# This file was formerly a part of Julia. License is MIT: https://julialang.org/license

#
# The Computer Language Benchmarks Game
# nbody benchmark
# http://shootout.alioth.debian.org/u32/performance.php?test=nbody
#
# A vectorized version of the Java port from nbody.jl
#

module NBodyVec

# 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

function advance(bodies, dt)
    for i = 1:length(bodies)
        for j = i+1:length(bodies)
            delta = bodies[i].pos - bodies[j].pos
            dsq = sum(delta .^ 2)
            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
    for i = 1:length(bodies)
        e += 0.5 * bodies[i].mass *
             sum(bodies[i].v .^ 2)
        for j = i+1:length(bodies)
            delta = bodies[i].pos - bodies[j].pos
            distance = sqrt(sum(delta .^ 2))
            e -= (bodies[i].mass * bodies[j].mass) / distance
        end
    end
    e
end


function nbody_vec(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
        advance(bodies, 0.01)
    end
#    @printf("%.9f\n", energy(bodies))
end

end # module
