view: add infrastructure for delayed cursor destruction

At least one cursor (referred to as primary or main cursor) has always
to exist. In the sam command language implementation we might want to
dispose a cursor even if it is the primary one before later commands
will create different ones (e.g. `:x/pattern/ { i/>>>/ a/<<</ }`).

This commit introduces view_cursors_dispose_force. If called on
the last remaining cursor, its selection is cleared and it is marked for
destruction as soon as a new cursor is created. view_cursor_disposed
returns the cursor marked for deletion (if any) and clears the descruction
flag.

view.c

@@ -80,6 +80,7 @@ struct View {
 	Line *lastline;     /* last currently used line, always <= bottomline */
 	Line *bottomline;   /* bottom of view, might be unused if lastline < bottomline */
 	Cursor *cursor;     /* main cursor, always placed within the visible viewport */
+	Cursor *cursor_dead;/* main cursor which was disposed, will be removed when another cursor is created */
 	int cursor_count;   /* how many cursors do currently exist */
 	Line *line;         /* used while drawing view content, line where next char will be drawn */
 	int col;            /* used while drawing view content, column where next char will be drawn */
@@ -972,7 +973,8 @@ static Cursor *cursors_new(View *view, size_t pos, bool force) {
 		view->cursors = c;
 	}
 	view->cursor = c;
-	view->cursor_count++;;
+	view->cursor_count++;
+	view_cursors_dispose(view->cursor_dead);
 	view_cursors_to(c, pos);
 	return c;
 err:
@@ -1067,13 +1069,15 @@ static void view_cursors_free(Cursor *c) {
 		c->view->cursors = c->next;
 	if (c->view->cursor == c)
 		c->view->cursor = c->next ? c->next : c->prev;
+	if (c->view->cursor_dead == c)
+		c->view->cursor_dead = NULL;
 	c->view->cursor_count--;
 	free(c);
 }
 
 bool view_cursors_dispose(Cursor *c) {
 	if (!c)
-		return false;
+		return true;
 	View *view = c->view;
 	if (!view->cursors || !view->cursors->next)
 		return false;
@@ -1083,6 +1087,23 @@ bool view_cursors_dispose(Cursor *c) {
 	return true;
 }
 
+bool view_cursors_dispose_force(Cursor *c) {
+	if (view_cursors_dispose(c))
+		return true;
+	View *view = c->view;
+	if (view->cursor_dead)
+		return false;
+	view_cursors_selection_clear(c);
+	view->cursor_dead = c;
+	return true;
+}
+
+Cursor *view_cursor_disposed(View *view) {
+	Cursor *c = view->cursor_dead;
+	view->cursor_dead = NULL;
+	return c;
+}
+
 Cursor *view_cursors(View *view) {
 	view->cursor_generation++;
 	return view->cursors;

view.h

@@ -132,6 +132,13 @@ bool view_cursors_multiple(View*);
 /* dispose an existing cursor with its associated selection (if any),
  * not applicaple for the last existing cursor */
 bool view_cursors_dispose(Cursor*);
+/* if called for the last existing cursor its selection is destroyed
+ * and the cursor is marked for destruction and will be disposed as
+ * soon as a new cursor is created. */
+bool view_cursors_dispose_force(Cursor*);
+/* if the primary cursor was marked for destruction (by means of
+ * view_cursors_dispose_force) return it and clear descruction flag */
+Cursor *view_cursor_disposed(View*);
 /* only keep the main cursor, release all others together with their
  * selections (if any) */
 void view_cursors_clear(View*);