items: introduce Item_GetIndex

This provides a common function to retrieve an item index.

src/libtrx/game/items.c

@@ -26,6 +26,11 @@ void Item_TakeDamage(
     }
 }
 
+int16_t Item_GetIndex(const ITEM *const item)
+{
+    return item - Item_Get(0);
+}
+
 ITEM *Item_Find(const GAME_OBJECT_ID obj_id)
 {
     for (int32_t item_num = 0; item_num < Item_GetTotalCount(); item_num++) {

src/libtrx/include/libtrx/game/items/common.h

@@ -4,6 +4,7 @@
 #include "./types.h"
 
 ITEM *Item_Get(int16_t num);
+int16_t Item_GetIndex(const ITEM *item);
 ITEM *Item_Find(GAME_OBJECT_ID obj_id);
 int32_t Item_GetTotalCount(void);
 int32_t Item_GetDistance(const ITEM *item, const XYZ_32 *target);

src/tr1/game/lara/cheat.c

@@ -448,7 +448,7 @@ bool Lara_Cheat_Teleport(int32_t x, int32_t y, int32_t z)
     g_LaraItem->floor = height;
 
     if (g_LaraItem->room_num != room_num) {
-        const int16_t item_num = g_LaraItem - Item_Get(0);
+        const int16_t item_num = Item_GetIndex(g_LaraItem);
         Item_NewRoom(item_num, room_num);
     }
 

src/tr1/game/overlay.c

@@ -569,7 +569,7 @@ static void M_BarDrawEnemy(void)
         break;
 
     case BSM_BOSS_ONLY:
-        m_EnemyBar.show = Creature_IsBoss(g_Lara.target - Item_Get(0));
+        m_EnemyBar.show = Creature_IsBoss(Item_GetIndex(g_Lara.target));
         break;
     }
 

src/tr2/game/lara/cheat.c

@@ -373,7 +373,7 @@ bool Lara_Cheat_Teleport(int32_t x, int32_t y, int32_t z)
     g_LaraItem->floor = height;
 
     if (g_LaraItem->room_num != room_num) {
-        const int16_t item_num = g_LaraItem - Item_Get(0);
+        const int16_t item_num = Item_GetIndex(g_LaraItem);
         Item_NewRoom(item_num, room_num);
     }
 

src/tr2/game/objects/creatures/skidoo_driver.c

@@ -40,8 +40,7 @@ static int16_t M_ControlAlive(ITEM *driver_item, ITEM *skidoo_item);
 
 static void M_KillDriver(ITEM *const driver_item)
 {
-    // TODO: Item_GetIndex(const ITEM *item)
-    const int32_t driver_item_num = driver_item - Item_Get(0);
+    const int32_t driver_item_num = Item_GetIndex(driver_item);
     Item_RemoveActive(driver_item_num);
     driver_item->collidable = 0;
     driver_item->flags |= IF_ONE_SHOT;
@@ -50,7 +49,7 @@ static void M_KillDriver(ITEM *const driver_item)
 
 static void M_MakeMountable(ITEM *const skidoo_item)
 {
-    const int32_t skidoo_item_num = skidoo_item - Item_Get(0);
+    const int32_t skidoo_item_num = Item_GetIndex(skidoo_item);
     LOT_DisableBaddieAI(skidoo_item_num);
     skidoo_item->object_id = O_SKIDOO_FAST;
     skidoo_item->status = IS_DEACTIVATED;