# The Computer LanguageBenchmarks Game

## n-body Matz's Interpreter #2 program

### source code

```# The Computer Language Benchmarks Game
# http://benchmarksgame.alioth.debian.org
#
# Optimized for Ruby by Jesse Millikan
# From version ported by Michael Neumann from the C gcc version,
# which was written by Christoph Bauer.

SOLAR_MASS = 4 * Math::PI**2
DAYS_PER_YEAR = 365.24

class Planet
attr_accessor :x, :y, :z, :vx, :vy, :vz, :mass

def initialize(x, y, z, vx, vy, vz, mass)
@x, @y, @z = x, y, z
@vx, @vy, @vz = vx * DAYS_PER_YEAR, vy * DAYS_PER_YEAR, vz * DAYS_PER_YEAR
@mass = mass * SOLAR_MASS
end

def move_from_i(bodies, nbodies, dt, i)
while i < nbodies
b2 = bodies[i]
dx = @x - b2.x
dy = @y - b2.y
dz = @z - b2.z

distance = Math.sqrt(dx * dx + dy * dy + dz * dz)
mag = dt / (distance * distance * distance)
b_mass_mag, b2_mass_mag = @mass * mag, b2.mass * mag

@vx -= dx * b2_mass_mag
@vy -= dy * b2_mass_mag
@vz -= dz * b2_mass_mag
b2.vx += dx * b_mass_mag
b2.vy += dy * b_mass_mag
b2.vz += dz * b_mass_mag
i += 1
end

@x += dt * @vx
@y += dt * @vy
@z += dt * @vz
end
end

def energy(bodies)
e = 0.0
nbodies = bodies.size

for i in 0 ... nbodies
b = bodies[i]
e += 0.5 * b.mass * (b.vx * b.vx + b.vy * b.vy + b.vz * b.vz)
for j in (i + 1) ... nbodies
b2 = bodies[j]
dx = b.x - b2.x
dy = b.y - b2.y
dz = b.z - b2.z
distance = Math.sqrt(dx * dx + dy * dy + dz * dz)
e -= (b.mass * b2.mass) / distance
end
end
e
end

def offset_momentum(bodies)
px, py, pz = 0.0, 0.0, 0.0

for b in bodies
m = b.mass
px += b.vx * m
py += b.vy * m
pz += b.vz * m
end

b = bodies[0]
b.vx = - px / SOLAR_MASS
b.vy = - py / SOLAR_MASS
b.vz = - pz / SOLAR_MASS
end

BODIES = [
# sun
Planet.new(0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 1.0),

# jupiter
Planet.new(
4.84143144246472090e+00,
-1.16032004402742839e+00,
-1.03622044471123109e-01,
1.66007664274403694e-03,
7.69901118419740425e-03,
-6.90460016972063023e-05,
9.54791938424326609e-04),

# saturn
Planet.new(
8.34336671824457987e+00,
4.12479856412430479e+00,
-4.03523417114321381e-01,
-2.76742510726862411e-03,
4.99852801234917238e-03,
2.30417297573763929e-05,
2.85885980666130812e-04),

# uranus
Planet.new(
1.28943695621391310e+01,
-1.51111514016986312e+01,
-2.23307578892655734e-01,
2.96460137564761618e-03,
2.37847173959480950e-03,
-2.96589568540237556e-05,
4.36624404335156298e-05),

# neptune
Planet.new(
1.53796971148509165e+01,
-2.59193146099879641e+01,
1.79258772950371181e-01,
2.68067772490389322e-03,
1.62824170038242295e-03,
-9.51592254519715870e-05,
5.15138902046611451e-05)
]

n = Integer(ARGV[0])

offset_momentum(BODIES)

puts "%.9f" % energy(BODIES)

nbodies = BODIES.size
dt = 0.01

n.times do
i = 0
while i < nbodies
b = BODIES[i]
b.move_from_i(BODIES, nbodies, dt, i + 1)
i += 1
end
end

puts "%.9f" % energy(BODIES)
```

### notes, command-line, and program output

```NOTES: