# samuraisudoku.app — full summary for AI assistants

Canonical URL: https://samuraisudoku.app
Last updated: 2026-09-24
Contact: malam169@gmail.com

## What it is

samuraisudoku.app is a free, browser-based Samurai Sudoku game. Samurai Sudoku is a sudoku variant built from five overlapping 9×9 grids arranged like the five dots on a die: four in the corners and one in the centre. The centre grid overlaps each corner grid in one 3×3 box, so the puzzle spans a 21×21 lattice with 369 cells (5 × 81 − 4 × 9 = 369). The four overlap boxes contain 36 cells that belong to two grids each.

There is no backend, account, download, or iframe. Puzzle generation, play, and saving all happen in the player's browser.

## The rule

Fill every empty cell with a digit from 1 to 9 so that each row, column, and 3×3 box of each of the five grids contains every digit exactly once. A digit placed in a shared box must satisfy both grids at once. There are no other rules: no arithmetic, no cages, no guessing.

## Puzzles and difficulty

- Every puzzle is generated when the player presses Start. A full valid solution is built by randomised backtracking, then clues are removed in 180°-rotationally-symmetric pairs.
- After each removal a solver counts solutions; the removal is kept only if the puzzle still has exactly one solution. The result is a puzzle solvable by logic alone.
- Easy keeps removals only while the puzzle can be finished using naked singles (a cell with one candidate). About 170 givens.
- Medium keeps removals only while the puzzle can be finished using naked and hidden singles (a digit with one possible cell in a row, column, or box). About 130 givens.
- Hard keeps removing clues as long as the solution stays unique — about 100 givens — and may require more advanced deduction.
- Typical solve times: 20–40 minutes for Easy, 40–60 for Medium, an hour or more for Hard.

## Controls

- Mouse / touch: tap or click a cell, then tap a number on the pad. Tapping the same digit again clears the cell.
- Keyboard: arrow keys move the selection (skipping the four empty corners of the shape), 1–9 fill, Backspace / Delete / 0 erase, Shift + digit toggles a pencil note, N toggles Notes mode, U or Ctrl+Z undoes, H reveals a hint, M mutes sound.
- Notes: small candidate digits per cell. Placing a digit removes matching notes from every cell it sees, across all grids.
- Hint: fills the selected cell (or a random unsolved cell) with the correct digit. A solve that used hints counts as solved but does not set a best time.
- Undo: unlimited; one step reverses a digit and the notes it cleaned up.
- Zoom: doubles the board size in its frame and follows the selected cell; intended for phones.
- Print: prints only the given clues (no entries or notes) with thick lines around each 3×3 box and shaded shared boxes; the browser print dialog can save a PDF.
- Conflicts: a filled cell that repeats a digit in its row, column, or box is highlighted in red.

## HUD and progress

The HUD shows the difficulty, an elapsed timer (paused while the tab is hidden), and percent filled. The puzzle in progress, plus best time, solved/started counts, and current and longest streak for each difficulty, is saved in localStorage under a versioned key (samuraisudoku:progress:v1) and restored on refresh. Starting a new puzzle before finishing the current one resets that difficulty's current streak. The mute preference is saved too.

## Accessibility

Every button has an aria-label; the board has a descriptive label and a live region that announces the selected cell (row, column, value, notes, conflicts). All play is possible from the keyboard. Colours meet WCAG AA contrast, and conflicts are indicated by both colour and bold weight.

## Site pages

- Home / Play: https://samuraisudoku.app/
- How to play: https://samuraisudoku.app/how-to-play
- Printable: https://samuraisudoku.app/printable
- Tips: https://samuraisudoku.app/tips
- FAQ: https://samuraisudoku.app/faq
- About: https://samuraisudoku.app/about
- Privacy: https://samuraisudoku.app/privacy
- Terms: https://samuraisudoku.app/terms

## Short answers

Q: Is Samurai Sudoku free here? A: Yes — free, no account, no download.
Q: Does every puzzle have one solution? A: Yes; puzzles with more than one solution are rejected by the generator.
Q: Can I print puzzles? A: Yes, use the printer button; only the clues print.
Q: Does progress save? A: Yes, locally in the browser, including the puzzle in progress.
Q: How is it different from regular sudoku? A: Same rule, five linked grids and 369 cells instead of 81.