Starcraft AI blog

This post is about timing openings to see if they are good for their intended purpose. I started with one opening, assumed I could guess its purpose, and timed a range of related openings with the same purpose to see which one is best.

McRaveZ has been playing an outlandish opening in ZvZ games, such as McRaveZ-Killerbot by Marian Devecka. The opening is 5 pool... but no fast zerglings, a lair is next. The opening is so strange that it looks like a bug, but McRave has kept playing it after updates, so maybe not.

- 5 pool
- drones to 9
- extractor
- overlord
- drone (back to 9 drones)
- ... wait for the overlord to finish
- drone
- zerglings until there is enough gas for the lair (1 or 2 pairs)
- lair
- zerglings until the lair completes
- spire

The aim logically must be fast mutalisks with little regard for defense against enemy zerglings. Since the opponent often attacks with zerglings, McRaveZ loses a lot of ZvZ games. The spire does start somewhat early though, at about 3:40. Is there a chance that the opening could make sense in some situations? That’s the starting point for my analysis.

Let’s suppose that the goal is to rush mutalisks. A spire takes a long time to finish, and we have 9 drones, so if we choose we can save up enough resources and larvas to start 3 mutalisks when the spire finishes. Similarly, a lair takes a long time to finish, so while it is constructing we can save up enough for the spire. So measuring when the lair starts tells us how fast we can rush mutalisks with a given opening. We don’t have to measure the rest, it follows mathematically.

A lair has 3 prerequisites: 150 minerals, 100 gas, and a completed spawning pool. Intuitively, the reason to suspect that the 5 pool lair build is inefficient is that it gets the spawning pool long before the minerals and gas. Saving up for the spawning pool causes a gap in drone production that delays income, so (other things being equal) you prefer to get the spawning pool as late as possible without delaying the end goal. The rule of thumb is, in a rush build you prefer all the prerequisites on each step of the critical path for your rush to finish at close to the same time. For example, in a zergling rush with an early pool, you time it so that when the spawning pool finishes, you have 150 minerals and 3 larvas so you can make zerglings immediately.

I timed the lair start for a range of related builds. Each (x+1)PoolLair build gets 1 more drone before the spawning pool and 1 less after, compared to the xPoolLair before it. Otherwise the xPoolLair builds are identical, and they all have 9 drones when the lair starts. 9Pool8GasLair moves the drone before the extractor to after it, and moves the zerglings before the lair to after it to avoid delaying the lair (with a drone made later, the minerals are slightly lower). 9GasLair gets an extractor on 9 drones, then a drone, then the spawning pool. The optimized version gets just enough gas, then stops gas collection. ZvZ_Overgas9Pool is a standard Steamhammer build that wins a lot of games, which gets overlord, then gas, then spawning pool, and includes a good number of zerglings while still rushing for mutalisks. CAVEAT These numbers are based on only 1 run each! Different runs, even on the same map with the same starting position, will come out slightly different.

9Pool8GasLair gets 150 minerals and 100 gas for the lair at virtually the same time; the pool is already finished by then. I don’t see how to get a faster lair after 9 drones. The purpose of the gas-first measurements is to show how the gas-minerals tradeoff continues as you get earlier gas. I’m not sure about this, but it’s possible that fast gas could be worth it if you intend a scourge-ling unit mix, seeking initial air control with scourge and using the minerals you save (compared to mutalisks) to add hatcheries.

I timed 5PoolLair against 9Pool8GasLair in full games in Steamhammer. They both start the spire at the same supply, but 9Pool8GasLair reaches that point over 600 frames earlier, exactly as forecast from the lair timings. What do you know, math works! The tremendous advantage in zergling and mutalisk timing can put heavier pressure on the opponent.

The purpose of the ZvZ_Overgas9Pool entry is to remind us all, me included, that rushing for one unit type is an extreme game plan. An extreme plan is not necessarily bad, but a balanced mix of units with a productive economy is safer and more flexible. Steamhammer plays its overgas 9 pool more often than it plays its 5 pool or its 9 pool spire openings, and that is because it wins more.

Theoretically, the assumption that the goal is to rush mutalisks could be wrong. It can make sense to prepare a tech which you don’t intend to use right away: It gives you more options. Getting an early spawning pool means you can make emergency zerglings if you are unexpectedly rushed. However, getting a spawning pool at 9 drones is early enough to stop a zergling rush. The only reason I can think of to get a spawning pool on 5 and then go to 9 drones is to stop a worker rush, if your worker defense is poor (which it shouldn’t be). You won’t scout the enemy in time to notice a vulnerability to the fast pool and change plans.

Bottom line: McRaveZ’s opening looks bad. I do not see a purpose that it is well optimized for. I still suspect that it is a bug, not an intended opening.

One of my goals for Steamhammer, you may remember, is to time its own openings. I want it to keep data about its openings, and reason during the game about which one is good for what purpose. Ideally, it should be able to figure out when one opening dominates another, in the same way that 9Pool8GasLair dominates 5PoolLair: It is better in some ways, and worse in none. An opening which is dominated need never be played or explored further.