feat: make death drops extendable (#9755)

* feat: support appendable monster death drops

Assisted-by: pi:gpt-5.5
Co-authored-by: chatgpt-codex-connector[bot] <199175422+chatgpt-codex-connector[bot]@users.noreply.github.com>

* docs: translate appendable death drop docs

Assisted-by: pi:gpt-5.5
Co-authored-by: chatgpt-codex-connector[bot] <199175422+chatgpt-codex-connector[bot]@users.noreply.github.com>

---------

Co-authored-by: chatgpt-codex-connector[bot] <199175422+chatgpt-codex-connector[bot]@users.noreply.github.com>

Author: scarf005

data/mods/TEST_DATA/monsters.json

@@ -25,5 +25,45 @@
     "species": [ "ZOMBIE", "HUMAN" ],
     "symbol": "Z",
     "upgrades": { "age_grow": 14, "into": "mon_zombie" }
+  },
+  {
+    "type": "item_group",
+    "id": "test_monster_death_drop_base",
+    "subtype": "collection",
+    "items": [ "rock" ]
+  },
+  {
+    "type": "item_group",
+    "id": "test_monster_death_drop_append",
+    "subtype": "collection",
+    "items": [ "stick" ]
+  },
+  {
+    "id": "mon_test_death_drops_append",
+    "copy-from": "debug_mon",
+    "type": "MONSTER",
+    "name": { "str": "death drop append test monster" },
+    "description": "This monster exists only for testing purposes.",
+    "death_drops": "test_monster_death_drop_base"
+  },
+  {
+    "id": "mon_test_death_drops_append",
+    "copy-from": "mon_test_death_drops_append",
+    "type": "MONSTER",
+    "extend": { "death_drops": "test_monster_death_drop_append" }
+  },
+  {
+    "id": "mon_test_death_drops_clear",
+    "copy-from": "debug_mon",
+    "type": "MONSTER",
+    "name": { "str": "death drop clear test monster" },
+    "description": "This monster exists only for testing purposes.",
+    "death_drops": "test_monster_death_drop_base"
+  },
+  {
+    "id": "mon_test_death_drops_clear",
+    "copy-from": "mon_test_death_drops_clear",
+    "type": "MONSTER",
+    "death_drops": ""
   }
 ]

docs/en/mod/json/reference/creatures/monsters.md

@@ -386,6 +386,8 @@ Can freely be a collection to imply two weapons wielded in both hands, as items
 An item group that is used to spawn items when the monster dies. This can be an inlined item group,
 see ITEM_SPAWN.md. The default subtype is "distribution".
 
+Use `"extend": { "death_drops": ... }` in an inherited or overridden monster to add another drop group without replacing existing death drops. A top-level `"death_drops"` member replaces inherited death drops.
+
 ## "death_function"
 
 (array of strings, optional)

docs/ja/mod/json/reference/creatures/monsters.md

@@ -287,6 +287,8 @@ BNのゲームでは、以下の種別が使用可能です： HUMAN(人間)、R
 
 モンスター死亡時にアイテムを生成するために使用されるアイテムグループです。インライン形式のアイテムグループも使用可能です(詳細は ITEM_SPAWN.md を参照)。デフォルトのサブタイプは "distribution"(分布)です。
 
+継承または上書きされたモンスターで `"extend": { "death_drops": ... }` を使うと、既存の死亡時ドロップを置き換えずに別のドロップグループを追加できます。トップレベルの `"death_drops"` メンバーは、継承された死亡時ドロップを置き換えます。
+
 ## "death_function"
 
 (文字列の配列、オプション)

docs/ko/mod/json/reference/creatures/monsters.md

@@ -288,6 +288,8 @@ Ctxt는 동음이의어(같은 이름을 가진 두 가지 다른 것)의 경우
 
 몬스터가 죽을 때 아이템을 스폰하는 데 사용되는 아이템 그룹. 이것은 인라인 아이템 그룹이 될 수 있습니다. ITEM_SPAWN.md를 참조하세요. 기본 하위 유형은 "distribution"입니다.
 
+상속되거나 오버라이드된 몬스터에서 `"extend": { "death_drops": ... }`를 사용하면 기존 사망 드롭을 대체하지 않고 다른 드롭 그룹을 추가합니다. 최상위 `"death_drops"` 멤버는 상속된 사망 드롭을 대체합니다.
+
 ## "death_function"
 
 (array of strings, optional)

src/monster.cpp

@@ -2,9 +2,11 @@
 
 #include <algorithm>
 #include <cmath>
+#include <iterator>
 #include <limits>
 #include <memory>
 #include <optional>
+#include <ranges>
 #include <tuple>
 #include <unordered_map>
 #include <unordered_set>
@@ -3514,12 +3516,17 @@ void monster::drop_items_on_death()
     if( is_hallucination() ) {
         return;
     }
-    if( !type->death_drops ) {
+    if( type->death_drops.empty() ) {
         return;
     }
 
-    auto items = item_group::items_from( type->death_drops,
+    auto items = item_group::items_from( type->death_drops.front(),
                                          calendar::start_of_cataclysm );
+    for( const item_group_id &death_drop : type->death_drops | std::views::drop( 1 ) ) {
+        auto group_items = item_group::items_from( death_drop,
+                           calendar::start_of_cataclysm );
+        std::ranges::move( group_items, std::back_inserter( items ) );
+    }
 
     // Apply both global and category-specific spawn rates
     const auto global_spawn_rate = get_option<float>( "ITEM_SPAWNRATE" );

src/monstergenerator.cpp

@@ -7,6 +7,7 @@
 #include <optional>
 #include <set>
 #include <utility>
+#include <vector>
 
