Sudoku Cracker

81 digits in reading order, with 0 or . for blanks.

Scan to import a puzzle from a newspaper, book or screenshot: take or choose a photo, check the digits it read (unsure ones are orange - tap to fix), then Import. If it misses the grid, use Adjust corners.

Or enter digits yourself: tap a cell, then a number. On a keyboard you can type digits and use the arrow keys. Or pick one from Puzzles.

The line under the grid shows the digits the selected cell could be. This updates automatically whenever a cell changes.

Save names the puzzle and adds it to Puzzles, so you can come back to it. Reset takes the puzzle back to its start: as it was scanned, imported or opened, or the digits you typed in before running any searches.

Solve

Use a combination of different search methods to solve the puzzle. While one search is running the others are disabled; tap it again to stop it. Digits are coloured by the method that found them.

• Not Search - For every blank, read across its row, column and box and remove every digit found from its 'maybes' list. If only one remains, enter it.
• Not Check - Additional method that checks all the affected cells whenever a value is entered to see if they can now be only one thing. Basically makes 'Not Search' redundant.
• Box Search - For each box, create list of cells where each digit could go. If only one place, enter it.
• Line Check - Additional check after box search, if digit can only be in 2 or 3 cells on the same row or column, then it can't be anywhere else on that line
• Column Search - For each column, create list of cells where each digit could go. If only one place, enter it.
• Row Search - For each row, create list of cells where each digit could go. If only one place, enter it.
• Solve - Not, Box, Column and Row search, each repeated while it finds anything, until solved or stuck. This is the 'All-rounder' approach on the Grade tab.
• Tree Search - Used by 'Solve' when it gets stuck: picks the blank with the fewest options, tries one and carries on. If that leads to a contradiction it backs up and rules that digit out.

Use ‹ and › to step backwards and forwards digit by digit, or Undo to go back to before the last search. On Slow or Fast the Working panel explains each digit as it is found; on Ultra only the result is shown. You can switch speed part way through.

Grade

Every approach counts its looks: reading a square, checking whether a digit fits, or rubbing out a pencil mark. When an approach gets stuck it borrows the other searches, easiest first, and goes back to its own way as soon as one finds something. Only when none of them can make progress does it guess (Tree Search), and all the work spent on wrong guesses counts too.

Guessing is hard for people, so a puzzle that can't be solved without it is rated at least Tough whatever its effort.

The puzzle's difficulty is the geometric mean of the looks each approach needed. Some puzzles suit one approach far better than another; the approach that needed fewest looks is the one the puzzle favours. 'Stuck' counts how often an approach ran dry and had to borrow another search.

More about how it was made, and its history since 2016, on the project page.