namespace nbody

{

/* The Computer Language Benchmarks Game

https://salsa.debian.org/benchmarksgame-team/benchmarksgame/

contributed by Isaac Gouy

modified by Robert F. Tobler

modified by Eric P. Nusbaum

*/

using System;

public class NBody

{

public static void Main(String[] args)

{

int n = args.Length > 0 ? Int32.Parse(args[0]) : 1000;

NBodySystem sys = new NBodySystem();

sys.OffsetMomentum();

Console.WriteLine("{0:f9}", sys.Energy());

for (int i = 0; i < n; i++) sys.Advance(0.01);

Console.WriteLine("{0:f9}", sys.Energy());

}

}

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

public class NBodySystem

{

private Body[] _bodies;

const byte bodyCount = 5;

const double Pi = 3.141592653589793;

const double Solarmass = 4 * Pi * Pi;

const double DaysPeryear = 365.24;

public NBodySystem()

{

_bodies = new[]

{

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,

},

};

}

public void OffsetMomentum()

{

double px = 0, py = 0, pz = 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)

{

for (var i = 0; i < bodyCount; i++)

{

var bi = _bodies[i];

var x = bi.x;

var y = bi.y;

var z = bi.z;

var vx = bi.vx;

var vy = bi.vy;

var vz = bi.vz;

var mi = bi.mass;

for (var j = i + 1; j < bodyCount; j++)

{

var bj = _bodies[j];

var dx = x - bj.x;

var dy = y - bj.y;

var dz = z - bj.z;

var d2 = dx * dx + dy * dy + dz * dz;

var mag = dt / (d2 * Math.Sqrt(d2));

var bj_m_mag = bj.mass * mag;

vx -= dx * bj_m_mag;

vy -= dy * bj_m_mag;

vz -= dz * bj_m_mag;

var bi_m_mag = mi * mag;

bj.vx += dx * bi_m_mag;

bj.vy += dy * bi_m_mag;

bj.vz += dz * bi_m_mag;

}

bi.vx = vx;

bi.vy = vy;

bi.vz = vz;

bi.x += vx * dt;

bi.y += vy * dt;

bi.z += vz * dt;

}

}

public double Energy()

{

double e = 0.0;

for (int i = 0; i < bodyCount; 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 < bodyCount; 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;

}

}

}