A few years ago I found a thread on the Wolfram forum about swarm intelligence and emergent behavior, two things that I enjoy a lot. The title of the thread was something like ‘‘Dancing With Friends and Enemies’’ and it does not exist anymore as far as I can tell. However, I was able to find a copy here. The idea is pretty simple and below you can see my reinterpretation of it.

The behavior of the original system is based on a very simple set of rules, which I will quote from Simon Woods here:

1000 dancers assume random positions on the dance-floor. Each randomly chooses one “friend” and one “enemy”. At each step every dancer moves 0.5% closer to the centre of the floor, then takes a large step towards their friend and a small step away from their enemy. At random intervals one dancer re-chooses their friend and enemy. Here is the dance…

And the original Mathematica code is the following:

n = 5000;
r := RandomInteger[{1, n}];
f := (#/(.007 + Sqrt[#.#])) & /@ (x[[#]] - x) &;
s := With[{r1 = r}, p[[r1]] = r; q[[r1]] = r];
x = RandomReal[{-1, 1}, {n, 2}];
{p, q} = RandomInteger[{1, n}, {2, n}];
Graphics[{PointSize[0.008], Opacity[.5], Dynamic[If[r < 100, s];
Point[x = 0.995 x + 0.02 f[p] - 0.01 f[q]]]}, PlotRange -> 2]

Now, when I reimplemented it, all was done from the top of my head. I could not find the original post or anything else mentioning it. All I could remember was that the particles moved towards friends and away from enemies. Also, they moved towards the center a little bit. While on the original each particle had one enemie, I set each particle to have 30 enemies. This, in my opinion, made the system more dynamic, where one or a set of particles can become isolated from the others. Another difference is that the particles are attracted both to the center of the room and the center of mass of all particles. This helps them to stay grouped but nevertheless move around. I also used matter.js, a physics engine. The use of it allows for the particle to have mass, friction, partially elastic collisions, inertia and more, which gave the system further complexity. Furthermore, the use of the physics engine and the rectangular shape for the particles allows them to bump and even get stuck at each other, adding an extra dimension to their collective behavior.

The system is full of complex behaviors. Initially, the system just looks like a mess of particles running around. While some little time it is possible to see that the particles are chasing one another. Sometimes a circle-like pattern appears. And sometimes it looks like there is one particle running away while several particles are chasing after it. These two are my favorite patterns so far. The chasing behavior led a friend of mine to say ‘‘It looks like boids, but trying to kill each other’’.

A special situation appears when the system reassigns the friends and enemies of the particles. Visually it looks like the particles running away got tired of fleeing and are taking revenge upon their previous chasers. Since the reassignment happens gradually, it looks like a single particle is taking revenge and charging at the other particles. And then inspiring the others to do the same.

So far this project has been very amusing. I spent hours already just looking at the behavior of the swarm and tweaking their behavior. My reinterpretation / reimplementation full source code is available here. It is written in JavaScript and it uses matter.js for the physics and p5js for the graphics. Enjoy and thank you.