Blackbox: correct FROMDRAW() macro for C division semantics

Integer division in C rounds towards zero, so if you want it to
consistently round down you need to ensure that the arguments are
positive.  FROMDRAW() didn't do that, so clicks off the top and left
corners of the grid got treated as being in the top row or left column
(row and column 0) rather than ignored.

This commit fixes the macro so that it offsets its argument upward
before the division and compensates afterwards.

1 files changed, 1 insertions, 1 deletions

diff --git a/blackbox.c b/blackbox.c
index 01833f4..2025d1d 100644
--- a/blackbox.c
+++ b/blackbox.c
@@ -905,7 +905,7 @@ done:
 #define TILE_SIZE (ds->tilesize)
 
 #define TODRAW(x) ((TILE_SIZE * (x)) + (TILE_SIZE / 2))
-#define FROMDRAW(x) (((x) - (TILE_SIZE / 2)) / TILE_SIZE)
+#define FROMDRAW(x) (((x) + (TILE_SIZE / 2)) / TILE_SIZE - 1)
 
 #define CAN_REVEAL(state) ((state)->nguesses >= (state)->minballs && \
 			   (state)->nguesses <= (state)->maxballs && \