Practice problems

1.1

Watch the replay below. What is significant about the unit composition?

Hint: look for what is missing. A unit mix tells you what a player chose not to build as clearly as what they did, and the absence is usually where the decision was.

The game. Zerg against Terran on a Lost Temple observer map, 16 minutes 41 seconds at fastest speed. The two players with bases are RiZi[aR] on Zerg and Aesthetic[x.PG] on Terran. The other two names in the file are observers.

1.2

Pick one branch of one race's tech tree and write it up.

Choose from twelve: Zerg pre lair, lair tech, hive and buildings; Protoss gateway, robotics, stargate and buildings; Terran barracks, factory, starport and buildings. For every unit in your branch, record cost, build time, damage, health, shields, armour, upgrades, tech requirements and every special ability. Then find a professional replay where one of those units is used well, and write one sentence on what that player did that you would not have.

That last sentence is the point. The table is how you earn the right to write it.

1.3

Find a nickname for each of ten players, and the unit each was known for.

The ten: Boxer, Reach, iloveoov, Savior, XellOS, Bisu, NaL_rA, Anytime, Garimto, Jaedong. Liquipedia has all of them.

It reads like trivia and is not. In this scene a nickname is a compressed scouting report: it tells you what a player does to you.

2.1

A commentator says a Zerg is safe while the Terran has fewer medics than the defender has sunken colonies. Medics have no attack. Why would their number decide a sunken break, and when does it stop mattering?

Answer. A sunken colony puts out a fixed rate of damage against marines, so once the number of medics matches the number of sunkens, that damage can be healed away for as long as the medics have energy. Two medics also let a full group of marines stim without running the healing dry. It stops mattering when the break is done with siege tanks instead, or when the attacking force is big enough that the healing micro costs more attention than it returns.

2.2

List everything a Terran barracks is good for, on the ground and in the air.

Answer. On the ground: a wall, a production building, a standing threat if it is in the enemy's base, and a body to put in front of tanks. Lifted: a scout, a shield, a deterrent against drops, a sight blocker so your tech stays hidden, a way to sit on mineral patches so they cannot be mined, and extra vision so tanks can shoot their full range.

That second list is the lesson: a unit is rarely only the thing it says on the tin.

2.3

You went mass hydralisk. The Protoss scouted it and is coming with mass speed zealots. How do your hydralisks do, and what do you do about it?

Answer. Badly, because of the damage type. Hydralisk damage is explosive: it hits shields at full value but a zealot's health, since a zealot is small, at half. Morph into lurkers. Their damage is normal, and the line splash lands on zealots that clump on their way in.

2.4

Cost out a dark archon as the answer to a pile of lurkers. Compare it with just building an observer.

Answer. The dark archon is far more expensive: two dark templar, a templar archives behind them, then the research, then waiting for energy that may not arrive before the lurkers do. Against that, a robotics facility and an observatory. The one argument for it: once it exists it can take an overlord, and get detection that way.

2.5

List uses for a probe other than mining.

Answer. Building, scouting, worker harassment, defending, blocking with pylons, stealing gas, standing on a ramp to stop scouts, the forward pylon, and the cannon rush.

2.6

What do you gain and what do you lose by selecting two high templar and casting storms that overlap?

Answer. Two storms on the same spot do not stack, so the second one is wasted. What two selected templar are good for is laying storms side by side, which is how carpet storming works.

2.7

Why is marine and medic rare against Protoss?

Answer. Protoss deals damage in bigger pieces than Zerg, and healing cannot keep up with a big piece. A reaver scarab kills a marine in one shot, dark templar kill one per hit, storm clears a block of them, and zealots do damage faster than a medic restores it. The exception is against carriers, where marines exist to shoot interceptors.

2.8

Work out what a sieged tank does to a zergling and to a hydralisk, then work out how attack or carapace upgrades change the number of shots.

Answer. A sieged tank shot is 70 explosive damage. Against a zergling (small, 35 health) it does 35: one shot. Against a hydralisk (medium, 80 health) it does 52: two shots, and no upgrade on either side changes that.

Careful: a sieged tank does not one shot a hydralisk. It needs two, so a tank line against hydralisks needs more tanks than you think. And one carapace upgrade takes the zergling to two shots, until the tank gets its first attack upgrade back.

