What this does
Paste a sudoku — or load the daily puzzle — and this solves it in front of you, one step at a time. Unlike a solver that just prints the answer, every step is labelled with the reason for it: a hidden single, a pointing pair, an XY-Wing, or an outright guess followed by a backtrack when that guess fails. You can play it at four speeds, step through one move at a time, or drag the slider to any point in the search.
The counts underneath are the interesting part. A well-formed puzzle is solved almost entirely by logical deductions with zero guesses — that is what "fair" means in sudoku. When you see the guess and backtrack counters climbing, the grid has reached a point where no single-step technique applies and the only way forward is to try a candidate and find out.
How to use it
Click any square and type a digit, or paste 81 characters using digits for clues and dots or zeroes for blanks — the format most sudoku sites export. The grid is checked as you type, so a digit repeated in a row, column or box is flagged before you try to solve. Press Solve it and the animation starts.
Very sparse grids can branch further than is worth watching, so the search stops at a budget and says so rather than freezing. Long searches keep their beginning and end and skip the middle, with the number of omitted steps shown — you always see how the search started and how it finished.
