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, 3 insertions, 3 deletions

diff --git a/untangle.c b/untangle.c
index ac40418..4dd93be 100644
--- a/untangle.c
+++ b/untangle.c
@@ -1117,14 +1117,14 @@ static char *interpret_move(const game_state *state, game_ui *ui,
 	    ui->newpoint.x = x;
 	    ui->newpoint.y = y;
 	    ui->newpoint.d = ds->tilesize;
-	    return "";
+	    return UI_UPDATE;
 	}
 
     } else if (IS_MOUSE_DRAG(button) && ui->dragpoint >= 0) {
 	ui->newpoint.x = x;
 	ui->newpoint.y = y;
 	ui->newpoint.d = ds->tilesize;
-	return "";
+	return UI_UPDATE;
     } else if (IS_MOUSE_RELEASE(button) && ui->dragpoint >= 0) {
 	int p = ui->dragpoint;
 	char buf[80];
@@ -1139,7 +1139,7 @@ static char *interpret_move(const game_state *state, game_ui *ui,
             ui->newpoint.x >= (long)state->w*ui->newpoint.d ||
 	    ui->newpoint.y < 0 ||
             ui->newpoint.y >= (long)state->h*ui->newpoint.d)
-	    return "";
+	    return UI_UPDATE;
 
 	/*
 	 * We aren't cancelling the drag. Construct a move string