Mongroup: subgroup spawning fixes

[dseguin]

Summary

None

Purpose of change

• Fixes Nested monster groups don't respect start/end dates #59174
• Fixes Monster groups in monster groups are not evaluated correctly #59276

Describe the solution

When spawning from a monster group, evaluate subgroups at the top level for validating spawn conditions (start/end dates, weight, season, etc.)

(Note: pack_size)

---

Pack size is a bit of a special case. The pack sizes in monster groups are deliberately weighted:

Cataclysm-DDA/data/json/monstergroups/triffid.json

Lines 31 to 41 in 554df77

	{
	"type": "monstergroup",
	"name": "GROUP_TRIFFID_HEARTGUARDS",
	"default": "mon_triffid",
	"monsters": [
	{ "monster": "mon_triffid", "weight": 280, "cost_multiplier": 0 },
	{ "monster": "mon_triffid", "weight": 300, "cost_multiplier": 0, "pack_size": [ 4, 6 ] },
	{ "monster": "mon_vinebeast", "weight": 260, "cost_multiplier": 2 },
	{ "monster": "mon_triffid_queen", "weight": 160, "cost_multiplier": 5 }
	]
	},

So a parent group defining a pack size for this subgroup would select that many sub-entries.

{
  "type": "monstergroup",
  "name": "GROUP_TRIFFID_HEART",
  "monsters": [ { "group": "GROUP_TRIFFID_HEARTGUARDS", "pack_size": [ 10, 12 ] } ]
}

In this case, 10-12 entries from GROUP_TRIFFID_HEARTGUARDS will spawn here

---

Describe alternatives you've considered

Testing

Added a unit test to check the spawn probabilities of monster groups at different levels of nesting:

./tests/cata_test --rng-seed time "Nested monster groups spawn chance"

Additional context

This was an implementation bug from #52323 (yes, it's been broken for that long)

[Venera3]

Took it for a test ride - start/end and conditions definitely work, weights seemed to as well (but those are a bit harder to check). One thing I ran into is that the top-level pack size definition doesn't seem to do anything, with the group-level pack size carrying over.

[dseguin]

Ah, I see what's wrong with pack size. I'll add a test case for that as well.

Thanks for checking!

[Venera3]

Pack size works, however do you think it would be possible to instead of multiplying the rolled mob with the top-level pack size you'd get pack_size rolls on the nested group?
Ex:

  {
    "type": "monstergroup",
    "name": "GROUP_ZOMBIE_TEST",
    "monsters": [
      { "group": "GROUP_ZOMBIE_FAMILY", "pack_size": [ 1, 5 ], "weight": 1000 }
    ]
  },
  {
    "type": "monstergroup",
    "name": "GROUP_ZOMBIE_FAMILY",
    "monsters": [
      { "monster": "mon_zombie", "weight": 200 },
      { "monster": "mon_zombie_fat", "weight": 100 },
      { "monster": "mon_zombie_child", "weight": 300 },
      { "monster": "mon_zombie_dog", "weight": 100, "pack_size": [ 1, 2 ] }
    ]
  },

As it stands, this spawns either 1-5 zombies/fat zeds/zed kids or 1-10 dogs (but only one type at a time). However, if the top-level pack size would instead count as the number of entries to roll you potentially could place a whole family with one spawn - and one spawn is all you'll get for overmap_terrain level spawns (like random monsters). For the mapgen-level it's less of a problem since you can just repeat the group spawn without pack size.

[dseguin]

do you think it would be possible to instead of multiplying the rolled mob with the top-level pack size you'd get pack_size rolls on the nested group?

Done, just needed a bit of refactoring to make it work.