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+/*
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ *
+ * The Original Code is Copyright (C) 2014 by Blender Foundation.
+ * All rights reserved.
+ */
+#ifndef __BKE_LIB_QUERY_H__
+#define __BKE_LIB_QUERY_H__
+
+/** \file
+ * \ingroup bke
+ *
+ * API to perform operations over all ID pointers used by a given data-block.
+ *
+ * \note `BKE_lib_` files are for operations over data-blocks themselves, although they might
+ * alter Main as well (when creating/renaming/deleting an ID e.g.).
+ *
+ * \section Function Names
+ *
+ * \warning Descriptions below is ideal goal, current status of naming does not yet fully follow it
+ * (this is WIP).
+ *
+ * - `BKE_lib_query_` should be used for functions in that file.
+ */
+
+struct ID;
+struct Main;
+
+/* Tips for the callback for cases it's gonna to modify the pointer. */
+enum {
+  IDWALK_CB_NOP = 0,
+  IDWALK_CB_NEVER_NULL = (1 << 0),
+  IDWALK_CB_NEVER_SELF = (1 << 1),
+
+  /**
+   * Indicates whether this is direct (i.e. by local data) or indirect (i.e. by linked data) usage.
+   * \note Object proxies are half-local, half-linked...
+   */
+  IDWALK_CB_INDIRECT_USAGE = (1 << 2),
+
+  /**
+   * That ID is used as mere sub-data by its owner
+   * (only case currently: those f***ing nodetrees in materials etc.).
+   * This means callback shall not *do* anything,
+   * only use this as informative data if it needs it.
+   */
+  IDWALK_CB_PRIVATE = (1 << 3),
+
+  /** That ID is not really used by its owner, it's just an internal hint/helper.
+   * This addresses Their Highest Ugliness the 'from' pointers: Object->from_proxy and Key->from.
+   * How to handle that kind of cases totally depends on what caller code is doing... */
+  IDWALK_CB_LOOPBACK = (1 << 4),
+
+  /** That ID is used as library override's reference by its owner. */
+  IDWALK_CB_OVERRIDE_LIBRARY_REFERENCE = (1 << 5),
+
+  /**
+   * Adjusts #ID.us reference-count.
+   * \note keep in sync with 'newlibadr_us' use in readfile.c
+   */
+  IDWALK_CB_USER = (1 << 8),
+  /** Ensure #ID.us is at least 1 on use. */
+  IDWALK_CB_USER_ONE = (1 << 9),
+};
+
+enum {
+  IDWALK_RET_NOP = 0,
+  /** Completely stop iteration. */
+  IDWALK_RET_STOP_ITER = 1 << 0,
+  /** Stop recursion, that is, do not loop over ID used by current one. */
+  IDWALK_RET_STOP_RECURSION = 1 << 1,
+};
+
+/**
+ * Call a callback for each ID link which the given ID uses.
+ *
+ * \return a set of flags to control further iteration (0 to keep going).
+ */
+typedef int (*LibraryIDLinkCallback)(void *user_data,
+                                     struct ID *id_self,
+                                     struct ID **id_pointer,
+                                     int cb_flag);
+
+/* Flags for the foreach function itself. */
+enum {
+  IDWALK_NOP = 0,
+  IDWALK_READONLY = (1 << 0),
+  IDWALK_RECURSE = (1 << 1), /* Also implies IDWALK_READONLY. */
+
+  IDWALK_NO_INDIRECT_PROXY_DATA_USAGE = (1 << 8), /* Ugly special case :(((( */
+};
+
+/* Loop over all of the ID's this datablock links to. */
+void BKE_library_foreach_ID_link(
+    struct Main *bmain, struct ID *id, LibraryIDLinkCallback callback, void *user_data, int flag);
+void BKE_library_update_ID_link_user(struct ID *id_dst, struct ID *id_src, const int cb_flag);
+
+int BKE_library_ID_use_ID(struct ID *id_user, struct ID *id_used);
+
+bool BKE_library_id_can_use_idtype(struct ID *id_owner, const short id_type_used);
+
+bool BKE_library_ID_is_locally_used(struct Main *bmain, void *idv);
+bool BKE_library_ID_is_indirectly_used(struct Main *bmain, void *idv);
+void BKE_library_ID_test_usages(struct Main *bmain,
+                                void *idv,
+                                bool *is_used_local,
+                                bool *is_used_linked);
+
+void BKE_library_unused_linked_data_set_tag(struct Main *bmain, const bool do_init_tag);
+void BKE_library_indirectly_used_data_tag_clear(struct Main *bmain);
+
+#endif /* __BKE_LIB_QUERY_H__ */