The Right Side of the Boat » Magic auto-drafter

Magic auto-drafter

August 29th, 2009 | Category: Games, Programming

This is kind of cool.

I’ve had an idea for a while to make an automatic draft picker for Magic: the Gathering. The idea was to use a sort of naive Bayes approach, using the records of previous drafts to estimate:

the probability that a player, given a pack with cards A and B, will pick A over B;
the probability that a player, given a pack with card A and having already drafted card B, will pick A over the other cards in the pack.

The idea is that the former gives an estimate of card quality, and the latter estimates the influence of earlier picks on each card. To use this in a draft, you (that is to say, the program that’s doing it for you) just multiply all the probabilities together for each card, and pick the one with the highest score.

So I finished coding it up this morning. Then I trained it using four Magic 2010 drafts from the MtG website (e.g. this one), then re-ran our draft from a couple of weeks ago, to see what would have happened if we were all replaced by auto-drafters.

The results are below the fold. And it’s not completely ridiculous. It’s probably less focused in colours than it could be; maybe there’s some tweaking to be done by weighting the influence of previous picks. But at a glance, I can’t see anyone’s card pool that couldn’t be made into, say, a two-colour deck with a third splash. (Edit: Now that I think about it, this is probably true of any 45 random cards. Hmm. Well, most of the pools have a strong leaning towards two or at most three colours, and Andrew C could almost get away with mono-green.)

Bizarrely, it’s drafted a red-blue deck for me instead of the white-green deck I actually played (although I did feel bad about not using that first-pick Lightning Bolt in the second pack).

There are a few hiccups because the training set of four drafts wasn’t big enough – there’s no way Ajani should have gone fifth pick, for example. I’m optimistic that throwing more data at it would make it better. In an ideal world, I’d somehow get hold of the results of every Magic Online draft to pump through it, and it would represent the collective wisdom of every online player everywhere.

In fact, if anyone from Wizards would like to do this, let me know.

Actually, that’s not as silly as it sounds. (Well, letting me do it probably is.) There’s already an AI of some kind controlling the online draft simulator. That could be something similar to my system, or it could just be a hand-crafted pick order along with some kind of futzing to get it to pick roughly consistent colours. It’d be interesting to find out how it works. (But they’ll never tell us.)

Interestingly, some kind of auto-picker could be useful for Magic Online. If you don’t pick a card within the time limit in an online draft, it randomly picks one for you. (At least, it was random last time I checked. I haven’t drafted online for a while.) MO has recurring problems with lag and dropped connections, so it would make a lot of people very happy to have an auto-drafter that makes vaguely intelligent choices when they can’t do it themselves.

Now, it can’t be too good, or you could start relying on it to do the draft for you. This could be a showstopper, since the outcome of a draft is supposed to depend on the players’ skill. (In fact, there’s a good chance that someone in Wizards has already had this conversation.) But maybe an auto-drafter that’s not a genius, but knows not to take the basic land and will generally pick playable cards in your colours, would be just enough of a safety net that people wouldn’t be driven to mass killings when their connection drops. Maybe.

I might put the code up later (after I do a cleanup and/or rewrite – it’s pretty ugly right now). Meanwhile, keep reading for the results of the re-draft.

Edit 2: I tried doubling, then quadrupling the effect of the already-drafted cards on the next pick. This puts nearly everyone solidly into two colours. (Unfortunately it leaves Mr Shellshear with an impressive blue-white deck, and anyone who remembers our first Lorwyn draft will realise what a bad idea that is. Apparently some people do just open all the good cards.) I’d post the new results, but I’m wary of picking at this too much, and would rather just leave the original results as they are until I get around to cleaning up the code.