Battle UFO spawning behavior

- MT_BATTLEUFO_SPAWNER args[0] is the ID
- Spawn a random UFO from the list spawner at the start of
  Battle
- UFO spawns 200 units above the spawner
- After destroyig a UFO, wait 25 seconds before spawning
  the next UFO (next ID in the list)

src/k_battle.c

@@ -350,6 +350,11 @@ void K_RunPaperItemSpawners(void)
 		return;
 	}
 
+	if (leveltime == g_battleufo.due)
+	{
+		Obj_SpawnBattleUFOFromSpawner();
+	}
+
 	if (!IsOnInterval(interval))
 	{
 		return;
@@ -797,6 +802,9 @@ void K_BattleInit(boolean singleplayercontext)
 			K_SpawnPlayerBattleBumpers(players+i);
 		}
 	}
+
+	g_battleufo.due = starttime;
+	g_battleufo.previousId = Obj_GetFirstBattleUFOSpawnerID();
 }
 
 UINT8 K_Bumpers(player_t *player)

src/k_battle.h

@@ -11,6 +11,7 @@ extern "C" {
 #define BATTLE_SPAWN_INTERVAL (4*TICRATE)
 #define BATTLE_DESPAWN_TIME (15*TICRATE)
 #define BATTLE_POWERUP_TIME (20*TICRATE)
+#define BATTLE_UFO_TIME (25*TICRATE)
 
 extern struct battleovertime
 {

src/k_objects.h

@@ -166,6 +166,11 @@ void Obj_BattleUFOLegThink(mobj_t *leg);
 void Obj_BattleUFOThink(mobj_t *ufo);
 void Obj_SpawnBattleUFOLegs(mobj_t *ufo);
 void Obj_BattleUFODeath(mobj_t *ufo);
+void Obj_LinkBattleUFOSpawner(mobj_t *spawner);
+void Obj_UnlinkBattleUFOSpawner(mobj_t *spawner);
+void Obj_SpawnBattleUFOFromSpawner(void);
+INT32 Obj_GetFirstBattleUFOSpawnerID(void);
+void Obj_ResetUFOSpawners(void);
 
 /* Power-Up Aura */
 void Obj_SpawnPowerUpAura(player_t* player);

src/m_random.h

@@ -74,6 +74,8 @@ typedef enum
 
 	PR_MOVINGTARGET, // Randomised moving targets
 
+	PR_BATTLEUFO, // Battle UFO spawning
+
 	PR_BOTS, // Bot spawning
 
 	PRNUMCLASS

src/objects/CMakeLists.txt

@@ -22,6 +22,6 @@ target_sources(SRB2SDL2 PRIVATE
 	gachabom-rebound.cpp
 	servant-hand.c
 	super-flicky.cpp
-	battle-ufo.c
+	battle-ufo.cpp
 	powerup-aura.cpp
 )

src/objects/battle-ufo.cpp

@@ -1,5 +1,11 @@
+#include <algorithm>
+#include <iterator>
+#include <set>
+
 #include "../doomdef.h"
+#include "../m_random.h"
 #include "../p_local.h"
+#include "../k_battle.h"
 #include "../k_objects.h"
 
 #define BATTLEUFO_LEG_ZOFFS (3*FRACUNIT) // Spawn height offset from the body
@@ -7,6 +13,102 @@
 #define BATTLEUFO_BOB_AMP (4) // UFO bob strength
 #define BATTLEUFO_BOB_SPEED (TICRATE*2) // UFO bob speed
 
