Cleanup: use enum for undo step direction

The step direction was confusingly 1 for undo, -1 for redo.
This convention is from 2004 (562d6958cb).

Use enum for ed_undo.c for better readability.

source/blender/editors/gpencil/gpencil_undo.c

@@ -67,7 +67,7 @@ int ED_undo_gpencil_step(bContext *C, int step, const char *name)
 
   gpd_ptr = ED_gpencil_data_get_pointers(C, NULL);
 
-  if (step == 1) { /* undo */
+  if (step == -1) { /* undo */
     // printf("\t\tGP - undo step\n");
     if (cur_node->prev) {
       if (!name || STREQ(cur_node->name, name)) {
@@ -76,7 +76,7 @@ int ED_undo_gpencil_step(bContext *C, int step, const char *name)
       }
     }
   }
-  else if (step == -1) {
+  else if (step == 1) {
     // printf("\t\tGP - redo step\n");
     if (cur_node->next) {
       if (!name || STREQ(cur_node->name, name)) {

source/blender/editors/undo/ed_undo.c

@@ -70,6 +70,17 @@
 /** We only need this locally. */
 static CLG_LogRef LOG = {"ed.undo"};
 
+/**
+ * \warning Values are used in #ED_undo_gpencil_step,
+ * which should eventually be replaced with the undo-system.
+ */
+enum eUndoStepDir {
+  STEP_REDO = 1,
+  STEP_UNDO = -1,
+  /** Only used when the undo name is passed to #ed_undo_step_impl. */
+  STEP_NONE = 0,
+};
+
 /* -------------------------------------------------------------------- */
 /** \name Generic Undo System Access
  *
@@ -172,11 +183,11 @@ void ED_undo_push(bContext *C, const char *str)
  * \note Also check #undo_history_exec in bottom if you change notifiers.
  */
 static int ed_undo_step_impl(
-    bContext *C, int step, const char *undoname, int undo_index, ReportList *reports)
+    bContext *C, enum eUndoStepDir step, const char *undoname, int undo_index, ReportList *reports)
 {
   /* Mutually exclusives, ensure correct input. */
-  BLI_assert(((undoname || undo_index != -1) && !step) ||
-             (!(undoname || undo_index != -1) && step));
+  BLI_assert(((undoname || undo_index != -1) && (step == STEP_NONE)) ||
+             (!(undoname || undo_index != -1) && (step != STEP_NONE)));
   CLOG_INFO(&LOG, 1, "name='%s', step=%d", undoname, step);
   wmWindowManager *wm = CTX_wm_manager(C);
   Scene *scene = CTX_data_scene(C);
@@ -197,7 +208,7 @@ static int ed_undo_step_impl(
   /* TODO(campbell): undo_system: use undo system */
   /* grease pencil can be can be used in plenty of spaces, so check it first */
   if (ED_gpencil_session_active()) {
-    return ED_undo_gpencil_step(C, step, undoname);
+    return ED_undo_gpencil_step(C, (int)step, undoname);
   }
   if (area && (area->spacetype == SPACE_VIEW3D)) {
     Object *obact = CTX_data_active_object(C);
@@ -207,7 +218,7 @@ static int ed_undo_step_impl(
   }
 
   UndoStep *step_data_from_name = NULL;
-  int step_for_callback = step;
+  enum eUndoStepDir step_for_callback = step;
   if (undoname != NULL) {
     step_data_from_name = BKE_undosys_step_find_by_name(wm->undo_stack, undoname);
     if (step_data_from_name == NULL) {
@@ -218,14 +229,14 @@ static int ed_undo_step_impl(
     /* Pointers match on redo. */
     step_for_callback = (BLI_findindex(&wm->undo_stack->steps, step_data_from_name) <
                          BLI_findindex(&wm->undo_stack->steps, wm->undo_stack->step_active)) ?
-                            1 :
-                            -1;
+                            STEP_UNDO :
+                            STEP_REDO;
   }
   else if (undo_index != -1) {
     step_for_callback = (undo_index <
                          BLI_findindex(&wm->undo_stack->steps, wm->undo_stack->step_active)) ?
-                            1 :
-                            -1;
+                            STEP_UNDO :
+                            STEP_REDO;
   }
 
   /* App-Handlers (pre). */
@@ -247,7 +258,7 @@ static int ed_undo_step_impl(
       BKE_undosys_step_undo_from_index(wm->undo_stack, C, undo_index);
     }
     else {
-      if (step == 1) {
+      if (step == STEP_UNDO) {
         BKE_undosys_step_undo(wm->undo_stack, C);
       }
       else {
@@ -310,19 +321,19 @@ static int ed_undo_step_impl(
   return OPERATOR_FINISHED;
 }
 
-static int ed_undo_step_direction(bContext *C, int step, ReportList *reports)
+static int ed_undo_step_direction(bContext *C, enum eUndoStepDir step, ReportList *reports)
 {
   return ed_undo_step_impl(C, step, NULL, -1, reports);
 }
 
 static int ed_undo_step_by_name(bContext *C, const char *undo_name, ReportList *reports)
 {
-  return ed_undo_step_impl(C, 0, undo_name, -1, reports);
+  return ed_undo_step_impl(C, STEP_NONE, undo_name, -1, reports);
 }
 
 static int ed_undo_step_by_index(bContext *C, int index, ReportList *reports)
 {
-  return ed_undo_step_impl(C, 0, NULL, index, reports);
+  return ed_undo_step_impl(C, STEP_NONE, NULL, index, reports);
 }
 
 void ED_undo_grouped_push(bContext *C, const char *str)
@@ -340,11 +351,11 @@ void ED_undo_grouped_push(bContext *C, const char *str)
 
 void ED_undo_pop(bContext *C)
 {
-  ed_undo_step_direction(C, 1, NULL);
+  ed_undo_step_direction(C, STEP_UNDO, NULL);
 }
 void ED_undo_redo(bContext *C)
 {
-  ed_undo_step_direction(C, -1, NULL);
+  ed_undo_step_direction(C, STEP_REDO, NULL);
 }
 
 void ED_undo_push_op(bContext *C, wmOperator *op)
@@ -448,7 +459,7 @@ static int ed_undo_exec(bContext *C, wmOperator *op)
 {
   /* "last operator" should disappear, later we can tie this with undo stack nicer */
   WM_operator_stack_clear(CTX_wm_manager(C));
-  int ret = ed_undo_step_direction(C, 1, op->reports);
+  int ret = ed_undo_step_direction(C, STEP_UNDO, op->reports);
   if (ret & OPERATOR_FINISHED) {
     /* Keep button under the cursor active. */
     WM_event_add_mousemove(CTX_wm_window(C));
@@ -477,7 +488,7 @@ static int ed_undo_push_exec(bContext *C, wmOperator *op)
 
 static int ed_redo_exec(bContext *C, wmOperator *op)
 {
-  int ret = ed_undo_step_direction(C, -1, op->reports);
+  int ret = ed_undo_step_direction(C, STEP_REDO, op->reports);
   if (ret & OPERATOR_FINISHED) {
     /* Keep button under the cursor active. */
     WM_event_add_mousemove(CTX_wm_window(C));