Port of C version of nbody to D from https://benchmarksgame-team.pages.debian.net/benchmarksgame/program/nbody-gcc-1.html

nbody.d

import std.stdio;                                                                                                                                                                                                                                                         [59/187]
// import std.math;
import core.stdc.stdio;
import core.stdc.math;

enum pi = 3.141592653589793;
enum solar_mass = (4 * pi * pi);
enum days_per_year = 365.24;

struct Planet {
    double x, y, z;
    double vx, vy, vz;
    double mass;
}

void advance(Planet[] bodies, double dt)
{
    foreach (i; 0 .. bodies.length) {
        auto b = &(bodies[i]);
        foreach (j; i+1 .. bodies.length) {
            auto b2 = &(bodies[j]);
            auto dx = b.x - b2.x;
            auto dy = b.y - b2.y;
            auto dz = b.z - b2.z;
            auto distance = sqrt(dx * dx + dy * dy + dz * dz);
            auto mag = dt / (distance * distance * distance);
            b.vx -= dx * b2.mass * mag;
            b.vy -= dy * b2.mass * mag;
            b.vz -= dz * b2.mass * mag;
            b2.vx += dx * b.mass * mag;
            b2.vy += dy * b.mass * mag;
            b2.vz += dz * b.mass * mag;
        }
    }
    foreach (i; 0 .. bodies.length) {
        auto b = &(bodies[i]);
        b.x += dt * b.vx;
        b.y += dt * b.vy;
        b.z += dt * b.vz;
    }
}

double energy(Planet[] bodies)
{
    double e = 0.0;
    foreach (i; 0 .. bodies.length) {
        auto b = &(bodies[i]);
        e += 0.5 * b.mass * (b.vx * b.vx + b.vy * b.vy + b.vz * b.vz);
        foreach (j; i+1 .. bodies.length) {
            auto b2 = &(bodies[j]);
            auto dx = b.x - b2.x;
            auto dy = b.y - b2.y;
            auto dz = b.z - b2.z;
            auto distance = sqrt(dx * dx + dy * dy + dz * dz);
            e -= (b.mass * b2.mass) / distance;
        }
    }
    return e;
}

void offset_momentum(Planet[] bodies)
{
    double px = 0.0, py = 0.0, pz = 0.0;
    foreach (i; 0 .. bodies.length) {
        px += bodies[i].vx * bodies[i].mass;
        py += bodies[i].vy * bodies[i].mass;
        pz += bodies[i].vz * bodies[i].mass;
    }
    bodies[0].vx = - px / solar_mass;
    bodies[0].vy = - py / solar_mass;
    bodies[0].vz = - pz / solar_mass;
}

enum NBODIES = 5;

Planet[NBODIES] bodies = [
    {                               /* sun */
        x: 0.0, y: 0.0, z: 0.0, vx: 0.0, vy: 0.0, vz: 0.0, mass:solar_mass
    },
    {                               /* jupiter */
        4.84143144246472090e+00,
        -1.16032004402742839e+00,
        -1.03622044471123109e-01,
        1.66007664274403694e-03 * days_per_year,
        7.69901118419740425e-03 * days_per_year,
        -6.90460016972063023e-05 * days_per_year,
        9.54791938424326609e-04 * solar_mass
    },
    {                               /* saturn */
        8.34336671824457987e+00,
        4.12479856412430479e+00,
        -4.03523417114321381e-01,
        -2.76742510726862411e-03 * days_per_year,
        4.99852801234917238e-03 * days_per_year,
        2.30417297573763929e-05 * days_per_year,
        2.85885980666130812e-04 * solar_mass
    },
    {                               /* uranus */
        1.28943695621391310e+01,
        -1.51111514016986312e+01,
        -2.23307578892655734e-01,
        2.96460137564761618e-03 * days_per_year,
        2.37847173959480950e-03 * days_per_year,
        -2.96589568540237556e-05 * days_per_year,
        4.36624404335156298e-05 * solar_mass
    },
    {                               /* neptune */
        1.53796971148509165e+01,
        -2.59193146099879641e+01,
        1.79258772950371181e-01,
        2.68067772490389322e-03 * days_per_year,
        1.62824170038242295e-03 * days_per_year,
        -9.51592254519715870e-05 * days_per_year,
        5.15138902046611451e-05 * solar_mass
    }
];

void main(string[] args)
{
    import std.conv;
    int n = to!(int)(args[1]);

    offset_momentum(bodies);
    printf("%.9f\n", energy(bodies));
    foreach (i; 0 .. n)
        advance(bodies, 0.01);
    printf("%.9f\n", energy(bodies));
}