+#define spawner_id(o) ((o)->args[0])
+
+#define ufo_spawner(o) ((o)->target)
+
+namespace
+{
+
+struct Spawner : mobj_t
+{
+	INT32 id() const { return spawner_id(this); }
+};
+
+struct UFO : mobj_t
+{
+	Spawner* spawner() const { return static_cast<Spawner*>(ufo_spawner(this)); }
+	void spawner(Spawner* n) { P_SetTarget(&ufo_spawner(this), n); }
+};
+
+struct SpawnerCompare
+{
+	bool operator()(const Spawner* a, const Spawner* b) const
+	{
+		return a->id() < b->id();
+	}
+};
+
+class SpawnerList
+{
+private:
+	std::set<Spawner*, SpawnerCompare> set_;
+
+public:
+	void insert(Spawner* spawner)
+	{
+		auto [it, inserted] = set_.insert(spawner);
+
+		if (inserted)
+		{
+			mobj_t* dummy = nullptr;
+			P_SetTarget(&dummy, spawner);
+		}
+	}
+
+	void erase(Spawner* spawner)
+	{
+		if (set_.erase(spawner))
+		{
+			mobj_t* dummy = spawner;
+			P_SetTarget(&dummy, nullptr);
+		}
+	}
+
+	Spawner* next(INT32 order) const
+	{
+		auto it = std::upper_bound(
+			set_.begin(),
+			set_.end(),
+			order,
+			[](INT32 a, const Spawner* b) { return a < b->id(); }
+		);
+
+		return it != set_.end() ? *it : *set_.begin();
+	}
+
+	INT32 random_id() const
+	{
+		if (set_.empty())
+		{
+			return 0;
+		}
+
+		auto it = set_.begin();
+
+		std::advance(it, P_RandomKey(PR_BATTLEUFO, set_.size()));
+
+		return (*std::prev(it == set_.begin() ? set_.end() : it))->id();
+	}
+
+	void spawn_ufo() const
+	{
+		if (set_.empty())
+		{
+			return;
+		}
+
+		Spawner* spawner = next(g_battleufo.previousId);
+		UFO* ufo = static_cast<UFO*>(P_SpawnMobjFromMobj(spawner, 0, 0, 200*FRACUNIT, MT_BATTLEUFO));
+
+		ufo->spawner(spawner);
+	}
+};
+
+SpawnerList g_spawners;
+
+}; // namespace
+
 void Obj_BattleUFOThink(mobj_t *ufo)
 {
 	// Copied and slightly modified from k_kart.c
@@ -15,10 +117,18 @@ void Obj_BattleUFOThink(mobj_t *ufo)
 	ufo->momz = targz;
 }
 
-void Obj_BattleUFODeath(mobj_t *ufo)
+void Obj_BattleUFODeath(mobj_t *mobj)
 {
+	UFO* ufo = static_cast<UFO*>(mobj);
+
 	ufo->momz = -(8*mapobjectscale)/2;
 	ufo->fuse = TICRATE;
+
+	if (ufo->spawner())
+	{
+		g_battleufo.previousId = ufo->spawner()->id();
+		g_battleufo.due = leveltime + BATTLE_UFO_TIME;
+	}
 }
 
 void Obj_SpawnBattleUFOLegs(mobj_t *ufo)
@@ -66,3 +176,28 @@ void Obj_BattleUFOLegThink(mobj_t *leg)
 		leg->eflags |= (leg->target->eflags & MFE_DAMAGEHITLAG);
 	}
 }
+
+void Obj_LinkBattleUFOSpawner(mobj_t *spawner)
+{
+	g_spawners.insert(static_cast<Spawner*>(spawner));
+}
+
+void Obj_UnlinkBattleUFOSpawner(mobj_t *spawner)
+{
+	g_spawners.erase(static_cast<Spawner*>(spawner));
+}
+
+void Obj_SpawnBattleUFOFromSpawner(void)
+{
+	g_spawners.spawn_ufo();
+}
+
+INT32 Obj_GetFirstBattleUFOSpawnerID(void)
+{
+	return g_spawners.random_id();
+}
+
+void Obj_ResetUFOSpawners(void)
+{
+	g_spawners = {};
+}

src/p_mobj.c

@@ -11175,6 +11175,11 @@ void P_RemoveMobj(mobj_t *mobj)
 			Obj_RingShooterDelete(mobj);
 			break;
 		}
+		case MT_BATTLEUFO_SPAWNER:
+		{
+			Obj_UnlinkBattleUFOSpawner(mobj);
+			break;
+		}
 		default:
 		{
 			break;
@@ -13442,6 +13447,11 @@ static boolean P_SetupSpawnedMapThing(mapthing_t *mthing, mobj_t *mobj)
 		Obj_InitLoopCenter(mobj);
 		break;
 	}
+	case MT_BATTLEUFO_SPAWNER:
+	{
+		Obj_LinkBattleUFOSpawner(mobj);
+		break;
+	}
 	default:
 		break;
 	}

src/p_tick.c

@@ -41,6 +41,7 @@
 #include "k_director.h"
 #include "k_specialstage.h"
 #include "acs/interface.h"
+#include "k_objects.h"
 
 #ifdef PARANOIA
 #include "deh_tables.h" // MOBJTYPE_LIST
@@ -230,6 +231,8 @@ void P_InitThinkers(void)
 	{
 		skyboxcenterpnts[i] = skyboxviewpnts[i] = NULL;
 	}
+
+	Obj_ResetUFOSpawners();
 }
 
 // Adds a new thinker at the end of the list.