Corpses can decay into bones, etc.

doc/MONSTERS.md

@@ -94,6 +94,8 @@ Property                 | Description
 `path_settings`          | (object) How monster may find a path, open doors, avoid traps, or bash obstacles
 `biosignature`           | (object) Droppings or feces left by the animal or monster
 `harvest`                | (string) ID of a "harvest" type describing what can be harvested from the corpse
+`dissect`                | (string) (Optional) ID of a "harvest" type describing what is returned when a corpse of this monster is dissected
+`decay`                  | (string) (Optional) ID of a "harvest" type describing what is left when a corpse of this monster rots away
 `zombify_into`           | (string) mtype_id this monster zombifies into after it's death
 `fungalize_into`         | (string) mtype_id this monster turns into when fungalized by spores
 `shearing`               | (array of objects) Items produced when the monster is sheared

src/item.cpp

@@ -14234,6 +14234,9 @@ bool item::process_internal( map &here, Character *carrier, const tripoint &pos,
             if( is_comestible() ) {
                 here.rotten_item_spawn( *this, pos );
             }
+            if( is_corpse() ) {
+                here.handle_decayed_corpse( *this, here.getglobal( pos ) );
+            }
             return true;
         }
     } else {
@@ -14620,6 +14623,11 @@ std::string item::type_name( unsigned int quantity, bool use_variant, bool use_c
     return ret_name;
 }
 
+const mtype *item::get_corpse_mon() const
+{
+    return corpse;
+}
+
 std::string item::get_corpse_name() const
 {
     return corpse_name;

src/item.h

@@ -3081,6 +3081,7 @@ class item : public visitable
         item_components components;
         /** What faults (if any) currently apply to this item */
         cata::heap<std::set<fault_id>> faults;
+        const mtype *get_corpse_mon() const;
 
     private:
         item_contents contents;

src/map.cpp

@@ -8414,6 +8414,56 @@ void map::loadn( const point &grid, bool update_vehicles )
     }
 }
 
+void map::handle_decayed_corpse( const item &it, const tripoint_abs_ms &pnt )
+{
+    if( !it.is_corpse() ) {
+        debugmsg( "Tried to decay a non-corpse item %s. Aborted", it.tname() );
+        return;
+    }
+    const mtype *dead_monster = it.get_corpse_mon();
+    if( !dead_monster ) {
+        debugmsg( "Corpse at abs_ms %s has no associated monster?!", pnt.to_string() );
+        return;
+    }
+
+    if( dead_monster->decay.is_null() ) {
+        return;
+    }
+    int decayed_weight_grams = to_gram( dead_monster->weight ); // corpse might have stuff in it!
+    decayed_weight_grams += std::round( decayed_weight_grams * rng_float( -0.1, 0.1 ) );
+    bool notify_player = false;
+    if( calendar::once_every( 30_minutes ) ) {
+        //one corpse max in 30 minutes will notify if seen, for *all* the items it left
+        notify_player = true;
+    }
+
+    bool anything_left = false;
+    for( const harvest_entry &entry : dead_monster->decay.obj() ) {
+        item harvest = item( entry.drop );
+        const float random_decay_modifier = rng_float( 0.0f, static_cast<float>( MAX_SKILL ) );
+        const float min_num = entry.scale_num.first * random_decay_modifier + entry.base_num.first;
+        const float max_num = entry.scale_num.second * random_decay_modifier + entry.base_num.second;
+        int roll = 0;
+        if( entry.mass_ratio != 0.00f ) {
+            roll = static_cast<int>( std::round( entry.mass_ratio * decayed_weight_grams ) );
+            roll = std::ceil( static_cast<double>( roll ) / to_gram( harvest.type->weight ) );
+        } else {
+            roll = std::min<int>( entry.max, std::round( rng_float( min_num, max_num ) ) );
+        }
+        anything_left = roll > 0;
+        for( int i = 0; i < roll; i++ ) {
+            if( harvest.has_temperature() ) {
+                harvest.set_item_temperature( get_weather().get_temperature( project_to<coords::omt>( pnt ) ) );
+            }
+            add_item_or_charges( bub_from_abs( pnt ), harvest, false );
+            if( anything_left && notify_player ) {
+                add_msg_if_player_sees( getlocal( pnt ), _( "You notice a %1$s has rotted away, leaving a %2$s." ),
+                                        it.tname(), harvest.tname() );
+            }
+        }
+    }
+}
+
 void map::rotten_item_spawn( const item &item, const tripoint &pnt )
 {
     if( get_creature_tracker().creature_at( pnt ) != nullptr ) {

src/map.h

@@ -2033,6 +2033,13 @@ class map
          */
         void spawn_monsters( bool ignore_sight, bool spawn_nonlocal = false );
 
+        /**
+        * Checks to see if the corpse that is rotting away generates items when it does.
+        * @param it item that is spawning creatures
+        * @param pnt The point on this map where the item is and where bones/etc will be
+        */
+        void handle_decayed_corpse( const item &it, const tripoint_abs_ms &pnt );
+
         /**
         * Checks to see if the item that is rotting away generates a creature when it does.
         * @param item item that is spawning creatures

src/monstergenerator.cpp

@@ -1080,6 +1080,8 @@ void mtype::load( const JsonObject &jo, const std::string &src )
 
     optional( jo, was_loaded, "dissect", dissect );
 
+    optional( jo, was_loaded, "decay", decay );
+
     if( jo.has_array( "shearing" ) ) {
         std::vector<shearing_entry> entries;
         for( JsonObject shearing_entry : jo.get_array( "shearing" ) ) {

src/mtype.cpp

@@ -19,6 +19,7 @@
 #include "weakpoint.h"
 
 static const harvest_id harvest_list_human( "human" );
+static const harvest_id harvest_list_null( "null" );
 
 static const itype_id itype_bone( "bone" );
 static const itype_id itype_bone_tainted( "bone_tainted" );
@@ -319,6 +320,7 @@ mtype::mtype()
     sp_defense = nullptr;
     melee_training_cap = MAX_SKILL;
     harvest = harvest_list_human;
+    decay = harvest_list_null;
     luminance = 0;
     bash_skill = 0;
 

src/mtype.h

@@ -302,6 +302,7 @@ struct mtype {
         mfaction_str_id default_faction;
         harvest_id harvest;
         harvest_id dissect;
+        harvest_id decay;
         speed_description_id speed_desc;
         // Monster upgrade variables
         mtype_id upgrade_into;