Need some general direction for turn-based planning AI

Question

I'm planning to make a turn-based fleet battle game. While I think I can figure out most of the things I need, I have no idea about the AI handling this sort of game. I need some general direction about how should I deal with it.

None of the things I described below is currently done - except maybe unit movement stuffs.

The combat resembles Frozen Synapse or Steambirds, in which you and your opponent plan their unit's actions and then execute it to see how it resolves. Since the units are naval warships, their movement is restricted by current direction and speed, etc etc. They attack when you order them to do so(not automatic).

I think I can give scores to each move(like Chess AI), depending on its location, direction, possible movement area, known enemy positions and things like that. Then I can use Minimax algorithm to let the AI select the best move.

Now, I'm asking because I can't figure out

1. which situation should the score be based on. Should the AI make a plan assuming the enemies are all moving straight, predict the best enemy plan based on it, and then write the actual plan based on the prediction? Is it going to demand a lot of processing power? Or is there simpler way using Minimax?

2. Unlike grid-based tactical games, the area which a unit can move is a (semi-)continual area. If I try to give score to all possible positions, that might take too much processing power. Is there a way to handle this issue neatly?

3. There might be more efficient/effective alternatives to the Minimax on this particular game which I do not know.

So... i'm asking for general direction about how should I handle the AI. Details will be in when I actually make it.

Answer 1

When the game is parallel turn-based, you can calculate your moves while the player is making theirs. This gives your AI quite a lot of time. So performance is not that much of a concern as it would be in a real-time game.

In many games it is not actually necessary to plan ahead more than one round. It is often possible to write a passable AI which doesn't. It will certainly be weaker than any moderately experienced player, but you can compensate for that by giving the AI a material advantage in form of more and stronger units. Yes, this is a lazy solution, but it is the solution used by the majority of titles on the market, among them many bestsellers. The expectations of the players are quite low in this regard.

When you decide that you do want to calculate more than one move ahead, the number of calculations increases exponentially. In an average turn-based strategy game, there is a very large number of possible moves each unit can perform per round. So even when your AI has up to a minute per move, this might become too much to handle. You can not assume that the player will set idle, so you have to also calculate the possible moves the player could perform and evaluate them like you evaluate those of the AI. This creates a really, really broad game-tree, which means some alpha-beta pruning will be necessary.

You can easily do that by reducing the moves you evaluate down to those which fulfill some tactical objective. First, check which tactical objectives are available for each unit. Which objectives are worth considering depends on your game mechanics, but it could be things like "rush enemy unit A", "snipe enemy unit A", "secure objective A" or "protect self". Take the one most obvious move to fulfill each objective, and use these moves as possible candidates by calculating the strategic value of each.

A completely different approach is to not focus on creating a strategically smart AI, but rather one which has units act impulsive like you would expect people to act in a real-time environment. Have units only react on what they can see and make them do decisions depending on a state machine governed by emotions like "anger" or "fear", not on what would be the decision with the strategically best outcome. The result will be an AI which will definitely be far too weak to stand any chance against a player when put on equal terms, but one which will be quite enjoyable to play against when a small number of player units is put against an overwhelming enemy force.