LuaSurface.get_expansion_probability()

[IronCartographer]

I would like a way to read the probability with which the chunk at a location is a candidate for biter expansion.

While this is possible to emulate with lua, it would be much more efficient to do so using the built-in system for generating the metadata.

Motivation: A mod to dynamically adjust the biter expansion timer and party size in response to the intensity of expansion location availability, thus incentivizing isolated defenses rather than monolithic walls enclosing vast areas of empty land.

(It would be even more useful to also have a surface-wide metric of these probabilities in some fashion, but that's specific enough to mod logic to be left to the mod developer.)