# n-body Julia #2 program

## source code

```# The Computer Language Benchmarks Game
# https://salsa.debian.org/benchmarksgame-team/benchmarksgame/

# contributed by Steven G. Johnson, Alex Arslan,
#    Kristoffer Carlsson, Jarret Revels
# based on the Java version and vectorized

module NBody

using Printf

# Constants
const solar_mass = 4 * pi * pi
const days_per_year = 365.24

# A heavenly body in the system
mutable struct Body
pos::Array{Float64, 1}
v::Array{Float64, 1}
mass::Float64
end

function offset_momentum(b::Body, p)
b.v .-= p ./ solar_mass
end

function init_sun(bodies)
local p::Array{Float64,1} = [0.0, 0.0, 0.0]
for b in bodies
p .+= b.v .* b.mass
end
offset_momentum(bodies[1], p)
end

delta = [0.,0.,0.]
for i = 1:length(bodies)
for j = i+1:length(bodies)
delta .= bodies[i].pos .- bodies[j].pos
dsq = sum(abs2, delta)
distance = sqrt(dsq)
mag = dt / (dsq * distance)

bodies[i].v .-= delta .* (bodies[j].mass * mag)
bodies[j].v .+= delta .* (bodies[i].mass * mag)
end
end

for b in bodies
b.pos .+= dt .* b.v
end
end

function energy(bodies)
local e::Float64 = 0.0
delta = [0., 0., 0.]
for i = 1:length(bodies)
e += 0.5 * bodies[i].mass *
sum(abs2, bodies[i].v)
for j = i+1:length(bodies)
delta .= bodies[i].pos .- bodies[j].pos
distance = sqrt(sum(abs2, delta))
e -= (bodies[i].mass * bodies[j].mass) / distance
end
end
e
end

function perf_nbody(N::Int=1000)
jupiter = Body( [4.84143144246472090e+00,                  # pos[1] = x
-1.16032004402742839e+00,                 # pos[2] = y
-1.03622044471123109e-01],                # pos[3] = z
[1.66007664274403694e-03 * days_per_year,   # v[1] = vx
7.69901118419740425e-03 * days_per_year,   # v[2] = vy
-6.90460016972063023e-05 * days_per_year], # v[3] = vz
9.54791938424326609e-04 * solar_mass)       # mass

saturn = Body( [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 = Body( [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 = Body( [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)

sun = Body([0.0, 0.0, 0.0], [0.0, 0.0, 0.0], solar_mass)

bodies = [sun, jupiter, saturn, uranus, neptune]

init_sun(bodies)
@printf("%.9f\n", energy(bodies))
for i = 1:N
end
@printf("%.9f\n", energy(bodies))
end

end # module

n = parse(Int,ARGS[1])
NBody.perf_nbody(n)

```

## notes, command-line, and program output

```NOTES:
julia version 1.10.2

Mon, 04 Mar 2024 00:25:02 GMT

MAKE: