Wednesday, November 03, 2010

Starcraft 2 build order optimizer using genetic algorithm

Over on a forum called teamliquid, a user by the name of Lomilar posted a fairly long thread about a program he had written that optimized build orders for the zerg race in starcraft. He eventually cleaned up his code and posted the code to googlecode. The program is called EvolutionChamber (a clever name, as it’s the name of one of the buildings in the game), and it uses genetic algorithms to find build orders.
A build order refers to the exact opening steps you take early in the game that best supports the strategy you are trying to conduct. These early games are all about balancing spending money on your economic foundation (making more workers), making units (rush!), or building new buildings or getting new upgrades (tech!).
Build orders generally only cover the very early game because once you’ve scouted the enemy, you have to begin to react to what he’s doing and modifying it as you go. In other words, and a perfect aphorism, no battle plan survives first contact with the enemy. In this way, build orders in real-time strategy are very much akin to openings in chess. They set up the soul of the entire game about to be played, and some players prefer to force certain types of games, depending on what kind of opening they choose to do.
One of the reasons build-order optimization is so important is that you can discover openings that “hard-counter” other openings. If I can get an army of N size into your base when you do opening X, you will always lose.
Enough with the abstract, here’s what you need to know:
  1. The program in question optimizes Zerg build orders (which is one race in starcraft), this is a rather significant choice because the mechanics of the zerg race are arguably the most difficult to manage (esp. for build order optimization).
  2. Of most interest are “rush” build orders. This means “how quickly can I get N of this type of unit?”.
  3. There are two primary resources that workers collect in starcraft: gas and minerals.
  4. Zerg also have a third de factor resource: larva. Larva are used to create ALL zerg units, including workers.  So long as you have less than three,  they regenerate at a fixed rate (note: this means any time spent at three larva delays all future larva production — very bad).
  5. Most units require some building to be constructed in order to be “unlocked” (and many of these buildings require others as prerequisites – this is the so-called tech tree)
  6. Creating a building causes you to lose the worker who creates it (so the longer you can wait, the more resources that worker can collect before building the building)
These “rules” provide for an extremely complicated search space to find optimal build orders. Essentially, you want to make the exact number of workers you need as quickly as possible (and then no more). Losing a worker when you make a building, and delaying all future larva when at three larva make the dynamics extremely complicated.

Here’s a video of someone doing the ‘all-in’ variant of this build against a real player:

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Massive-Man said...

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Massive-Man said...

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