Nuritwin Puzzle Generator

What this generator does

Grows regions of four to nine cells, draws a valid shading for them, reshapes regions where a second answer would fit, and prints only the block sizes needed for one answer — often none or one. Region tetromino puzzles place one fixed shape per region; here each region holds two matching blocks whose size you must work out.

How to use this tool

1. Choose the grid size and press Make a puzzle. Add a puzzle code to make the same puzzle again later.
2. Solve with pencil and paper or print the two pages.
3. Press Show the answer to check your shading.

Understanding the controls

Grid

6, 7 or 8 squares a side. Larger grids have more regions and take a few seconds longer to prove.

Puzzle code

Leave empty for a new puzzle from the browser's secure generator. With a code, the same code and size make the same puzzle again; the code is a seed, not a secret.

Worked examples

Code harbour, 6 × 6

6 regions and no numbers at all — the region shapes alone fix the answer; 14 cells shaded.

Code harbour, 8 × 8

11 regions, 4 of them numbered; 30 cells shaded.

Common use cases

• A shading puzzle where regions, not single cells, carry the logic
• Printed puzzle sheets with answers
• Shared puzzles for a club from one code

How this generator works

Regions are grown by random flood fill and small ones merged. For each region, every shading that makes exactly two equal blocks inside it is listed in full. A search picks one listed shading per region in turn, abandoning a branch the moment a 2 × 2 square is all shaded or a group of shaded cells is sealed off while others remain; run in a shuffled order it draws one genuine answer. With every region's block size given, the search counts answers again; while a second one fits, a cell where the two differ is moved into a neighbouring region (both regions staying joined and four to nine cells, and the drawn answer still obeying every rule), up to 40 moves. Then each number is removed in turn and put back if more than one answer appears. A search that reaches its step limit proves nothing and the layout is dropped. Afterwards blocks are found again by flood fill, the answer re-read against every rule, and the count re-run from the printed puzzle.

Randomness and fairness

Regions, the drawn shading, the reshaping moves and the choice of numbers come from a random sequence: the browser's secure generator, or a seeded one when you give a code. A code repeats a puzzle while the generator is unchanged. Codes are deterministic, not cryptographic.

For how randomness is produced across the whole site, see how Generate Random works.

Assumptions this tool makes

• Nikoli's published rules for Nuritwin, restated on the page.
• A number sits in the first cell of its region in reading order.

Limitations and good to know

• 6 × 6 to 8 × 8 only; proving one answer for larger grids takes too long in a browser.
• No difficulty grade is given; a puzzle with no numbers is not necessarily harder than one with several.
• Regions are generated and reshaped by a search, not hand-designed.

Common mistakes

Letting the two blocks of a region touch

Two blocks that touch side by side are one block. Leave a gap inside the region.

Forgetting blocks must join across walls

Every shaded cell connects to every other through shaded cells, crossing region walls.

Making a 2 × 2 shaded square

Look out for it where two regions' blocks meet at a wall.

Practical tips

• A four-cell region always holds two single shaded cells: two dominoes would fill it and join into one block.
• A region with a 1 holds two single shaded cells with a gap between them.
• Work from the edges: corner regions have fewest ways to reach the rest.

Privacy and your data

Nuritwin puzzles are built in your browser. With analytics consent, only the fact that a puzzle was made or failed is counted; the regions, numbers, answer and puzzle code are never sent.