So far most of the bugs are solved but there's still some edge cases in the inventory dance.

gui/dnd_loot.cpp

@@ -126,7 +126,7 @@ namespace gui {
 
     switch(ev) {
       case INV_SELECT:
-        commit_drop($loot_ui.$gui, $status_ui.$gui, $grab_source, data);
+        if(commit_drop($loot_ui.$gui, $status_ui.$gui, $grab_source, data))
         {
           END(Event::CLOSE);
         }

gui/status_ui.cpp

@@ -103,18 +103,16 @@ namespace gui {
     auto& slot_name = $slot_to_name.at(gui_id);
     auto& inventory = $level.world->get_the<inventory::Model>();
 
-    for(auto [ent, slot] : inventory.by_entity) {
+    inventory.dump();
-      fmt::println("BY_ENTITY: ent={}, slot={}", ent, slot);
-    }
 
-    for(auto [slot, ent] : inventory.by_slot) {
+    if(inventory.add(slot_name, world_entity)) {
-      fmt::println("BY_SLOT: ent={}, slot={}", ent, slot);
+      $level.world->make_constant(world_entity);
+      update();
+      return true;
+    }  else {
+      dbc::log("there's something there already");
+      return false;
     }
-
-    $level.world->make_constant(world_entity);
-    inventory.add(slot_name, world_entity);
-    update();
-    return true;
   }
 
   bool StatusUI::drop_item(DinkyECS::Entity item_id) {

inventory.cpp

@@ -1,17 +1,15 @@
 #include "inventory.hpp"
 
 namespace inventory {
-  void Model::add(const Slot &in_slot, DinkyECS::Entity ent) {
+  bool Model::add(const Slot &in_slot, DinkyECS::Entity ent) {
-    if(by_entity.contains(ent)) {
+    invariant();
-      // doing it this way so we can get the offending entity, otherwise it
+
-      // crashes on the by_entity.at when this test _passes_
+    if(by_slot.contains(in_slot)) return false;
-      dbc::sentinel(fmt::format("failed to add item to inventory, entity {} is already in the inventory slot {}", ent, by_entity.at(ent)));
-    }
 
     by_entity.insert_or_assign(ent, in_slot);
     by_slot.insert_or_assign(in_slot, ent);
 
-    invariant();
+    return true;
   }
 
   Slot& Model::get(DinkyECS::Entity ent) {
@@ -49,9 +47,6 @@ namespace inventory {
   }
 
   void Model::invariant() {
-    dbc::check(by_slot.size() == by_entity.size(), "by_slot and by_entity have differing sizes");
-    // std::unordered_map<DinkyECS::Entity, Slot> find_dupes;
-
     for(auto& [slot, ent] : by_slot) {
       dbc::check(by_entity.at(ent) == slot,
           fmt::format("mismatched slot {} in by_slot doesn't match entity {}", slot, ent));
@@ -61,5 +56,18 @@ namespace inventory {
       dbc::check(by_slot.at(slot) == ent,
           fmt::format("mismatched entity {} in by_entity doesn't match entity {}", ent, slot));
     }
+
+    dbc::check(by_slot.size() == by_entity.size(), "by_slot and by_entity have differing sizes");
+  }
+
+  void Model::dump() {
+    invariant();
+    fmt::println("INVENTORY has {} slots, sizes equal? {}, contents:",
+        by_entity.size(), by_entity.size() == by_slot.size());
+
+    for(auto [slot, ent] : by_slot) {
+      fmt::println("slot={}, ent={}, both={}, equal={}",
+          slot, ent, by_entity.contains(ent), by_entity.at(ent) == slot);
+    }
   }
 }

inventory.hpp

@@ -8,7 +8,7 @@ namespace inventory {
     std::unordered_map<Slot, DinkyECS::Entity> by_slot;
     std::unordered_map<DinkyECS::Entity, Slot> by_entity;
 
-    void add(const Slot &in_slot, DinkyECS::Entity ent);
+    bool add(const Slot &in_slot, DinkyECS::Entity ent);
     Slot& get(DinkyECS::Entity ent);
     DinkyECS::Entity get(const Slot& slot);
     bool has(DinkyECS::Entity ent);
@@ -17,5 +17,6 @@ namespace inventory {
     void remove(DinkyECS::Entity ent);
     void remove(const Slot& slot);
     void invariant();
+    void dump();
   };
 }

tests/inventory.cpp

@@ -9,12 +9,17 @@ TEST_CASE("base test", "[inventory]") {
   inventory::Model inv;
   DinkyECS::Entity test_ent = 1;
 
-  inv.add("hand_l", test_ent);
+  bool good = inv.add("hand_l", test_ent);
   inv.invariant();
+  REQUIRE(good);
 
   auto& slot = inv.get(test_ent);
   REQUIRE(slot == "hand_l");
 
+  // confirm that we get false when trying to do it again
+  good = inv.add("hand_l", test_ent);
+  REQUIRE(!good);
+
   auto ent = inv.get(slot);
   REQUIRE(ent == test_ent);
 
@@ -27,7 +32,8 @@ TEST_CASE("base test", "[inventory]") {
   REQUIRE(!inv.has(ent));
 
   // test remove just by slot
-  inv.add("hand_r", test_ent);
+  good = inv.add("hand_r", test_ent);
+  REQUIRE(good);
   REQUIRE(inv.has("hand_r"));
   REQUIRE(inv.has(test_ent));
   inv.invariant();
@@ -37,7 +43,8 @@ TEST_CASE("base test", "[inventory]") {
   REQUIRE(!inv.has(test_ent));
 
   // test remove just by entity
-  inv.add("pocket_l", test_ent);
+  good = inv.add("pocket_l", test_ent);
+  REQUIRE(good);
   REQUIRE(inv.has("pocket_l"));
   REQUIRE(inv.has(test_ent));
   inv.invariant();

worldbuilder.cpp

@@ -179,6 +179,7 @@ void WorldBuilder::configure_starting_items(DinkyECS::World &world) {
 
   auto &inventory = world.get_the<inventory::Model>();
   inventory.add("hand_r", torch_id);
+  world.make_constant(torch_id);
 }
 
 void WorldBuilder::place_entities(DinkyECS::World &world) {