SukakuExplainer VS Emerentius/sudoku

The SE rating system is based on a list of Sudoku techniques that have been ranked (somewhat subjectively) by how difficult they are to spot and apply. A computer solver uses that list and repeatedly applies the easiest available move from the list to the puzzle until it is solved. The difficulty of the hardest individual step un that path then determines the difficulty of the complete puzzle. Here is the list in question: https://github.com/SudokuMonster/SukakuExplainer/wiki/Difficulty-Ratings-in-Sukaku-Explainer-v1.17.8

The most complex function is handling the hint generation. Because I'm not smart enough to code the hint generation myself, the hard work is done using a Java tool called Sukaku Explainer which I use to generate one complete solution path for the whole puzzle in HTML. My Perl functions feed a queue of requests into a single instance of the Java program (my 1GB VPS couldn't handle more than one). The HTML is then munged into JSON like this for consumption by the frontend. The JSON is also saved to a file, so that if two people ask for hints for the same puzzle Nginx can directly return the contents of that file - that way the function only gets called if the file does not already exist.

Sudoku/Sukaku Explainer Rating, it's the second very established difficulty metric for Sudoku puzzles. It ranges from 1.0 (only Singles) to 12.7 (“Dynamic Forcing Chains(+Dynamic Forcing Chains) in Pencilmark Sudoku”).

If you always find out that the next step that you didn't spot is a WXYZ-Wing, that most likely means that you know all the techniques the computer solver uses for the difficulty level you currently use, except for WXYZ-Wings. I was at this exact point for quite some time (my subreddit flair references that struggle), and I took it as a challenge to get good at ALS-XZ-based techniques. The underlying reason was just the ordering of techniques used for SE rating (used in SudokuExchange.com for grading puzzles) though, not some truth inherent to Sudoku in general.

There are extreme differences between different apps. To compare difficulty, I will use the (well established) SE rating system as described here. It rates the hardest single step a sophisticated computer needed in order to solve a puzzle on a scale from 1.0 (easiest, only one empty cell left in a row/column/box) to 12.7 (basically impossible for humans, crazy chains within chains). The “Hard” puzzle of the New York Times is typically rated between 3.0 and 3.4.

Sukaku Explainer is a straight extension of the original Sudoku Explainer that also supports Sukaku (Pencilmark) puzzles. The JAR file from the “Releases” page can be started directly if you have Java installed.

SudokuExchange.com is always worth considering. It has all the advantages you mention about Sven's Sudoku Pad, plus a large collections of puzzles from trivial to brutally hard. The difficulty ratings and hints are delegated to Sukaku Explainer, which is a very mature and highl-quality solver. Some not-so-obvious features:

Wow, this is a very tough puzzle, especially for the “Expert” category. Which app is this from? The computer solver Sukaku Explainer needs multiple Forcing Chains and ALS-Wings to solve it and rates the hardest step 7.2 SE (out of 13). A normal “Expert” puzzle is not expected to require any technique above 5.5, and most never touch 4.5: https://github.com/SudokuMonster/SukakuExplainer/wiki/Difficulty-Ratings-in-Sukaku-Explainer-v1.17.8

If you want to avoid all Wings and harder techniques, you can hover over the yellow line under each puzzle on SudokuExchange.com to see its SE Rating. Everything below 4.0 (around the middle of the “Hard” section) is guaranteed not to require any Wings.