return e;
		}
	}


	static 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;
		}
	}

}