Function ensure_valid_tdir

plugins/dig-now.cpp:611–615

ensure we have at least two directions enabled (or 0) so we can find a matching tiletype. The game chooses to curve "end piece" walls into orthogonally adjacent hidden tiles, or uses a pillar if there are no such tiles. we take the easier, but not quite conformant, path here and always use a pillar for end pieces. If we want to become faithful to how the game does it, this code should be moved to

// longer adjacent to hidden tiles and convert them to pillars when we dig two
// tiles away from such a wall end and reveal their adjacent hidden tile.
static TileDirection ensure_valid_tdir(TileDirection tdir) {
    if (tdir.sum() < 2)
        tdir.whole = 0;
    return tdir;
}

// connects adjacent smooth walls to our new smooth wall
static TileDirection BLANK_TILE_DIRECTION;