Potion stacks in every player inventory and the chest, not in trader stock

The chest is inventory 0x11 and already saved as JSON; stacking now applies
to all inventories below 0x12. Traders keep selling single potions.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/potion-stacks.md

@@ -57,3 +57,13 @@ Not seen yet: a sale, a right-click drink.
 - Dropping a stack on the ground: it should split into single potions; for now
   the count stays with the object while the game runs, but a save keeps only
   the object (one potion).
+
+## Chest and other inventories (2026-10-17)
+
+Asked: potions stack in every kind of inventory; traders only sell single
+potions. Measured: the chest is inventory 0x11 (`cUI_Chest` draws window
++0x158), saved with the hero as chunk 0xc6 and in save games, both through the
+JSON path already (stack sizes survived a reload). Stacking now applies to all
+inventories below 0x12 (players and the chest); trader stock (0x12 and up)
+never stacks. Selling a stack to a trader still has to be handled (pay
+count x price, the trader keeps singles).

src/game/engine/items.h

@@ -42,7 +42,16 @@ constexpr size_t kInventoryGrid = 0x18;
 constexpr size_t kGridCellSize = 0xc;
 constexpr size_t kGridWidth = 0x115c;   // u16, 0x14
 constexpr size_t kGridHeight = 0x115e;  // u16, 8
-constexpr size_t kInventoryOwner = 0x1160;  // u16: player index (heroes < 0x12)
+constexpr size_t kInventoryOwner = 0x1160;  // u16: = the inventory's index (measured)
+
+// Inventories below 0x12 belong to the players (index = hero id) and the
+// chest (0x11, saved with the hero as chunk 0xc6); PlaceItem / RemoveAt only
+// update the hero below that. Traders use higher ones (22 measured).
+constexpr uint16_t kPlayerInventoryEnd = 0x12;
+constexpr uint16_t kChestInventory = 0x11;
+inline bool IsPlayerInventory(const uint8_t* inventory) {
+    return *reinterpret_cast<const uint16_t*>(inventory + kInventoryOwner) < kPlayerInventoryEnd;
+}
 
 inline unsigned GridCells(const uint8_t* inventory) {
     return *reinterpret_cast<const uint16_t*>(inventory + kGridWidth) *

src/mods/potionstack.cpp

@@ -5,8 +5,11 @@
 // id, with the item type to tell a reused id apart) and saved with the JSON
 // inventories (src/mods/inventory_save.cpp).
 //
-// - Entry: a potion placed into the hero's grid (picked up, bought, dropped
-//   from the cursor; cInventoryEntry::PlaceItem 0x5d9ec0) pours into other
+// Stacks exist in the players' inventories and the chest (inventory 0x11);
+// trader stock waits for stack prices.
+//
+// - Entry: a potion placed into a grid (picked up, bought, dropped from the
+//   cursor; cInventoryEntry::PlaceItem 0x5d9ec0) pours into other
 //   stacks of its type that have room, on the next engine tick (the caller
 //   still uses the object while placing it). An emptied object is removed and
 //   destroyed.
@@ -36,9 +39,7 @@
 #include "game/engine/events.h"
 #include "game/engine/items.h"
 #include "game/engine/objects.h"
-#include "game/engine/orders.h"
 #include "game/ui/text.h"
-#include "mods/skillslots.h"
 #include "re/hook.h"
 
 namespace mods::potionstack {
@@ -61,8 +62,12 @@ struct Stack {
 std::mutex g_lock;
 std::unordered_map<uint32_t, Stack> g_stacks;
 
-// Potions placed into the hero's grid, poured on the next tick.
-std::vector<uint32_t> g_entered;
+// Potions placed into a grid, poured on the next tick.
+struct Entry {
+    uint8_t* inventory;
+    uint32_t id;
+};
+std::vector<Entry> g_entered;
 // Inside cInventoryEntry::Sort (its re-placements are no entries).
 thread_local bool g_sorting = false;
 // The stack a drink just took a potion from (per thread: drinks come from the
@@ -75,14 +80,10 @@ uint32_t TypeOf(uint32_t id) {
     return item ? game::ItemType(item) : 0;
 }
 
-uint8_t* HeroInventory() {
-    void* hero = game::GetPlayerHero();
-    return hero ? skillslots::Inventory(static_cast<uint32_t>(game::CreatureField(hero, game::kCreatureId)))
-                : nullptr;
-}
-
-bool IsHeroInventory(const void* inventory) {
-    return inventory && inventory == HeroInventory();
+// Potions stack in the players' inventories and the chest; trader stock not
+// yet (buying from a stack would sell the whole stack for one price).
+bool Stacks(const void* inventory) {
+    return inventory && game::IsPlayerInventory(static_cast<const uint8_t*>(inventory));
 }
 
 bool InGrid(const uint8_t* inventory, uint32_t id) {
@@ -139,18 +140,14 @@ void StackAll(uint8_t* inventory) {
 }
 
 void OnTick(uint8_t*) {
-    std::vector<uint32_t> entered;
+    std::vector<Entry> entered;
     {
         std::lock_guard guard(g_lock);
         entered.swap(g_entered);
     }
-    uint8_t* inventory = entered.empty() ? nullptr : HeroInventory();
-    if (!inventory) {
-        return;
-    }
-    for (const uint32_t id : entered) {
-        if (InGrid(inventory, id)) {
-            Pour(inventory, id);
+    for (const Entry& entry : entered) {
+        if (InGrid(entry.inventory, entry.id)) {
+            Pour(entry.inventory, entry.id);
         }
     }
 }
@@ -249,9 +246,9 @@ int __fastcall Inv_PlaceItem(void* self, void* edx, uint32_t cell, uint32_t id,
 RE_HOOK(0x005d9ec0, Inv_PlaceItem);
 int __fastcall Inv_PlaceItem(void* self, void* edx, uint32_t cell, uint32_t id, uint32_t w, uint32_t h, uint32_t notify) {
     const int previous = Inv_PlaceItem_original(self, edx, cell, id, w, h, notify);
-    if (!g_sorting && PotionKindOf(id) && IsHeroInventory(self)) {
+    if (!g_sorting && PotionKindOf(id) && Stacks(self)) {
         std::lock_guard guard(g_lock);
-        g_entered.push_back(id);
+        g_entered.push_back({static_cast<uint8_t*>(self), id});
     }
     return previous;
 }
@@ -296,11 +293,11 @@ char __fastcall Creature_UseItem(void* self, void* edx, uint32_t id) {
     return Creature_UseItem_original(self, edx, id);
 }
 
-// cInventoryEntry::Sort (0x5db670, ret 4): stack the hero's potions first.
+// cInventoryEntry::Sort (0x5db670, ret 4): stack the potions first.
 void __fastcall Inv_Sort(void* self, void* edx, uint32_t mode);
 RE_HOOK(0x005db670, Inv_Sort);
 void __fastcall Inv_Sort(void* self, void* edx, uint32_t mode) {
-    if (IsHeroInventory(self)) {
+    if (Stacks(self)) {
         StackAll(static_cast<uint8_t*>(self));
     }
     g_sorting = true;