Much better, @mia_.

While Pico-8 is pretty fast, you might use the DEL command for any circles that are no longer visible in being overwritten by their predecessor to ensure a stable speed.

In your SWAP function:

unction swap(arr, a, b)
    tmp = arr[a]
    arr[a] = arr[b]
    arr[b] = tmp

end

You can shorten this to one line.

arr[a],arr[b]=arr[b],arr[a]

Your coding style strikes me as someone who is familiar with BASIC. I think you'll find Pico-8 has all kinds of curious shortcuts not all available to other programming languages. :)

Here is a STAR to get you started.

simple but cool