NBody Benchmark

NBody.cs

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

   contributed by Isaac Gouy, optimization and use of more C# idioms by Robert F. Tobler
*/

using System;

public static class Program {
    public static void Main () {
        int n = 10000;
        NBodySystem bodies = new NBodySystem();
      
        Console.WriteLine("{0:f9}", bodies.Energy());
      
        for (int i = 0; i < n; i++)
            bodies.Advance(0.01);
      
        Console.WriteLine("{0:f9}", bodies.Energy());
    }
}

class Body { public double x, y, z, vx, vy, vz, mass; }
class Pair { public Body bi, bj; }

class NBodySystem {
    private Body[] bodies;
    private Pair[] pairs;

    const double Pi = 3.141592653589793;
    const double Solarmass = 4 * Pi * Pi;
    const double DaysPeryear = 365.24;

    public NBodySystem () {
        bodies = new Body[] {
            new Body() { // Sun
                mass = Solarmass,
            },
            new Body() { // Jupiter
                x = 4.84143144246472090e+00,
                y = -1.16032004402742839e+00,
                z = -1.03622044471123109e-01,
                vx = 1.66007664274403694e-03 * DaysPeryear,
                vy = 7.69901118419740425e-03 * DaysPeryear,
                vz = -6.90460016972063023e-05 * DaysPeryear,
                mass = 9.54791938424326609e-04 * Solarmass,
            },
            new Body() { // Saturn
                x = 8.34336671824457987e+00,
                y = 4.12479856412430479e+00,
                z = -4.03523417114321381e-01,
                vx = -2.76742510726862411e-03 * DaysPeryear,
                vy = 4.99852801234917238e-03 * DaysPeryear,
                vz = 2.30417297573763929e-05 * DaysPeryear,
                mass = 2.85885980666130812e-04 * Solarmass,
            },
            new Body() { // Uranus
                x = 1.28943695621391310e+01,
                y = -1.51111514016986312e+01,
                z = -2.23307578892655734e-01,
                vx = 2.96460137564761618e-03 * DaysPeryear,
                vy = 2.37847173959480950e-03 * DaysPeryear,
                vz = -2.96589568540237556e-05 * DaysPeryear,
                mass = 4.36624404335156298e-05 * Solarmass,
            },
            new Body() { // Neptune
                x = 1.53796971148509165e+01,
                y = -2.59193146099879641e+01,
                z = 1.79258772950371181e-01,
                vx = 2.68067772490389322e-03 * DaysPeryear,
                vy = 1.62824170038242295e-03 * DaysPeryear,
                vz = -9.51592254519715870e-05 * DaysPeryear,
                mass = 5.15138902046611451e-05 * Solarmass,
            },
        };

        pairs = new Pair[bodies.Length * (bodies.Length - 1) / 2];
        int pi = 0;
        for (int i = 0; i < bodies.Length - 1; i++)
            for (int j = i + 1; j < bodies.Length; j++)
                pairs[pi++] = new Pair() { bi = bodies[i], bj = bodies[j] };

        double px = 0.0, py = 0.0, pz = 0.0;
        foreach (var b in bodies) {
            px += b.vx * b.mass; py += b.vy * b.mass; pz += b.vz * b.mass;
        }
        var sol = bodies[0];
        sol.vx = -px / Solarmass; sol.vy = -py / Solarmass; sol.vz = -pz / Solarmass;
    }

    public void Advance (double dt) {
        foreach (var p in pairs) {
            Body bi = p.bi, bj = p.bj;
            double dx = bi.x - bj.x, dy = bi.y - bj.y, dz = bi.z - bj.z;
            double d2 = dx * dx + dy * dy + dz * dz;
            double mag = dt / (d2 * Math.Sqrt(d2));
            bi.vx -= dx * bj.mass * mag; bj.vx += dx * bi.mass * mag;
            bi.vy -= dy * bj.mass * mag; bj.vy += dy * bi.mass * mag;
            bi.vz -= dz * bj.mass * mag; bj.vz += dz * bi.mass * mag;
        }
        foreach (var b in bodies) {
            b.x += dt * b.vx; b.y += dt * b.vy; b.z += dt * b.vz;
        }
    }

    public double Energy () {
        double e = 0.0;
        for (int i = 0; i < bodies.Length; i++) {
            var bi = bodies[i];
            e += 0.5 * bi.mass * (bi.vx * bi.vx + bi.vy * bi.vy + bi.vz * bi.vz);
            for (int j = i + 1; j < bodies.Length; j++) {
                var bj = bodies[j];
                double dx = bi.x - bj.x, dy = bi.y - bj.y, dz = bi.z - bj.z;
                e -= (bi.mass * bj.mass) / Math.Sqrt(dx * dx + dy * dy + dz * dz);
            }
        }
        return e;
    }
}