doc: Mapgen elements "density" and "roof" of "terrain".

[free-coffee]

Summary

SUMMARY: None

(documentation, density is in mapgen.cpp/139-193)

[Hirmuolio]

I too have tried to dechipher the meaning of these for the pull I have.

It is pretty strange and I don't think I understand it all yet. But I am pretty sure that density and chance are independent. If the !one_in( chance ) ) is false everything ends and no spawn is done.

As far as I can see the number of spawned monsters comes from something like this
num = probabilistic_round( density * spawn_density * rng_float( 10.0f, 50.0f ) )

After each spawn cost_multiplier * pack_size is subtracted from num
Once num is not positive anymore the spawning stops.

So I think the total spawns is (with roundings removed) about number_of_monsters = density * spawn_density * rng_float( 10.0f, 50.0f ) / cost_multiplier but this may be wrong.

[free-coffee]

@Hirmulolio: Maybe i missed the float multiplication? Im certain the overmap densities (spawn_density) were added and not multiplied though. And /cost_multiplier would be wrong, since different monsters with different costs can spawn & subtract from the pool.
I won't look at this for some time - feel free to update after #33256.

[Hirmuolio]

Edit: Oh it is different code that looks similar and does kind of the same thing but is actually different.

map::generate VS class jmapgen_monster_group with map::place_spawns