/* nbody.us */

final import Math.sqrt;
final import Number.parse as num;
final import Shell.args as args;

var PI	 	        = 3.141592653589793;
var SOLAR_MASS 	    = 4 * PI * PI;
var DAYS_PER_YEAR	= 365.24;

var sun = {
    x: 0,
    y: 0,
    z: 0,
    vx: 0,
    vy: 0,
    vz: 0,
    mass: SOLAR_MASS
};

function advance(bodies, nbody, dt) {
    for (var i = 0; i < nbody; i++) {
        var bi = bodies[i];
        for (var j = i + 1; j < nbody; j++) {
            var bj = bodies[j];
			var dx = bi.x - bj.x, dy = bi.y - bj.y, dz = bi.z - bj.z;
			var distance = sqrt(dx * dx + dy * dy + dz * dz);
			var mag = dt / (distance * distance * distance);
			var bim = bi.mass * mag, bjm = bj.mass * mag);
			bi.vx -= dx * bj.m;
			bi.vy -= dy * bj.m;
			bi.vz -= dz * bj.m;
			bj.vx += dx * bi.m;
			bj.vy += dy * bi.m;
			bj.vz += dz * bi.m;
		}
	}
	for (var i = 0; i < nbody; i++) {
		var bi = bodies[i];
		bi.x += dt * bi.vx;
		bi.y += dt * bi.vy;
		bi.z += dt * bi.vz;
	}
}

var N;
if (args.length > 0)
    N = args[0];
else
    N = 1000;

var bodies = [ Sun ];
var nbody = bodies.length;

offsetMomentum(bodies, nbody);

print(String.format("%0.9f\n", energy(bodies, nbody)));
for (var i = 0; i < N; i++)
    advance(bodies, nbody, 0.01);
print(String.format("%0.9f\n", energy(bodies, nbody)));