calculate store offset at compile time

src/entity.c

@@ -1,6 +1,9 @@
 #include "entity.h"
 #include "math.h"
 
+/* Offset in bytes from start of entities array to start of store struct */
+#define STORE_ENTITIES_OFFSET (sizeof(struct entity_store) + (sizeof(struct entity_store) % alignof(struct entity)))
+
 /* Accessed via entity_store_nil() */
 READONLY struct entity_store _g_entity_store_nil = {
     0
@@ -25,13 +28,7 @@ struct entity_store *entity_store_alloc(void)
     struct entity_store *store = arena_push_zero(&arena, struct entity_store);
     store->arena = arena;
     store->entities = arena_dry_push(&arena, struct entity);
-
+    ASSERT((u64)store->entities - (u64)store == STORE_ENTITIES_OFFSET);  /* Offset must be correct */
-    /* Special entity for keeping track of store offset */
-    struct entity *offset_entity = entity_alloc(store);
-    offset_entity->valid = false;
-    offset_entity->store_offset = (u64)store->entities - (u64)store;
-    ASSERT(offset_entity->handle.idx == 0);  /* Must be first entity in array */
-
     return store;
 }
 
@@ -44,12 +41,10 @@ void entity_store_copy_replace(struct entity_store *dest, struct entity_store *s
 {
     struct arena dest_arena = dest->arena;
     struct entity *dest_entities = dest->entities;
-    u64 dest_store_offset = dest_entities[0].store_offset;
     MEMCPY_STRUCT(dest, src);
     arena_copy_replace(&dest_arena, &src->arena);
     dest->arena = dest_arena;
     dest->entities = dest_entities;
-    dest->entities[0].store_offset = dest_store_offset;
 }
 
 /* ========================== *
@@ -89,6 +84,17 @@ void entity_release(struct entity_store *store, struct entity *entity)
  * Query
  * ==========================  */
 
+struct entity_store *entity_get_store(struct entity *ent)
+{
+    struct entity_store *store = entity_store_nil();
+    if (ent->valid) {
+        u64 first_entity_addr = (u64)(ent - ent->handle.idx);
+        store = (struct entity_store *)(first_entity_addr - STORE_ENTITIES_OFFSET);
+        ASSERT(store->entities == (struct entity *)first_entity_addr);
+    }
+    return store;
+}
+
 /* Returns a valid entity or nil entity. Always safe to read result, need to check `valid` to write. */
 struct entity *entity_from_handle(struct entity_store *store, struct entity_handle handle)
 {
@@ -140,26 +146,6 @@ struct entity *entity_find_first_match_all(struct entity_store *store, struct en
  * Xform
  * ========================== */
 
-
-
-
-
-/* TODO: Move this */
-INTERNAL struct entity_store *entity_get_store(struct entity *ent)
-{
-    struct entity_store *store = entity_store_nil();
-    if (ent->valid) {
-        struct entity *first_entity = ent - ent->handle.idx;
-        u64 store_offset = first_entity->store_offset;
-        store = (struct entity_store *)((u64)first_entity - store_offset);
-        ASSERT(store->entities == first_entity);
-    }
-    return store;
-}
-
-
-
-
 struct xform entity_get_local_xform(struct entity *ent)
 {
     return ent->xform;
@@ -196,8 +182,10 @@ void entity_set_global_xform(struct entity *ent, struct xform xf)
  * Tree
  * ========================== */
 
-void entity_link(struct entity_store *store, struct entity *parent, struct entity *child)
+void entity_link(struct entity *parent, struct entity *child)
 {
+    struct entity_store *store = entity_get_store(parent);
+
     struct entity *first_child = entity_from_handle(store, parent->first);
     struct entity *last_child = entity_from_handle(store, parent->last);
 

src/entity.h

@@ -181,11 +181,12 @@ struct xform entity_get_global_xform(struct entity *ent);
 void entity_set_global_xform(struct entity *ent, struct xform xf);
 
 /* Query */
+struct entity_store *entity_get_store(struct entity *ent);
 struct entity *entity_from_handle(struct entity_store *store, struct entity_handle handle);
 struct entity *entity_find_first_match_one(struct entity_store *store, enum entity_prop prop);
 struct entity *entity_find_first_match_all(struct entity_store *store, struct entity_prop_array props);
 
 /* Tree */
-void entity_link(struct entity_store *store, struct entity *parent, struct entity *child);
+void entity_link(struct entity *parent, struct entity *child);
 
 #endif