Sudoku Lab

Sudoku Lab is a sudoku that teaches you to solve, instead of handing you the
answer.

Most sudoku apps know one kind of hint: "There is a 7 here." You learn nothing
from that. Sudoku Lab explains the next logical step instead — in stages you
control yourself.

HINTS IN STAGES

1. First just the name of the technique: "Next step: X-Wing."
2. Then the area it happens in — can you find the pattern yourself?
3. Then the full derivation, step by step to click through. Each step marks
exactly the cells at issue.

Every claim can be checked on the board: either the blocking digits are marked,
or it says plainly that something follows from an earlier step. Nothing is
"known" from the solution.

For eliminations you can also have the logic shown as a chain of contradiction:
"Suppose the 5 really were here ..." — the app plays out the forced follow-up
moves until the contradiction becomes visible.

And if you only want to know why a digit cannot go somewhere: select the cell,
long-press the digit. The app explains it and marks the blockers.

35 TECHNIQUES, HONESTLY GRADED

From Full House and Naked Single through Locked Candidates, Pairs and Triples to
X-Wing, Skyscraper, Empty Rectangle, Swordfish, finned fish, XY-, XYZ-, W- and
WXYZ-Wing, Unique Rectangle types 1 to 4, Hidden Rectangle, BUG+1, Simple
Coloring, Sue de Coq, XY-Chain, ALS-XZ and AIC — the general chain form that
most named patterns are special cases of.

Every single one has practice positions. No node on the learning path is a dead
end.

The five difficulty levels are defined by the hardest technique required — not
by the number of givens, as is usual elsewhere. A "hard" here really does mean
triples and X-Wing, not an easy one with fewer numbers. And only puzzles the
hint engine can solve all the way through make it into the game: in every
position there is guaranteed to be an explainable next step.

LEARNING PATH

A route through every technique, one at a time: read the lesson, then practise
on positions where that technique is precisely the next step. No hunting, no
luck. A model keeps track of what you can do and suggests a refresher before you
forget it.

DAILY, STREAK, QUESTS

A puzzle every day, ramping from easy on Monday to expert on Sunday. You can
catch up on missed days at any time — no day is lost for good. A monthly
calendar with trophies, three quests a day, and a 3-minute mode for fast pattern
spotting.

The streak has forgiveness built in: earned freezes, and if it does break it
falls back rather than to zero. Freezes come only from playing, never from
paying.

CONTROLS THAT THINK ALONG

- Corner and centre notes, like on paper
- Cell-first or digit-first; in notes mode, swiping fills whole runs
- Unlimited undo and redo
- Error checking is your call: off, rule violations only, or a running
comparison with the solution
- Configurable panels below the board: progress, quests, pace, candidate preview
- Available in English, German and Portuguese, with screen-reader labels on
every cell

WHAT THERE ISN'T

No ads. No purchases. No mistake counter. No game over. No hearts, no energy, no
sign-up. No advertising ID, no tracking across app boundaries. The app works
offline — the 10,053 puzzles it contains are already on board.

You can help out anonymously if you like: we ask exactly once whether we may
count where people get stuck. It hangs on a random number, never on you, and
saying no costs you no feature.

Sudoku Lab is for everyone who wants not just to solve sudokus, but to get
better at them.