Just Friends/Snippet

Just Friends is a two-state rule created by David Bell and first described in June 2000, making it one of the first isotropic non-totalistic cellular automata considered. It normally stabilizes into a maze-like structure consisting of lines and oscillators with escaping 3-cell spaceships.

The original description goes as follows:

“ My idea was to investigate Life rules which had birth on only two

neighbors. Using totalistic rules, this is a lose (in my opinion) since objects explode when they get two cells adjacent vertically or horizontally. But if we abandon totalistic rules, then we can fix this problem. I was looking for the combination of nicest behavior and simplest rule I could find.

The most interesting rule I found was this one:

A cell stays alive only if it has 1 or 2 live neighbors (in any position). A cell is born only if it has exactly two live neighbors which are not adjacent vertically or horizontally.

So in the following figure, the central cell is born if there are live cells at any two cells marked ac, ad, ae, af, ag, bd, be, bf, bg, bh, ce, cf, cg, ch, df, dg, dh, eg, eh, or fh.

abc
hid
gfe

” — David Bell, Mon, 19 Jun 2000

In Hensel notation, the rule is written as B2-a/S12, where "B2-a" transition indicates that birth does not happen on a domino surface. This prevents the rule from being explosive like B2/S12. A related rulespace named "SansDomino" was briefly considered in 2012, in which Just Friends would be expressed as sansdomino_s12.