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, 5 insertions, 5 deletions

diff --git a/sixteen.c b/sixteen.c
index edc9771..562eeb8 100644
--- a/sixteen.c
+++ b/sixteen.c
@@ -618,7 +618,7 @@ static char *interpret_move(const game_state *state, game_ui *ui,
     if (IS_CURSOR_MOVE(button) || pad) {
         if (!ui->cur_visible) {
             ui->cur_visible = 1;
-            return "";
+            return UI_UPDATE;
         }
 
         if (control || shift || ui->cur_mode) {
@@ -673,7 +673,7 @@ static char *interpret_move(const game_state *state, game_ui *ui,
             }
 
             ui->cur_visible = 1;
-            return "";
+            return UI_UPDATE;
         }
     }
 
@@ -691,11 +691,11 @@ static char *interpret_move(const game_state *state, game_ui *ui,
                 const enum cursor_mode m = (button == CURSOR_SELECT2 ?
                                             lock_position : lock_tile);
                 ui->cur_mode = (ui->cur_mode == m ? unlocked : m);
-                return "";
+                return UI_UPDATE;
             }
         } else {
             ui->cur_visible = 1;
-            return "";
+            return UI_UPDATE;
         }
     } else {
 	return NULL;
@@ -710,7 +710,7 @@ static char *interpret_move(const game_state *state, game_ui *ui,
     else if (cy == state->h && cx >= 0 && cx < state->w)
         dy = +1, dx = 0;
     else
-        return "";                   /* invalid click location */
+        return UI_UPDATE;            /* invalid click location */
 
     /* reverse direction if right hand button is pressed */
     if (button == RIGHT_BUTTON || button == CURSOR_SELECT2) {