Tacoma Commune: fix caravans splitting

[hexagonrecursion]

Summary

Bugfixes "Tacoma Commune: fix caravans splitting"

Purpose of change

By talking to the crop overseer at the Tacoma Commune you can send
followers on a caravan to the refugee center. The caravans are sometimes
attacked by raiders. It is supposed to be possible to send multiple
followers in one caravan to make it better defended. The issue is that
when you recover followers from the caravan the game behaves as if each
follower went on a separate caravan and if one of the caravans is
attacked the rest don't help.

To reproduce:

1. Load the attached save file caravan.zip
2. Talk to the overseer and send all 8 followers on one caravan
3. Debug skip time forward 1 day
4. Recover the followers

Version:

• OS: Linux
  • OS Version: LSB Version: :core-4.1-amd64:core-4.1-noarch; Distributor ID: Fedora; Description: Fedora release 33 (Thirty Three); Release: 33; Codename: ThirtyThree;
• Game Version: cb86201 [64-bit]
• Graphics Version: Tiles
• Game Language: English [en]
• Mods loaded: [
  Dark Days Ahead [dda],
  Disable NPC Needs [no_npc_food],
  Bionic Professions [package_bionic_professions]
  ]

This code looks suspect:

Cataclysm-DDA/src/mission_companion.cpp

Lines 763 to 768 in 998d767

	for( auto &elem : npc_list ) {
	if( elem->companion_mission_time == calendar::before_time_starts ) {
	//Adds a 10% error in estimated travel time
	elem->companion_mission_time = calendar::turn + time * rng_float( -1.1, 1.1 );
	}
	}

Cataclysm-DDA/src/mission_companion.cpp

Lines 792 to 807 in 998d767

	//So we have chosen to return an individual or party who went on the mission
	//Everyone who was on the mission will have the same companion_mission_time
	//and will simulate the mission and return together
	std::vector<npc_ptr> caravan_party;
	std::vector<npc_ptr> bandit_party;
	std::vector<npc_ptr> npc_list = companion_list( p, id );
	const int rand_caravan_size = rng( 1, 3 );
	caravan_party.reserve( npc_list.size() + rand_caravan_size );
	for( int i = 0; i < rand_caravan_size; i++ ) {
	caravan_party.push_back( temp_npc( string_id<npc_template>( "commune_guard" ) ) );
	}
	for( auto &elem : npc_list ) {
	if( elem->companion_mission_time == comp->companion_mission_time ) {
	caravan_party.push_back( elem );
	}
	}

Describe the solution

Randomize the time once per caravan instead of once for each follower

Describe alternatives you've considered

None

Testing

1. Load the attached save file caravan.zip
2. Talk to the overseer and send all 8 followers on one caravan
3. Debug skip time forward 1 day
4. Recover the followers

Additional context

[hexagonrecursion]

For some reason this patch triggers a debug assertion if the caravan is attacked and some of the followers die:

 DEBUG    : overmapbuffer::remove_npc: NPC (53) not found.

 FUNCTION : shared_ptr_fast<npc> overmapbuffer::remove_npc(const character_id &)
 FILE     : src/overmapbuffer.cpp
 LINE     : 1110
 VERSION  : 0.F-1420-g96497661b9

[I-am-Erk]

Closing as abandoned, bookmarked for hopeful pickup

[hexagonrecursion]

This can be removed from Abandoned but Desired PRs: it was fixed by #55746