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Unity How to update tiles based on their neighbours

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Hello community i hope you can help me on this one.
I have a large hexagonal tile-map (150k tiles now might be more in future). I am working on an ecosystem simulation. Every hex has information about it's current state, and associated variables (like fertility). Tiles should be updated according to their neighboring tiles. I am facing some design and implementation problems here.
First problem is how to handle updating of two neighboring tiles. Let's say we have a tile. It's type is grass and it's fertility is 0.8, one neighbor is river, other is sand. River should rise grasses fertility, and sand should lower it, gradually until this system is balanced. If grasses fertility should drop under a certain threshold, that tile should change to sand (for example). How to design this situation in spirit of good oop.
Problem is I don't know what type is my neighbor, and so i don't know how to react to it. I could use something like double dispatch (but would two polymorphic calls incur too much overhead when world gets large?), or i could use some kind of ID system, and a switch statement. That way code could get pretty messy, as there will be more than 10 different types of tiles, and it would be nice to be able to add more later on.
Second problem is how to update a large system. Updating so much tiles could drop my frame-rate so I figured that it should be done asynchronously in a different thread. This is a big issue for me because i am a total threading noob. My idea is that worker thread updates once everything, then signals the main thread, main thread detects changes and accordingly displays results (if grass is no longer grass make it a dirt).
Also is there a way to organize my data to lower cache misses. I at first used bfs algorithm to traverse entire map, but that was about 20% slower than traversing them the way they are stored in memory. Problem with updating is that i need to access neighbors data that might bee "too far" in memory.

Any kind of help is mucho appreciato. Ideas, good design principles and code are all welcome.

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If you can generalize the type of tiles interactions, you may get away by not creating a class per type. You could have your tiles provide a modification factor to various properties and make an average and then update your tile. For example, sand could reduce fertility by 0.01 every update and river increase it by 0.01, balancing themselves. You then wouldn't need to know if your neighbor is a river or sand, just how it affects your fertility.

As for your threading problem, you could simply split your tiles in chunks and split them among your worker threads instead of doing your current loop through all tiles. I wouldn't have the update be done in parallel or you will have all sorts of fun problems to deal with later on.

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As for my threading problem what kind of problems could i get into? Also could you give me a word or two about worker thread method?

About tiles, you are right we could take some time to define our abstractions better, but lets say just hypotheticaly that i have to leave some room for adding new tile types that would break current hierarchy. What would be my approach in this case. (number of tiles and fast interaction still stand)

Sorry for the lack of comment but im on my mobile :/.

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You would lose determinism when compared to single-threaded code if you use a fixed timestep approach. For example, if your update loop is :
- updateEcosystem()
- doOtherStuff()

If you take out updateEcosystem() and place it in a concurrent thread, doOtherStuff() is no longer guaranteed to work with the up to date data. It may or may not be up to date and may become up to date in the middle of the function. That means you can no longer rely on the ecosystem being up to date in your algorithms. This is where bugs come from.

What you can do instead in the updateEcosystem() function is split all the tiles into chunks. Then you send the chunks to your worker threads and wait until they complete before exiting the function. That way, you retain the same determinism than with single threaded code.

For your tiles, I'm not sure I understand the problem. It seems to me the tiles can be abstracted with a set of values from 0 to 1 and each type affects its neighbors' value in some way, probably by trying to create uniformity. If you must have more complex logic, why not keep the type as a variable and centralize all processing in some common function? You can compare the types directly without calling polymorphic functions and all that. It's not really OOP, but if that's really your bottleneck, you may have to break some rules. It's too vague to properly answer.

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What you're implementing is essentially a cellular automata. Each cell will transition to a new state, based on its current state and that of its neighbors. Automata get a little more complex when you introduce supplementary data such as the "fertility" value you mention, but it's still possible. Let's assume you have a single integer value "F" for each cell in addition to its state "S". The F-value can have different interpretations depending on its state; say, for city tiles F would be population, and for farmland tiles F would be fertility.

In these terms, you need to define the transition function that takes as input current S, current F, and current S of each adjacent tile, and outputs a new S and new F value. Now, in whatever ways you like, you need to work out your rules and define the transition functions. Keep them simple, and have them generate a large lookup table.

Finally, because a cellular automata updates atomically from one step to another, you need to keep two copies of your grid; one "current", and one "updated". After updating, swap them, making the "updated" into "current" and vice-versa. Now you can have any number of worker threads running on your grid, in whatever order ends up being best optimized; they cannot interfere with eachother, because they all read from one grid and write to non-overlapping portions of another grid.

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