New name UI_UPDATE for interpret_move's return "".

Now midend.c directly tests the returned pointer for equality to this
value, instead of checking whether it's the empty string.

A minor effect of this is that games may now return a dynamically
allocated empty string from interpret_move() and treat it as just
another legal move description. But I don't expect anyone to be
perverse enough to actually do that! The main purpose is that it
avoids returning a string literal from a function whose return type is
a pointer to _non-const_ char, i.e. we are now one step closer to
being able to make this code base clean under -Wwrite-strings.

1 files changed, 2 insertions, 2 deletions

diff --git a/twiddle.c b/twiddle.c
index 6e05f4d..f52e999 100644
--- a/twiddle.c
+++ b/twiddle.c
@@ -661,7 +661,7 @@ static char *interpret_move(const game_state *state, game_ui *ui,
         if (button == CURSOR_DOWN && (ui->cur_y+n) < (h))
             ui->cur_y++;
         ui->cur_visible = 1;
-        return "";
+        return UI_UPDATE;
     }
 
     if (button == LEFT_BUTTON || button == RIGHT_BUTTON) {
@@ -685,7 +685,7 @@ static char *interpret_move(const game_state *state, game_ui *ui,
             dir = (button == CURSOR_SELECT2) ? -1 : +1;
         } else {
             ui->cur_visible = 1;
-            return "";
+            return UI_UPDATE;
         }
     } else if (button == 'a' || button == 'A' || button==MOD_NUM_KEYPAD+'7') {
         x = y = 0;