This is the question that becomes a tool. Once you can ask "how many hits, at which upgrade level" for any pair of units, you can ask it for every pair. The pairs where the count drops are called breakpoints, and set 3 opens on them.

3.1

A sieged tank hits for 70, and another sieged tank has 150 health and 1 armour. Two attack upgrades take it from three hits to two. Find the other combinations that matter, including mixed groups of units, not just upgrade levels.

Answer. A list, not a rule, and that is the right shape. Three zealot attacks kill a zergling, or two zealot attacks and a probe hit. Two upgraded tanks against a tank. Two wraiths and a tank shot against a tank. Two lurker hits against a marine, three if the marine has one armour upgrade. One scourge and two mutalisk volleys against a mutalisk. And many more.

That "many more" is the best question in this page, because it generalises. The number you want is not the damage. It is the hit count, and the upgrade level where the hit count drops.

3.2

You are attacking a sunken defence with marines and medics. What goes in front, and why?

Answer. The medics. They take the first hits, and they are tougher than they look: a medic has 60 health and 1 armour, so a sunken needs four hits on it against two on a marine. From the front they are also already next to the marines that get hurt. Behind the marines they are healing nobody.

3.3

Is it better to attack in a line or to wrap around the enemy army?

Answer. Wrap around, almost always. A line only brings the units at the front into range. An arc around the enemy brings far more of your army into range at once, while the enemy's own clump can only fire outward from its edge. Lesson 4 works out exactly how much, and the Flux Lab lets you count it.

3.4

Explain leapfrogging with lurkers or siege tanks, when it is dangerous, and when you should simply move instead.

Answer. Leapfrogging moves units that only fight when they stand still. Half of them stay set up and cover while the other half walks. The cost is speed: you can only advance about half your attack range at a time, because the moving half has to stay under the cover of the half that is shooting. Moving everything at once is much faster, and much worse if you are caught, because unsieged tanks and unburrowed lurkers are nearly defenceless.

3.5

Medics change the rate at which marines die. Write down how, with no stim and nothing unusual shooting at you.

Answer. A small model. Take the rate at which damage arrives, subtract the rate at which the medics heal it, and divide by a marine's health to get a death rate. Two cases fall out. When incoming damage is much bigger than the healing, as under a storm or a line of lurkers, the subtraction barely changes anything and the medics do not save you. When the two rates are close, the death rate collapses, and it can reach zero.

The lesson underneath: healing is not a fixed amount of value. It is a rate that either keeps up or does not.

3.6

Why does it matter so much that you have enough mutalisks to kill your target in a single volley?

Answer. A well grouped flock behaves like one unit that attacks in lumps. If the lump is big enough, the target dies on the first pass. If not, you come back for a second, and in between the target can be healed or repaired while your flock takes another round of damage. Getting it wrong roughly halves what your mutalisks are worth.

4.1

Zerg often sits on less supply than the other races in the midgame, which looks like supply efficiency. So why does a maxed Zerg army lose to a Terran ball on less supply?

Hint: this is starkest in Terran against Zerg, where a 150 supply ball can beat a bigger Zerg army outright. Efficiency per supply is not what decides a fight, because not every unit you own is shooting.

4.2

A Zerg army is closing on a Protoss ball, sixty squares of arc against twenty squares of front. The Zerg player misjudges the Protoss position by ten squares. How much earlier does one side of the arc arrive than the other, what does that cost in casualties, and, if you are the Protoss and you see the charge coming, how far do you have to move to cut the surround in half before the far side gets there?

Three questions, and the third is the real one, because it turns a geometry answer into a decision you make in about a second.

4.3

How do you decide where to put your army in the early game, the midgame and the late game? What information drives each answer?

Hint: early, you position against the attack you cannot yet rule out. Midgame, you position to protect what you just took and to cut the paths to it. Late, you position where your army fights with the most units in range and theirs with the fewest.

4.4

Explain mobility, and say which tech trees have it.

Hint: mobility is how quickly your army can be where the fight is, including the fight you did not expect. Think about speed upgrades, air units, transports and recall, and about what each one lets you ignore.