+ else if (button == LEFT_BUTTON) {
+ /*
+ * Find the bearing of the click point from the current
+ * square's centre.
+ */
+ int cx, cy;
+ double angle;
+
+ cx = from->squares[from->current].x * GRID_SCALE + ds->ox;
+ cy = from->squares[from->current].y * GRID_SCALE + ds->oy;
+
+ if (x == cx && y == cy)
+ return NULL; /* clicked in exact centre! */
+ angle = atan2(y - cy, x - cx);
+
+ /*
+ * There are three possibilities.
+ *
+ * - This square is a square, so we choose between UP,
+ * DOWN, LEFT and RIGHT by dividing the available angle
+ * at the 45-degree points.
+ *
+ * - This square is an up-pointing triangle, so we choose
+ * between DOWN, LEFT and RIGHT by dividing into
+ * 120-degree arcs.
+ *
+ * - This square is a down-pointing triangle, so we choose
+ * between UP, LEFT and RIGHT in the inverse manner.
+ *
+ * Don't forget that since our y-coordinates increase
+ * downwards, `angle' is measured _clockwise_ from the
+ * x-axis, not anticlockwise as most mathematicians would
+ * instinctively assume.
+ */
+ if (from->squares[from->current].npoints == 4) {
+ /* Square. */
+ if (fabs(angle) > 3*PI/4)
+ direction = LEFT;
+ else if (fabs(angle) < PI/4)
+ direction = RIGHT;
+ else if (angle > 0)
+ direction = DOWN;
+ else
+ direction = UP;
+ } else if (from->squares[from->current].directions[UP] == 0) {
+ /* Up-pointing triangle. */
+ if (angle < -PI/2 || angle > 5*PI/6)
+ direction = LEFT;
+ else if (angle > PI/6)
+ direction = DOWN;
+ else
+ direction = RIGHT;
+ } else {
+ /* Down-pointing triangle. */
+ assert(from->squares[from->current].directions[DOWN] == 0);
+ if (angle > PI/2 || angle < -5*PI/6)
+ direction = LEFT;
+ else if (angle < -PI/6)
+ direction = UP;
+ else
+ direction = RIGHT;
+ }
+ } else