 #include "assign.h"
 #include "bodypart.h"
@@ -220,6 +221,25 @@ std::string enum_to_string<m_flag>( m_flag data )
 
 } // namespace io
 
+namespace
+{
+
+auto add_death_drop_group( std::vector<item_group_id> &death_drops,
+                           const JsonObject &jo, const std::string &member_name ) -> void
+{
+    if( !jo.has_member( member_name ) ) {
+        return;
+    }
+
+    const auto death_drop = item_group::load_item_group( jo.get_member( member_name ),
+                            "distribution" );
+    if( death_drop ) {
+        death_drops.push_back( death_drop );
+    }
+}
+
+} // namespace
+
 // TODO: Make this like any other generic factory so we can use type_id_implement
 /** @relates string_id */
 template<>
@@ -892,8 +912,13 @@ void mtype::load( const JsonObject &jo, const std::string &src )
                          "distribution" );
     }
     if( jo.has_member( "death_drops" ) ) {
-        death_drops = item_group::load_item_group( jo.get_member( "death_drops" ),
-                      "distribution" );
+        death_drops.clear();
+        add_death_drop_group( death_drops, jo, "death_drops" );
+    }
+    if( jo.has_object( "extend" ) ) {
+        auto tmp = jo.get_object( "extend" );
+        tmp.allow_omitted_members();
+        add_death_drop_group( death_drops, tmp, "death_drops" );
     }
 
     assign( jo, "harvest", harvest );
@@ -1561,9 +1586,11 @@ void MonsterGenerator::check_monster_definitions() const
                 debugmsg( "monster %s has invalid species %s", mon.id.c_str(), spec.c_str() );
             }
         }
-        if( mon.death_drops && !item_group::group_is_defined( mon.death_drops ) ) {
-            debugmsg( "monster %s has unknown death drop item group: %s", mon.id.c_str(),
-                      mon.death_drops.c_str() );
+        for( const item_group_id &death_drop : mon.death_drops ) {
+            if( !item_group::group_is_defined( death_drop ) ) {
+                debugmsg( "monster %s has unknown death drop item group: %s", mon.id.c_str(),
+                          death_drop.c_str() );
+            }
         }
         for( auto &m : mon.mat ) {
             if( m.str() == "null" || !m.is_valid() ) {

src/mtype.h

@@ -288,8 +288,8 @@ struct mtype {
         mtype_id id;
 
         std::map<itype_id, int> starting_ammo; // Amount of ammo the monster spawns with.
-        // Name of item group that is used to create item dropped upon death, or empty.
-        item_group_id death_drops;
+        // Names of item groups used to create items dropped upon death.
+        std::vector<item_group_id> death_drops;
 
         /** Stores effect data for effects placed on attack */
         std::vector<mon_effect_data> atk_effs;

tests/monster_test.cpp

@@ -18,10 +18,12 @@
 #include "player.h"
 #include "state_helpers.h"
 #include "test_statistics.h"
+#include "type_id.h"
 #include "vehicle.h"
 #include "vehicle_part.h"
 #include "vpart_position.h"
 
+#include <algorithm>
 #include <cmath>
 #include <fstream>
 #include <list>
@@ -35,6 +37,55 @@
 
 using move_statistics = statistics<int>;
 
+namespace {
+
+auto count_items_at(const tripoint_bub_ms& pos, const itype_id& type) -> int {
+    return std::ranges::count_if(get_map().i_at(pos), [&type](const auto* it) {
+        return it->typeId() == type;
+    });
+}
+
+} // namespace
+
+TEST_CASE("extended monster death drops append to inherited drops", "[monster][death_drops]") {
+    clear_all_state();
+    const auto global_spawn_rate = override_option("ITEM_SPAWNRATE", "1.0");
+    const auto rock_spawn_rate = override_option("SPAWN_RATE_rocks", "1.0");
+    const auto wood_spawn_rate = override_option("SPAWN_RATE_scrap_wood", "1.0");
+    static_cast<void>(global_spawn_rate);
+    static_cast<void>(rock_spawn_rate);
+    static_cast<void>(wood_spawn_rate);
+    move_player_out_of_the_way();
+
+    auto& here = get_map();
+    build_test_map(ter_id("t_floor"));
+
+    const auto monster_pos = tripoint_bub_ms(60, 60, 0);
+    here.i_clear(monster_pos);
+
+    auto& test_monster = spawn_test_monster("mon_test_death_drops_append", monster_pos);
+    test_monster.drop_items_on_death();
+
+    CHECK(count_items_at(monster_pos, itype_id("rock")) == 1);
+    CHECK(count_items_at(monster_pos, itype_id("stick")) == 1);
+}
+
+TEST_CASE("empty top-level monster death drops replace inherited drops", "[monster][death_drops]") {
+    clear_all_state();
+    move_player_out_of_the_way();
+
+    auto& here = get_map();
+    build_test_map(ter_id("t_floor"));
+
+    const auto monster_pos = tripoint_bub_ms(60, 60, 0);
+    here.i_clear(monster_pos);
+
+    auto& test_monster = spawn_test_monster("mon_test_death_drops_clear", monster_pos);
+    test_monster.drop_items_on_death();
+
+    CHECK(here.i_at(monster_pos).empty());
+}
+
 TEST_CASE("hallucination_monsters_do_not_open_real_doors", "[monster][hallucination]") {
     clear_all_state();
     move_player_out_of_the_way();