Cleanup/refactor: Undosys: Get rid of the magic values for undo direction.

Move `eUndoStepDir` to `BKE_undo_system.h` and use its values
everywhere.

Note that this also introduce the `STEP_INVALID` value in that enum.

Finally, kept the matching struct members in some lower-level readfile
code as an `int` to avoid having to include `BKE_undo_system.h` in a lot
of unrelated files.

3 files changed, 21 insertions, 15 deletions

diff --git a/source/blender/blenkernel/intern/blender_undo.c b/source/blender/blenkernel/intern/blender_undo.c
index 9e061b1ac69..d826aaf24e3 100644
--- a/source/blender/blenkernel/intern/blender_undo.c
+++ b/source/blender/blenkernel/intern/blender_undo.c
@@ -48,6 +48,7 @@
 #include "BKE_context.h"
 #include "BKE_global.h"
 #include "BKE_main.h"
+#include "BKE_undo_system.h"
 
 #include "BLO_readfile.h"
 #include "BLO_undofile.h"
@@ -62,7 +63,7 @@
 #define UNDO_DISK 0
 
 bool BKE_memfile_undo_decode(MemFileUndoData *mfu,
-                             const int undo_direction,
+                             const eUndoStepDir undo_direction,
                              const bool use_old_bmain_data,
                              bContext *C)
 {
@@ -80,7 +81,7 @@ bool BKE_memfile_undo_decode(MemFileUndoData *mfu,
   }
   else {
     struct BlendFileReadParams params = {0};
-    params.undo_direction = undo_direction > 0 ? 1 : -1;
+    params.undo_direction = undo_direction;
     if (!use_old_bmain_data) {
       params.skip_flags |= BLO_READ_SKIP_UNDO_OLD_MAIN;
     }
diff --git a/source/blender/blenkernel/intern/blendfile.c b/source/blender/blenkernel/intern/blendfile.c
index 49475c014cd..32710c4fa60 100644
--- a/source/blender/blenkernel/intern/blendfile.c
+++ b/source/blender/blenkernel/intern/blendfile.c
@@ -57,6 +57,7 @@
 #include "BKE_scene.h"
 #include "BKE_screen.h"
 #include "BKE_studiolight.h"
+#include "BKE_undo_system.h"
 #include "BKE_workspace.h"
 
 #include "BLO_readfile.h"
@@ -148,7 +149,7 @@ static void setup_app_data(bContext *C,
     LOAD_UNDO,
   } mode;
 
-  if (params->undo_direction != 0) {
+  if (params->undo_direction != STEP_INVALID) {
     BLI_assert(bfd->curscene != NULL);
     mode = LOAD_UNDO;
   }
diff --git a/source/blender/blenkernel/intern/undo_system.c b/source/blender/blenkernel/intern/undo_system.c
index 643510cf652..07e7d07f1ef 100644
--- a/source/blender/blenkernel/intern/undo_system.c
+++ b/source/blender/blenkernel/intern/undo_system.c
@@ -176,8 +176,12 @@ static bool undosys_step_encode(bContext *C, Main *bmain, UndoStack *ustack, Und
   return ok;
 }
 
-static void undosys_step_decode(
-    bContext *C, Main *bmain, UndoStack *ustack, UndoStep *us, int dir, bool is_final)
+static void undosys_step_decode(bContext *C,
+                                Main *bmain,
+                                UndoStack *ustack,
+                                UndoStep *us,
+                                const eUndoStepDir dir,
+                                bool is_final)
 {
   CLOG_INFO(&LOG, 2, "addr=%p, name='%s', type='%s'", us, us->name, us->type->name);
 
@@ -677,9 +681,9 @@ UndoStep *BKE_undosys_step_find_by_type(UndoStack *ustack, const UndoType *ut)
  *
  * \return -1 for undo, 1 for redo, 0 in case of error.
  */
-int BKE_undosys_step_calc_direction(const UndoStack *ustack,
-                                    const UndoStep *us_target,
-                                    const UndoStep *us_reference)
+eUndoStepDir BKE_undosys_step_calc_direction(const UndoStack *ustack,
+                                             const UndoStep *us_target,
+                                             const UndoStep *us_reference)
 {
   if (us_reference == NULL) {
     us_reference = ustack->step_active;
@@ -694,26 +698,26 @@ int BKE_undosys_step_calc_direction(const UndoStack *ustack,
    *    to the end of the list, rather than its start. */
   /* NOTE: in case target step is the active one, we assume we are in an undo case... */
   if (ELEM(us_target, us_reference, us_reference->prev)) {
-    return -1;
+    return STEP_UNDO;
   }
   if (us_target == us_reference->next) {
-    return 1;
+    return STEP_REDO;
   }
 
   /* Search forward, and then backward. */
   for (UndoStep *us_iter = us_reference->next; us_iter != NULL; us_iter = us_iter->next) {
     if (us_iter == us_target) {
-      return 1;
+      return STEP_REDO;
     }
   }
   for (UndoStep *us_iter = us_reference->prev; us_iter != NULL; us_iter = us_iter->prev) {
     if (us_iter == us_target) {
-      return -1;
+      return STEP_UNDO;
     }
   }
 
   BLI_assert(!"Target undo step not found, this should not happen and may indicate an undo stack corruption");
-  return 0;
+  return STEP_INVALID;
 }
 
 /**
@@ -752,8 +756,8 @@ bool BKE_undosys_step_load_data_ex(UndoStack *ustack,
   }
 
   /* This considers we are in undo case if both `us_target` and `us_reference` are the same. */
-  const int undo_dir = BKE_undosys_step_calc_direction(ustack, us_target, us_reference);
-  BLI_assert(undo_dir != 0);
+  const eUndoStepDir undo_dir = BKE_undosys_step_calc_direction(ustack, us_target, us_reference);
+  BLI_assert(undo_dir != STEP_INVALID);
 
   /* This will be the active step once the undo process is complete.
    *