1 files changed, 41 insertions, 9 deletions

diff --git a/source/blender/blenkernel/BKE_main.h b/source/blender/blenkernel/BKE_main.h
index 8106607572b..b6116b32ca5 100644
--- a/source/blender/blenkernel/BKE_main.h
+++ b/source/blender/blenkernel/BKE_main.h
@@ -60,22 +60,52 @@ typedef struct BlendThumbnail {
 } BlendThumbnail;
 
 /* Structs caching relations between data-blocks in a given Main. */
+typedef struct MainIDRelationsEntryItem {
+  struct MainIDRelationsEntryItem *next;
+
+  union {
+    /* For `from_ids` list, a user of the hashed ID. */
+    struct ID *from;
+    /* For `to_ids` list, an ID used by the hashed ID. */
+    struct ID **to;
+  } id_pointer;
+  /* Session uuid of the `id_pointer`. */
+  uint session_uuid;
+
+  int usage_flag; /* Using IDWALK_ enums, defined in BKE_lib_query.h */
+} MainIDRelationsEntryItem;
+
 typedef struct MainIDRelationsEntry {
-  struct MainIDRelationsEntry *next;
-  /* WARNING! for user_to_used,
-   * that pointer is really an ID** one, but for used_to_user, it’s only an ID* one! */
-  struct ID **id_pointer;
-  int usage_flag; /* Using IDWALK_ enums, in BKE_lib_query.h */
+  /* Linked list of IDs using that ID. */
+  struct MainIDRelationsEntryItem *from_ids;
+  /* Linked list of IDs used by that ID. */
+  struct MainIDRelationsEntryItem *to_ids;
+
+  /* Session uuid of the ID matching that entry. */
+  uint session_uuid;
+
+  /* Runtime tags, users should ensure those are reset after usage. */
+  uint tags;
 } MainIDRelationsEntry;
 
+/* MainIDRelationsEntry.tags */
+typedef enum MainIDRelationsEntryTags {
+  /* Generic tag marking the entry as to be processed. */
+  MAINIDRELATIONS_ENTRY_TAGS_DOIT = 1 << 0,
+  /* Generic tag marking the entry as processed. */
+  MAINIDRELATIONS_ENTRY_TAGS_PROCESSED = 1 << 1,
+} MainIDRelationsEntryTags;
+
 typedef struct MainIDRelations {
-  struct GHash *id_user_to_used;
-  struct GHash *id_used_to_user;
+  /* Mapping from an ID pointer to all of its parents (IDs using it) and children (IDs it uses).
+   * Values are `MainIDRelationsEntry` pointers. */
+  struct GHash *relations_from_pointers;
+  /* Note: we could add more mappings when needed (e.g. from session uuid?). */
 
   short flag;
 
   /* Private... */
-  struct BLI_mempool *entry_pool;
+  struct BLI_mempool *entry_items_pool;
 } MainIDRelations;
 
 enum {
@@ -172,7 +202,9 @@ void BKE_main_unlock(struct Main *bmain);
 
 void BKE_main_relations_create(struct Main *bmain, const short flag);
 void BKE_main_relations_free(struct Main *bmain);
-void BKE_main_relations_ID_remove(struct Main *bmain, struct ID *id);
+void BKE_main_relations_tag_set(struct Main *bmain,
+                                const MainIDRelationsEntryTags tag,
+                                const bool value);
 
 struct GSet *BKE_main_gset_create(struct Main *bmain, struct GSet *gset);