X-Sums Sudoku
X-Sums Sudoku is one of the most elegant sudoku variants. The base is
familiar: every row, column and 3×3 block holds the digits 1–9 without
repeats. But several rows and columns got numbers at the grid edge, and each
number is the sum of the first X cells from the edge. And how many cells
that is, X, hides in the very first cell at the clue.
Example: a clue of 14 at the top of the third column. If the digit in the
first cell of that column is 3, then the sum of the first three cells of the
column must be 14. But who said it is 3? That is exactly what remains to
find out — the sum and the length are linked in a loop, and it only unravels
together with the grid.
Hence the signature tricks of the genre: a clue above 30 almost shouts for
a large X (even 9+8+7+6+5 = 35), and a clue of 1 or 3 definitively reveals
the first cell. Every round is generated on the fly: the full solution comes
first, clues are computed from it honestly, and open digits are removed so
that the solution stays unique.
Arrows at the edge show the direction of each sum. Input is tuned for
phones: tap a cell, an on-screen digit pad, erase with a single button.
«Check» highlights mistakes, «Hint» places a correct digit. Three
difficulties: «Easy» — more open digits, «Hard» — up to eight edge sums and
a nearly bare field.