How are alien bases spawned right now?

[Gammro]

In previous versions, alien bases were spawned in newly discovered lands, which had advantage of knowing where and how to build your defenses. Now since 0.7.1, bases and worms keep appearing in my base.

I have setup a rather wide perimeter though, so I have enough room left for expansion and train stations that need a lot of room. I have filled all the gaps on my perimeter, but they keep appearing in my base!
I am thinking I also need to defend the water, since they often seem to spawn near the lake which my factory borders on, but I'd like to know with some certainty how new bases and worms are spawned.


Edit:
I have found some information in the 0.7.0 discussion:

two things please,
firstly could you give a general explanation to the logic of how the enemy base expansion is intended to work? i.e what factors you use to calculate new areas for the biters bases.

and secondly, would it be possible to add in an option to choose how rapidly and aggressively the biters bases expand?
i just got killed trying to clear out some biters which spawned in an open area near my power generation zone which i hadn't set up with turrets because it was protected by a water barrier and i would love an option to restrict spawning near my infrastructure. biters getting into my base because of poor planning i can deal with, having my base destroyed from the inside because i left room for expansion in a closed off area just feels like i'm being punished for trying to plan ahead.

You can have a look at data/base/prototypes/map-settings.lua (in the enemy_expansion section). The whole thing works as follows:
1. The algorithm keeps the set of possible candidate places.
2. These are selected to be "not too close from the existing enemy bases" (min_base_spacing), "not too far from the enemy bases" (max_expansion_distance) and not too close to the player's structure (min_player_base_distance - this is new in 0.7.1).
3. The cooldown (expansion_cooldown) governs when the expansion happens. At the moment it comes out to be +- once in 15 minuts on medium sized maps. But we will have to playtest it more, since this gets more frequent for bigger factories.
4. One of the possible locations is selected and a group of biters is sent there.
5. They walk around the place for a while and then they die and buildings appear.

So the update in 0.7.1 is that the expansion is going to avoid players structures, meaning that enemies will not try to build bases inside your factory. Which I agree is more annoying than fun.

Apparently, I have setup my base too wide

[FreeER]

lol, you should be able to hit F7 and look at the map to see the expansion candidates, or F4 (for debug menu) and set it up so you can see only what you want (the pollution values are nice to see, but it looks weird on the minimap), F5 seems to be basic, F6 detailed, and F7 full.

[slpwnd]

The logic for spawning new bases (expansion) is guarded by parameters in the data/base/prototype/map-settings.lua.

min_base_spacing = how "dense" the bases are
max_expansion_distance = how far from the existing base they can expand
min_player_base_distance = how far from the player base they can expand

All this is in "chunks" (1 chunk = 32 tiles).

FreeER is right. Looking into the map and switching on the debug format you can see green circles which define where the enemies can expand (they can also be in the water - it is just a candidate set based on above parameters).

You can also try F4 and customize what you see in F5 / F6 / F7 modes. The value you are looking for in the F4 menu is "show_enemy_expansion_candidate_chunks".