# Sudoku - credits and notes

**The puzzle.** Sudoku is the logic puzzle in which a 9x9 grid is filled so that every row, every column
and every 3x3 box contains the digits 1 to 9 exactly once. It was first published as Number Place by
Dell Magazines in 1979 (designed by Howard Garns), named Sudoku by the Japanese publisher Nikoli in 1986,
and became a worldwide newspaper feature after Wayne Gould brought it to The Times in 2004. A proper
Sudoku has exactly one solution; the fewest clues a proper Sudoku can have is 17.

**The reference behaviour.** The feature set that today's most popular Sudoku apps (Sudoku.com by
Easybrain, Microsoft Sudoku) made standard: a difficulty ladder Easy / Medium / Hard / Expert / Master /
Extreme, a mistakes counter with game over at three and a second chance, three hints per game, pencil
notes with automatic removal, unlimited undo, a number pad with remaining counts, highlighting of the
selected row, column, box and same digits, auto-check, a timer with pause hiding the board, statistics
per level with streaks, a Daily Challenge with a calendar, and light and dark themes.

**This implementation** (https://sudoku.skillsafe.ai/) is written from scratch. Every one of its 720
puzzles (120 per level) was generated at build time, proved to have exactly one solution by a
backtracking solver (and cross-checked with an independent Dancing Links solver), and graded by the
hardest human technique it needs: Easy = naked singles only; Medium = hidden singles; Hard = naked and
hidden pairs and triples, pointing pairs, box-line reduction; Expert = X-Wing, Swordfish or XY-Wing;
Master = beyond all of those; Extreme = Master with 22 or fewer givens. Hints run the same technique
ladder and explain the step they found.

Our own choices, labelled as such: the grading rule above; puzzle numbers such as #hard-42 with
shareable links (this page's own numbering, not that of any other program); the Daily Challenge
schedule (Monday Easy through Saturday Extreme, Sunday Hard; one bank puzzle per UTC day, relabelled
and permuted, so it is the same for everyone); the second chance resetting the counter to 2/3;
mistakes being counted against the solution while Auto-check only controls the red colour; earlier
Daily Challenges playable from the calendar; Shift+digit for a note; and the game in progress
surviving a reload.

Everything here - the board design, the icons, the synthesized sounds, the help text and the code - is
original work. Nothing was copied from Sudoku.com, Microsoft Sudoku, Nikoli or any other Sudoku
product, and this page is not affiliated with or endorsed by Easybrain, Microsoft or Nikoli. "Sudoku"
is a registered trademark of Nikoli in Japan; the name is used here generically for the puzzle.

The code is released under the MIT licence; see LICENSE.txt.

References (all read through the MediaWiki API on 2026-09-05): Wikipedia, "Sudoku" (rules, history,
minimum of 17 clues, grid counts); "Sudoku solving algorithms" (backtracking, exact cover / dancing
links, the 17-clue puzzle built against brute force whose solution begins 987654321); "Glossary of
Sudoku" (naked and hidden singles, pairs, triples, intersection removal, N-fish, validity-preserving
transformations); "Mathematics of Sudoku" (McGuire, Tugemann and Civario 2014, no 16-clue Sudoku).
The 17-clue test puzzle string comes from the Wikimedia Commons description of the file
"Sudoku puzzle hard for brute force.svg" (Rico Alan, 2008).