Cellular Chronometer Part 1: Reversing the Game of Life

I’m designing an LED matrix clock that plays Conway’s Game of Life to evolve a representation of the current time. See the whole series of blog posts about the project here.

The very first step of making the Cellular Chronometer is figuring out how I’m going to make the clock display the time. As a quick refresher, Conway’s Game of Life is what’s called a “cellular automaton“, which is a set of simple rules that’s played over a grid of cells, each of which are alive or dead. You process the grid with the rules and get a new grid with a different set of cells. Cellular automata are interesting because simple starting configurations can lead to really wild phenomena like self-replicating patterns and apparent movement.

But what I want to do is be able to get the game to display specific states. I want to populate the grid with a state that I know will evolve into a digital display of the current minute. The only reasonable way I could think to do this is to craft my desired state in advance and then do a search backwards – that is, in the reverse direction of the regular Game of Life – to find prior states. This happens to be a nontrivial problem. Played forwards, the Game of Life has very simple deterministic rules, but played backwards, there are an immense number of possible configurations to examine. In computer science terms, this problem is bounded by 2^n, where n is the number of cells in the grid. 2^n gets really big really fast. Not good.

Luckily, like so many other aspects of the Game of Life, work has been done trying to efficiently search for prior game states. I figured this out after two weeks of fruitless genetic algorithms experiments when a desperate Google search led me here. While the code didn’t prove useful to me, I managed to suss out that there are several existing algorithms that do what I need. The one that I best understood is called Duparc’s Method. The key win is that instead of being 2^n in the size of the pattern, Duparc’s method is only 2^n in the height of the grid! Since my grid is only five cells tall, this makes the search into a tractable problem, even if it does turn my MacBook into a proper lap-roaster.

duparc_illustrationIt’s hard to be 100% sure if I took the same approach as Duparc, since his original research paper is written in French. The way my algorithm works is to divide the grid in half vertically again and again until I’m working with single columns, at which point I essentially rotate the column into a row apply the same subdivision algorithm again until I reach a single cell. Single cells – both alive and dead – have a nice, cacheable method for determining all their prior states. Once I have the partial answers for a single cell, I intersect them with the answers from the neighboring cell by checking the overlap between the two. If the cells shared by the two possible solutions match, then the combined solution is also viable. Partial solutions that don’t match neighbors are discarded. This same process of merging based on overlapping cells continues back up the search tree until we reach the root, at which point we have a set of possible prior generations.

Something that took me a long time to grasp fully is that the algorithm naturally has to search in a grid bigger than the one you’re starting with. The reason for this is that while true “gardens of eden” are extremely rare, if you constrain the grid to a finite number of cells, it’s not at all hard to come up with a pattern that isn’t produceable. I ultimately came up with this sort-of logical proof to convince myself on this one:

  • In a grid of N cells, there are 2^N unique configurations
  • Some of these 2^N configurations will lead to non-unique next generations (ie, all cells dead is pretty common)
  • Therefore, it cannot be possible to generate each of the 2^N initial configurations from one of the other initial configurations.

In practice, this just means that you have to allow a “gutter” of arbitrary-valued cells around your desired image. If it’s really important to trim the excess, you just select ultimate solutions that have the extra cells as dead, since that’s the same as the border on a finite grid. The risk is that you’ll filter all the possible solutions by doing so.

As I developed this algorithm, I found that the line between “ever finishes” and “never finishes” was very fine. Most of the problems stemmed from running out of memory on my 16GB MacBook. A given target grid can have millions of intermediate states, and they need to be cached to make the algorithm run in a reasonable amount of time. The tweak that I credit with getting me all the way over the line was to discard many of the intermediate answers that, while actually unique, had the same border regions as other answers. Since the merging algorithm examines only the region of overlap between two sets of sub-solutions, any non-overlapping sections don’t actually increase your chance of finding some solution, and as such they can be discarded. This does mean that not all prior states are found, but for my purposes, I only need one.

Screen Shot 2013-09-01 at 8.57.39 PM

Along with improving the algorithm, I found that I had to tweak my font. Early on I guessed that different fonts and digit spacings would have an effect on whether a given configuration was reversible. I was surprised by how many of the configurations I wanted to generate were impossible with the grid dimensions I was enforcing. I had the algorithm report a bunch of statistics while searching so that I could see (at an aggregate level) when all possible solutions were being eliminated. I learned that particular characters were difficult to reverse, like 4s, 5s, and 0s. But the one that most surprised me was the colon I added between the hours and minutes. That guy turned out to be stubborn in some of the worst cases. In the end, I had to compromise and hand-edit certain minutes so that they would reach a prior generation.

The end result of all this hard work is 720 unique start states that will each evolve into one of the minutes of the 12-hour day. I’ll embed these states directly into the firmware for the clock so that my tiny microcontroller won’t have to do any “real” work, just run the Game of Life from each start state and advance the pointer every minute.

You can find the Ruby code for this crazy search in my GitHub repo.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s