A Sudoku's difficulty level is often the first thing readers notice about a puzzle. For questions and answers showing a puzzle it would be nice to have a relatively objective linkable tester.

I've used Thonky.com's Sudoku Difficulty Estimator for this purpose, but according to user21820 it seems Thonky.com tends to be an overestimator:

The first one is not at all "very, very difficult". I only needed basic techniques and a binary branch-point at G7 (either 4 or 9) to solve it.

A further check of the puzzle in question using SudokuWiki.Org's Grader, (click on the orange "Grader" button), returns "diabolical", which agrees with the presumed overestimate of Thonky.

Is there something that does a better job at grading sudokus out there online? It should be:

  • linkable
  • no paywall or login
  • include some form of sudoku shorthand in the URL

Failing that, if anyone knows of a good command-line utility that does the job? Preferably a util that's already widely distributed, rather than something that needs compiling...

  • $\begingroup$ If you are good enough at programming, I can briefly describe how you can write a program that should be reasonably accurate at estimating how hard a sudoku puzzle will seem to a competent solver. I myself don't have the time to spend on this right now. $\endgroup$ – user21820 Jun 25 '19 at 14:03
  • $\begingroup$ @user21820, Sure that'd be fine, but to avoid re-inventing the wheel please wait a few days to see if some other reader doesn't already know of something that meets the spec. $\endgroup$ – agc Jun 25 '19 at 14:20
  • $\begingroup$ Roughly, use depth-first search while maintaining the number of possible digits that can go into each cell, at each step always filling in a cell with the minimum number of possibilities. But this doesn't find easy tactics, so if there are no 1-possibility cells, prune before each step by using a low-depth (say depth 3) complete search (without pruning) to find short contradictions to eliminate more possibilities. Measure difficulty by number of steps taken to find a solution (but excluding the steps taken during pruning). $\endgroup$ – user21820 Jun 25 '19 at 16:43
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    $\begingroup$ This meta site should be used for things pertaining to the site and/or its moderation. This question is properly in scope for the main site, not the meta. I'm migrating it accordingly. $\endgroup$ – Rubio Jun 29 '19 at 23:35

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