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@Arkoniak
Last active Dec 14, 2020
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NBabel with 4th dimension for simd calculations.
"""
This is an implementation of the NBabel N-body problem.
See nbabel.org for more information.
This is 'naive & native' Julia with type and loop annotations for
fastmath and vectorization, because this is really is no effort.
Without annotations the code will be as slow as naive Python/IDL.
include("nbabel.jl")
NBabel("IC/input128", show=true)
Julius Donnert INAF-IRA 2017
"""
using Printf
using DelimitedFiles
tbeg = 0.0
tend = 10.0
dt = 0.001
function NBabel(fname::String; tend = tend, dt = dt, show=false)
if show
println("Reading file : $fname")
end
ID, mass, pos, vel = read_ICs(fname)
return NBabelCalcs(ID, mass, pos, vel, tend, dt, show)
end
function NBabelCalcs(ID, mass, pos, vel, tend = tend, dt = dt, show=false)
acc = similar(vel)
acc = compute_acceleration(pos, mass, acc)
last_acc = copy(acc)
Ekin, Epot = compute_energy(pos, vel, mass)
Etot_ICs = Ekin + Epot
t = 0.0
nstep = 0
while t < tend
pos = update_positions(pos, vel, acc, dt)
acc, last_acc = last_acc, acc
acc = compute_acceleration(pos, mass, acc)
vel = update_velocities(vel, acc, last_acc, dt)
t += dt
nstep += 1
if show && nstep%100 == 0
Ekin, Epot = compute_energy(pos, vel, mass)
Etot = Ekin + Epot
dE = (Etot - Etot_ICs)/Etot_ICs
@printf "t = %g, Etot=%g, Ekin=%g, Epot=%g, dE=%g \n" t Etot Ekin Epot dE
end
end
Ekin, Epot = compute_energy(pos, vel, mass)
Etot = Ekin + Epot
return (; Ekin, Epot, Etot)
# return size(pos,1)
end
function update_positions(pos, vel, acc, dt)
N = length(pos)
@fastmath @inbounds @simd for i in 1:N
pos[i] = @. (0.5 * acc[i] * dt + vel[i])*dt + pos[i]
end
return pos
end
function update_velocities(vel, acc, last_acc, dt)
N = length(vel)
@fastmath @inbounds @simd for i in 1:N
vel[i] = @. vel[i] + 0.5 * dt * (acc[i] + last_acc[i])
end
return vel
end
"""
Force calculation.
"""
function compute_acceleration(pos, mass, acc)
N = length(pos)
@inbounds for i in 1:N
acc[i] = (0.0, 0.0, 0.0, 0.0)
end
@inbounds for i in 1:N
pos_i = pos[i]
m_i = mass[i]
@fastmath @inbounds @simd for j = i+1:N
dr = pos_i .- pos[j]
rinv3 = 1/sqrt(sum(dr .^ 2)) ^ 3
dr = rinv3 .* dr
acc[i] = @. acc[i] - mass[j] * dr
acc[j] = @. acc[j] + m_i * dr
end
end
return acc
end
"""
Kinetic and potential energy.
"""
function compute_energy(pos, vel, mass)
N = length(vel)
Ekin = 0.0
@simd for i = 1:N
@inbounds Ekin += 0.5 * mass[i] * sum(vel[i] .^ 2)
end
Epot = 0.0
@inbounds for i = 1:N-1
pos_i = pos[i]
@fastmath @inbounds for j = i+1:N
dr = pos_i .- pos[j]
rinv = 1/sqrt(sum(dr .^ 2))
Epot -= mass[i] * mass[j] * rinv
end
end
return Ekin, Epot
end
function read_ICs(fname)
ICs = readdlm(fname)
N = size(ICs,1)
pos = Vector{Tuple{Float64, Float64, Float64, Float64}}(undef, N)
vel = Vector{Tuple{Float64, Float64, Float64, Float64}}(undef, N)
id = @view ICs[:, 1]
mass = @view ICs[:, 2]
for i in axes(ICs, 1)
pos[i] = (ICs[i, 3], ICs[i, 4], ICs[i, 5], 0.0)
end
for i in axes(ICs, 1)
vel[i] = (ICs[i, 6], ICs[i, 7], ICs[i, 8], 0.0)
end
return id, mass, pos, vel
end
function main(args)
NBabel(args[1], show=true)
end
main(ARGS)
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