Kakurasu
Kakurasu is a Japanese puzzle that looks like cross-sums but plays quite
differently. On an N×N board, fill in cells so that every row and column
reaches its target sum. The twist is the cell weight: filling a cell adds
its column number to the row target and its row number to the column
target. A cell has a dual identity: to its row it is a «three», to its
column a «one».
Hence a simple but elegant tactic: a big row target pulls you toward the
right-hand columns, a big column target toward the bottom cells. A zero
target guarantees nothing needs filling there. And targets that decompose
into a single combination of weights are solved instantly, giving a
foothold for the rest.
Every round is generated on the fly: a hidden solution is scattered first,
targets are computed from it honestly, and some cells are revealed as
givens — dark, already filled. The puzzle is guaranteed solvable, and
usually in more than one way.
Targets are highlighted live: matched — teal, unmatched — red. Boards of
5×5 for a warm-up and 7×7 for a full challenge. Input is tuned for phones:
tap a cell, nothing else. Winning earns points — from 80 on the small board
to 100 on the large one.