Starcraft AI blog

PurpleTickles is a new bot in the Purple family. It plays a 4 probe rush, mining only with the 1 probe it makes with its initial 50 minerals. It never sets more probes mining, either, so the 4 probe rush hits hard but builds slowly.

On a 2 player map it’s a dangerous rush that gives trouble to strong defenders like Krasi0. The 4 probes arrive in a group and stick together to fight in coordination, so the defender needs skill to hold. The style is quite different from Stone’s SCVs, which act independently most of the time and gang up only when they spot an opportunity.

I haven’t seen a game on a 3 player map. On a 4 player map, the rush is naturally much weaker. PurpleTickles scouts with 2 probes and leaves 2 in the center to join in faster when the enemy base is located. Consider the 4 player scouting options for a 4 probe rush; there are 3 bases to scout:

1. Scout with 2 probes and leave 2 at home mining until the enemy is located. The worst case is when the enemy is at the unscouted base. That happens 1/3 of the time and has 4 probes arriving more or less simultaneously at the enemy. The extra minerals make the followup stronger.
2. Scout with 3 probes and leave 1 at home mining. This always gets 1 probe to the enemy base as soon as possible, and the others arrive more or less simultaneously. Compared to option 1 it hits a little harder 1/3 of the time in exchange for a slightly weaker followup.
3. Scout with 4 probes, sending 2 to one base. Compared to option 2, it hits harder 1/3 of the time when the double probes reach the enemy first, and the followup is a little weaker again.
4. Scout with 2 probes and leave 2 in the center, the PurpleTickles option. The center probes can join the fight sooner than a probe at a base, whether it is the home base or an empty base. So there’s a 2/3 chance that the enemy is hit by 1 probe, then 2 more after, then another; and a 1/3 chance that the enemy is hit a little later by 2 probes, then 2 more after.
5. Scout with 3 probes and leave 1 in the center. The enemy is hit by 1 probe, then 1 after, then the final 2 probes.

Intermediate plans are possible, with probes mining part of the time and staged in different locations depending on the map distances, but these 5 seem to be the main options. I don’t see much difference between them, but the PurpleTickles option seems plausible if you believe that hitting as hard and early as you can is best, and that the followup is less important.

I noticed that the rushing probes choose their attack goals along a straight line from the base entrance. They are more successful when they arrive at one end of the mineral line and attack from one end to the other, when they face few enemy workers at a time. They are less successful when the minerals lie at right angles to the line from the base entrance, so that the probes aim for the center of the mineral line and meet many enemy workers at once. An injection of smarts might improve that.

PurpleTickles is moderately successful overall, but it does have wins versus Iron (Iron apparently suffered a bug and never built a barracks) and versus Krasi0. Notice Krasi0’s trick of building its depot and barracks in an out-of-the-way location so that the attacking probes never bother them. It seems like a clever idea versus a probe rush.

Yuanheng Zhu coincidentally uploaded shortly after with nearly the same strategy, though the code is unrelated. On a 4 player map, it scouts with option 3 above. It also crashes when its probes arrive at their scout locations, so we haven’t seen how well it plays yet.

Update: Yuanheng Zhu succeeded in playing one game versus Sungguk Cha. Maybe it works on 2 player maps? The strategy turns out to be quite different from other worker rushes: 1. The 4 rush probes initially attack mostly buildings, not workers, presumably to stay safe for the followup. 2. The bot does not send more probes to attack, but sets new probes mining. 3. It builds a pylon and forge in its main, then adds a pylon and cannons in the enemy base, switching into a cannon rush. Most cool.

Other good stuff to build in or near the enemy base are a shield battery and gateways. I’m sure bots will do that too one of these days.