Animation of colliding particles based on 'particles' (Chapter 2 of Paul Smaldino’s book, Modeling Social Behavior)

particles.jl

using Distances
using Plots

function ninradius(indiv, radius, distances)
    findall(distances[indiv, :] .< radius)
end

function getdists(particlelist)
    pairwise(Euclidean(), reduce(hcat, [[p.x, p.y] for p in particlelist]), dims=2)
end

mutable struct Particle
    x
    y
    heading
    clash
end

nparticles = 150
particles = [
    Particle(
        rand() * 100,
        rand() * 100,
        rand() * 2 * pi,
        false
    ) for i in 1:nparticles
]

anim = @gif for iter in 1:400
    dists = getdists(particles)
    for i in 1:nparticles
        particles[i].clash = false
        nlist = ninradius(i, 2, dists)
        if length(nlist) > 1
            particles[i].heading = rand() * 2 * pi
            particles[i].clash = true
        end
    end

    # Update positions
    for i in 1:nparticles
        particles[i].x = mod(particles[i].x + cos(particles[i].heading), 100)
        particles[i].y = mod(particles[i].y + sin(particles[i].heading), 100)
    end

    scatter(
        [particle.x for particle in particles],
        [particle.y for particle in particles],
        color=[particle.clash ? :red : :blue for particle in particles],
        label=false, aspect_ratio=:equal, title="Iter $iter",
        xlims=(-5, 105), ylims=(-5, 105)
    )
end