123doku, the puzzle where every move can be proved

Field notes

The puzzle uses three stone weights and two simultaneous constraints: exact totals for every row and column, plus a ban on edge-to-edge contact between equal weights. The site describes grids from 5×5 through 8×8, with more than 140,000 puzzles installed for offline play. A hint system identifies both a cell and the rule that resolves it, while daily rankings and timed friend duels add a competitive layer.