# n-body Go #2 program

## source code

```/* The Computer Language Benchmarks Game
* https://salsa.debian.org/benchmarksgame-team/benchmarksgame/
*
* contributed by The Go Authors.
* based on C program by Christoph Bauer
* flag.Arg hack by Isaac Gouy
* modified by Bert Gijsbers.
*/

package main

import (
"flag"
"fmt"
"math"
"strconv"
)

type Body struct {
x, y, z, pad, vx, vy, vz, mass float64
}

const (
solarMass   = 4 * math.Pi * math.Pi
daysPerYear = 365.24
numBodies   = 5
)

var system = [numBodies]Body{sun, jupiter, saturn, uranus, neptune}

func momentum() {
var px, py, pz float64
for i := range system {
px += system[i].vx * system[i].mass
py += system[i].vy * system[i].mass
pz += system[i].vz * system[i].mass
}
system[0].vx = -px / solarMass
system[0].vy = -py / solarMass
system[0].vz = -pz / solarMass
}

func energy() float64 {
var e float64
for i := range system {
e += 0.5 * system[i].mass *
(system[i].vx*system[i].vx +
system[i].vy*system[i].vy +
system[i].vz*system[i].vz)
for j := i + 1; j < numBodies; j++ {
dx := system[i].x - system[j].x
dy := system[i].y - system[j].y
dz := system[i].z - system[j].z
distance := math.Sqrt(dx*dx + dy*dy + dz*dz)
e -= system[i].mass * system[j].mass / distance
}
}
return e
}

for i := range system {
for j := i + 1; j < numBodies; j++ {
dx := system[i].x - system[j].x
dy := system[i].y - system[j].y
dz := system[i].z - system[j].z

dSquared := dx*dx + dy*dy + dz*dz
distance := math.Sqrt(dSquared)
mag := dt / (dSquared * distance)

system[i].vx -= dx * system[j].mass * mag
system[i].vy -= dy * system[j].mass * mag
system[i].vz -= dz * system[j].mass * mag

system[j].vx += dx * system[i].mass * mag
system[j].vy += dy * system[i].mass * mag
system[j].vz += dz * system[i].mass * mag
}
}

for i := range system {
system[i].x += dt * system[i].vx
system[i].y += dt * system[i].vy
system[i].z += dt * system[i].vz
}
}

var (
jupiter = Body{
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,
}
saturn = Body{
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,
}
uranus = Body{
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,
}
neptune = Body{
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,
}
sun = Body{
mass: solarMass,
}
)

func main() {
n := 0
flag.Parse()
if flag.NArg() > 0 {
n, _ = strconv.Atoi(flag.Arg(0))
}

momentum()
fmt.Printf("%.9f\n", energy())
for i := 0; i < n; i++ {
}
fmt.Printf("%.9f\n", energy())
}
```

## notes, command-line, and program output

```NOTES:
go version go1.22.3 linux/amd64
GOAMD64=v2

Thu, 23 May 2024 02:06:16 GMT

MAKE:
/opt/src/go1.22.3/go/bin/go build -o nbody.go-2.go_run nbody.go-2.go

3.96s to complete and log all make actions

COMMAND LINE:
./nbody.go-2.go_run 50000000

PROGRAM OUTPUT:
-0.169075164
-0.169059907
```