Starcraft AI blog

What upgrades should you get? In what order? I often see bots get upgrades in an order that doesn’t make sense to me. I’ll mention 2 big examples.

If you’re terran and you’ll be making marines for a long time, you want the academy upgrades. Standard is to get stim first and then range. Stim helps in almost every situation, and it helps a lot, while range has a smaller effect and gives the greatest boost to large groups of marines. Stim also takes less time to research. When I added stim to Steamhammer, I found it clear that stim made the marines much more dangerous, despite the fact that (like most bots) Steamhammer tends to overstim and run down the energy of its medics. Probably the most important change needed to stim is not reducing overstim, but making sure that stim is always researched as early as possible. BOSS doesn’t understand priorities, and just throws research in whenever.

And yet I think most bots that get both stim and range, get range first. Even sophisticated Tscmoo gets range first. Why is that? Maybe there’s a good reason—every bot is different, and you want to tune for its skills. Or maybe it’s a mistake.

The other example is zerglings versus zealots in ZvP. Unless you’re going all air like XIMP by Tomas Vajda, protoss should get +1 ground attack as the first forge upgrade. Bots generally do that right. (The second forge upgrade should be +2 attack, and the third should be +3 attack. Get armor, and maybe then shields, only if you’re upgrading at multiple forges, or after +3 attack finishes.)

If you opened forge first, you may be able to start the upgrade very early. It’s worth it. A zergling has 35 health. A zealot does 16 base damage, so it takes 3 hits to kill a zergling. A zealot with +1 does 18 damage, and 18 * 2 = 36 so it kills a zergling in 2 hits (the zergling will regenerate 1 hp, so the 36 is exactly what’s needed). 2 hits versus 3 is a giant difference. Zerglings suddenly turn into tofu.

A zealot has damage factor 2, meaning it hits twice in each shot: It doesn’t do 16, it does 8 + 8. So a zergling with +1 carapace avoids 2 points of damage. If zerg can get as many carapace upgrades as protoss has attack upgrades, then zerglings remain zerglings and do not become bean curd.

CherryPi gets +1 zergling attack instead of +1 carapace. It is effective against a protoss that does not upgrade at all. Against a protoss that gets +1 attack, it is a mistake. I’ll do the arithmetic. A zergling does 5 damage; a +1 attack zergling does 6. A zealot has 60 shields: That’s 12 hits versus 10 to remove the shields. A zealot has 1 armor and 100 hp: That’s 25 hits (100 hp divided by 5-1=4 damage per hit) versus 20 to finish the zealot off. I’m ignoring shield regeneration, because it gets complicated. That makes 37 hits versus 30, a nice improvement. 30/37 is .81; it takes about 4/5 as many hits after the upgrade. But 2/3 is .67—the zealot’s attack upgrade outweighs the zergling’s attack upgrade.

There are other examples. These are the ones that jump out at me.