source code
/* The Computer Language Benchmarks game
https://salsa.debian.org/benchmarksgame-team/benchmarksgame/
Naive transliteration from Michael Ferguson's Chapel program
contributed by Isaac Gouy
*/
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#define pi 3.141592653589793
#define solar_mass (4 * pi * pi)
#define days_per_year 365.24
struct planet {
double x, y, z;
double vx, vy, vz;
double mass;
};
void offset_momentum(int nbodies, struct planet * bodies)
{
double px = 0.0, py = 0.0, pz = 0.0;
for (int i = 0; i < nbodies; i++) {
struct planet * b = &(bodies[i]);
px += b->vx * b->mass;
py += b->vy * b->mass;
pz += b->vz * b->mass;
}
struct planet * b = &(bodies[0]);
b->vx = - px / solar_mass;
b->vy = - py / solar_mass;
b->vz = - pz / solar_mass;
}
double energy(int nbodies, struct planet * bodies)
{
double e = 0.0;
for (int i = 0; i < nbodies; i++) {
struct planet * b = &(bodies[i]);
double sq = b->vx * b->vx + b->vy * b->vy + b->vz * b->vz;
e += 0.5 * b->mass * sq;
for (int j=i+1; j < nbodies; ++j) {
struct planet * b2 = &(bodies[j]);
double dx = b->x - b2->x;
double dy = b->y - b2->y;
double dz = b->z - b2->z;
double dsq = dx * dx + dy * dy + dz * dz;
e -= (b->mass * b2->mass) / sqrt(dsq);
}
}
return e;
}
void advance(int nbodies, struct planet * bodies, double dt)
{
for (int i = 0; i < nbodies; i++) {
struct planet * b = &(bodies[i]);
for (int j=i+1; j < nbodies; ++j) {
struct planet * b2 = &(bodies[j]);
double dx = b->x - b2->x;
double dy = b->y - b2->y;
double dz = b->z - b2->z;
double dsq = dx * dx + dy * dy + dz * dz;
double mag = dt / (dsq * sqrt(dsq));
double mj = b2->mass * mag;
b->vx -= dx * mj;
b->vy -= dy * mj;
b->vz -= dz * mj;
double mi = b->mass * mag;
b2->vx += dx * mi;
b2->vy += dy * mi;
b2->vz += dz * mi;
}
}
for (int i = 0; i < nbodies; i++) {
struct planet * b = &(bodies[i]);
b->x += b->vx * dt;
b->y += b->vy * dt;
b->z += b->vz * dt;
}
}
int nbody(int n)
{
#define NBODIES 5
struct planet bodies[NBODIES] = {
{ /* sun */
0, 0, 0, 0, 0, 0, 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
}
};
offset_momentum(NBODIES,bodies);
printf ("%.9f\n", energy(NBODIES,bodies));
for (int i=0; i<n; ++i)
advance(NBODIES,bodies, 0.01);
printf ("%.9f\n", energy(NBODIES,bodies));
}
int main(int argc, char *argv[])
{
int n = argc > 1 ? atoi(argv[1]) : 1000;
nbody(n);
return 0;
}