killer sudoku: no starting digits, just cage sums. every dashed region's digits are unique and add up to the number in its corner — combine that with normal sudoku rules and the board cracks open, one smug tabby cheering (or judging) every move.
a full solved grid is built with randomized backtracking, then chopped into 1–4 cell cages whose digits never repeat. a constraint-propagation solver (row/col/box rules + per-cage sum combinations, most-constrained- cell first) checks the cage layout has exactly one solution — if not, it reveals the fewest starting digits needed to force uniqueness, same trick real killer sudoku setters use. today's puzzle is seeded from the UTC date, so it's the same for everyone (and the same tabby judges everyone equally).