How to play Hyper Sudoku

Hyper Sudoku, also known as Windoku, is classic Sudoku with four extra regions. Inside the grid sit four shaded 3x3 windows, one towards each corner, and each window must also contain the digits 1 to 9 exactly once. The windows overlap four boxes each, so a single digit can now be ruled in or out from new directions - and the grid hides a few more surprises that the best players learn to use.

Every puzzle here is made fresh by the game when you start it, has exactly one solution, and can be solved by reasoning alone. You never need to guess. The four levels differ in which solving techniques a puzzle demands, from simple scanning on Easy to chains and wings on Crazy.

How to play on skill.games

When you open the game, the settings popup appears. Choose a level under DIFFICULTY, choose how mistakes are treated under MISTAKES, and press PLAY. A new puzzle is generated in a moment.

• The board takes up most of the screen. The four windows are shaded grey and outlined with a dashed frame, so you can pick them out even on a small phone. The digits printed in dark ink are the givens; they can't be changed. The digits you enter appear in blue.
• Choosing a cell: tap any cell. Its row, column and box are shaded - and its window too, if it lies in one - and every cell holding the same digit is highlighted.
• Entering a digit: with a cell chosen, tap a number on the number pad (under the board, or beside it when your screen is wider than it is tall). The small number under each key shows how many of that digit are still to place; a key fades out once all nine are on the board.
• NOTES: tap NOTES to switch notes on. Numbers you tap are now pencilled into the cell in small print as candidates, and tapping one again removes it. Tap NOTES again to go back to entering digits. When you enter a digit, it is removed automatically from the notes of every cell in its row, column, box and window.
• ERASE clears the chosen cell (its digit or its notes).
• UNDO takes back your last action - a digit, a note or an erase - all the way back to the start. Undo is free and unlimited.
• Highlighting a digit: with no cell chosen, tap a number on the pad to highlight every cell and note holding that digit. Tap it again to clear the highlight.

On a computer you can also use the keyboard: the arrow keys (or W, A, S, D) move between cells, 1 to 9 enter a digit, Shift and a digit pencil a note, Backspace, Delete or 0 erase, N switches notes on and off, and Z undoes.

The top line shows the clock, the level and your mistakes. The clock runs only while you play: it stops when the menu or a popup is open. The menu button in the top-left corner pauses the game, starts a new puzzle or switches the sound on or off. The game is saved after every move, so if you close the page you can press CONTINUE later and carry on with the same grid, notes, clock and undo history.

When the last cell is filled correctly, the board ripples and the results screen shows your time, your best time at that level (a star marks a new record), your mistakes and the puzzle's code, together with your wins, losses, win rate and streaks.

Rules

The grid. The board has 9 rows, 9 columns and 9 boxes of 3x3 cells, plus four windows of 3x3 cells: rows 2 to 4 and 6 to 8, crossed with columns 2 to 4 and 6 to 8. Every row, column, box and window must end up holding each digit from 1 to 9 exactly once.

Givens. The puzzle starts with some digits already placed. They are part of the puzzle and can't be changed. Every puzzle has exactly one way to complete it.

Mistakes - Unlimited (the default). If you enter a digit that is already in the same row, column, box or window, the clash is marked in red and the mistakes counter goes up by one. Nothing else is checked: a digit that doesn't clash with anything is not marked, even if it turns out to be wrong later - just as on paper. The puzzle is solved when every cell is filled and nothing clashes. Because the solution is unique, a full grid without clashes is always the right one.

Mistakes - 3 lives. Every digit you enter is checked against the solution at once. A wrong digit is marked in red and costs a life; the third mistake ends the game and counts as a loss. Undo can take a wrong digit back, but not the life it cost.

Abandoned puzzles. Starting a new puzzle before finishing the current one counts as a loss in your statistics for that level.

Solving techniques

These are the methods the game's own solver uses to grade every puzzle, from the simplest up. In Hyper Sudoku each one works on the windows exactly as it does on rows, columns and boxes.

• Hidden single: a digit that can only go in one cell of a row, column, box or window. Scan the windows as well as the boxes.
• Naked single: a cell where only one digit is still possible. Cells inside a window see more of the board than the others, so they run out of options sooner.
• Pointing and line reductions: if a digit's only places in a box or a window lie on one row or column, it can't go anywhere else on that line - and if a line's only places for a digit lie in one box or one window, it can't go anywhere else there. A window and a box share four cells: when a digit's places in one of them all fall inside that shared corner, it leaves the rest of the other.
• Naked and hidden pairs: two cells in a unit that can only hold the same two digits claim those digits; two digits that can only go in the same two cells claim those cells.
• Triples and the X-Wing: the same idea with three cells or digits; and a digit confined to the same two columns in two rows can be removed from the rest of those columns.
• XY-Wing: three two-candidate cells linked so that one digit must sit in one of the two outer cells - any cell seeing both can't hold it.
• Advanced: XYZ-Wing, Swordfish, quads, simple colouring and XY-Chains - longer chains of "if this, then that" reasoning, all still without guessing.

Strategy tips

• Work the overlaps. Each window shares a 2x2 corner with four boxes. A digit squeezed into that corner by the box often settles the window, and the other way round.
• Look for the hidden windows. The windows imply five more groups of nine. Take columns 2 to 4: they hold every digit three times, the two windows inside them hold every digit twice, so the cells left over - columns 2 to 4 of the top, middle and bottom rows - hold each digit exactly once. The same goes for columns 6 to 8, for the matching row bands, and for the nine cells where rows 1, 5 and 9 meet columns 1, 5 and 9. Strong players use these groups; the game never needs them, but they can make a step easier to see.
• Start where the windows are fullest. A window with many givens leaves few options, and its digits reach into four boxes at once.
• Use notes once singles run out. Pencil in the candidates of a window, then look for pairs and pointing digits among them.
• Watch the number pad. A key that shows 1 means a single copy of that digit is still missing - often easy to find.
• Don't guess. Every puzzle here can be solved by logic. If you feel stuck, check how the windows cross the boxes around them.

Difficulty levels

The game generates every puzzle on the spot and grades it with a solver that works the way a person does, always using the easiest step available. A puzzle is given to you only if its hardest required step matches the level you chose. The windows add information, so Hyper Sudoku puzzles start with fewer givens than classic Sudoku at the same level.

• Easy: at least half the board is filled in (41 of 81 cells); scanning for hidden and naked singles is all you need.
• Normal: at least 40% of the board is filled in (32 of 81 cells); at least one step needs a pointing pair, a line reduction or a naked or hidden pair.
• Hard: at least 30% of the board is filled in (24 of 81 cells); at least one step needs a triple, an X-Wing or an XY-Wing.
• Crazy: at least 20% of the board is filled in (16 of 81 cells); at least one step needs simple colouring, an XY-Chain, an XYZ-Wing, a Swordfish or a quad.

Statistics and best times are kept separately for each level. If you are new to Hyper Sudoku, start on Easy and get used to scanning the four windows before moving up.