UPDATE:

Hoduku pretty much does everything I was wanting to do and more: https://github.com/PseudoFish/Hodoku (which is fork of https://github.com/yoki123/hodoku, but with a better README at least)

https://www.sudokuwiki.org/sudoku.htm looks good too.

=========================================

Working on a general sudoko player/solver.

I think my favorite interface I've seen and used for solving puzzles on line is the one nytimes one, in auto candidate mode.

================

References: https://www.learn-sudoku.com/omission.html My reviews:

https://www.sudokuwiki.org/sudoku.htm Looks good, haven't looked in detail yet.

Cracking the cryptic - hard to find a good hub page!

https://www.nytimes.com/puzzles/sudoku Most usable interface, due to the auto candidate mode. Drawbacks: - no undo; the X button is sort of a halfhearted substitute, but it's not clear how it interacts with penciling I've done manuall. - too easy to mis-type, or click on a canditate and unintentially toggle candidacy, and then suffer forever - auto candidate is weird-- it's partly auto, but then lets me remove stuff additionally, without much feedback as to which is which, which is error prone - "check guesses when entered" is unfriendly: when it's on, I can't X out my most recent move to see what changed - I definitely like auto candidate mode; that helps avoid the tedious part of this. But there is other similar tedious logic that I think could be automated. https://www.latimes.com/games/sudoku I've used this once, it's horribly error prone; mistyping anything totally wrecks everything. Interface is super weird (or else I just don't understand it)-- very little idea how to control it, or what the various assist options mean. It has an accessible archive, but it shows a long ad before each puzzle.

https://1sudoku.com Nice! - Seems like a very polished interface - has cool variants like kids 6x6. - Hides the board when mouse isn't over the page, which makes for a reasonable timer experience. - hmm, have to explicitly press button to update candidates

http://sudoklue.com - https://www.learn-sudoku.com/hidden-singles.html points out that pressing ctrl- highlights all pencil marks of that number, making hidden singles easier to spot Oh jeez, it isn't on line :-(

Wish list in a solver:

• penciling (showing remaining candidates for each cell)
• also some way of showing remaining possible positions of a given digit in a given row
• arbitrary size (2x2, 3x3, 4x4, etc)

Logic:

• "naked single" or "lone single": When only one candidate left in a cell, that's the answer for the cell, and that digit can be removed from the rest of all 3 houses containing that cell
• "hidden single": When only one place left in a row/column/block for a given digit to go, it goes there. (most or all of the latimes hints and nytimes hints are of this form)

DONE: "naked pair": when two cells in same row/column/block have same two candidates, that rules out those candidates for the rest of that row/column/block. Very strong if sharing the same block and row-or-column simultaneously; this is called a "locked pair". DONE: "naked triple", "naked quad". probably increasingly hard to spot. It means (according to http://hodoku.sourceforge.net/en/tech_als.php) when N unsolved cells in a house together contain only N candidates.

• "visual elimination" of a digit - supposedly powerful, but I don't totally get it yet. https://www.learn-sudoku.com/visual-elimination.html "Most Sudoku puzzles printed in newspapers and magazines, even those rated with high levels of difficulty, are in fact quite easy to solve — at least from the perspective of someone who has mastered this technique." My current guess is it's just a way of visually scanning for the other patterns such as hidden singles (1-fish), xwings (2-fish), swordfish (3-fish), and other fish.

DONE: "hidden pairs" - https://www.learn-sudoku.com/hidden-pairs.html This is a hidden naked pair: if two digits occur in only two cells in a block (or row or column), then erase everything else in those two cells, making it a naked pair. DONE: "hidden triples", "hidden quads" - https://www.learn-sudoku.com/hidden-triplets.html analogous to hidden pairs. need to go back and review this

DONE: "omission"/"intersection"/"pointing"/"blocking"/"claiming" https://www.learn-sudoku.com/omission.html

• when a given digit's occurrances in block is limited to a given row or column, then that digit can be eliminated from the rest of that row or column (or, when a given digit's occurrances in a row or column is limited to a block, then the digit can be eliminated from the rest of that block) The general statement: when a given digit's occurrance in a first house is limited to a second house, then the digit can be eliminated from the rest of that second house. Supposedly lighting up a given number can help find these.

TODO: x-wing. references: https://www.youtube.com/watch?v=gVT786t1Kjk "This Sudoku Actually Teaches The X-Wing Technique!"

================================== Ideas for concise puzzle in url. Hmm, can't decide.

003020600900305001001806400008102900700000008006708200002609500800203009005010300 ..3.2.6..9..3.5..1..18.64....81.29..7.......8..67.82....26.95..8..2.3..9..5.1.3.. 003020600!900305001!001806400!008102900!700000008!006708200!002609500!800203009!005010300 ..3.2.6..!9..3.5..1!..18.64..!..81.29..!7.......8!..67.82..!..26.95..!8..2.3..9!..5.1.3.. ..3.2.6..-9..3.5..1-..18.64..-..81.29..-7.......8-..67.82..-..26.95..-8..2.3..9-..5.1.3.. ..3.2.6..=9..3.5..1=..18.64..=..81.29..=7.......8=..67.82..=..26.95..=8..2.3..9=..5.1.3.. 003020600.900305001.001806400.008102900.700000008.006708200.002609500.800203009.005010300 003020600-900305001-001806400-008102900-700000008-006708200-002609500-800203009-005010300 003020600=900305001=001806400=008102900=700000008=006708200=002609500=800203009=005010300 003-020-600=900-305-001=001-806-400=008-102-900=700-000-008=006-708-200=002-609-500=800-203-009=005-010-300 ..3-.2.-6..=9..-3.5-..1=..1-8.6-4..=..8-1.2-9..=7..-...-..8=..6-7.8-2..=..2-6.9-5..=8..-2.3-..9=..5-.1.-3.. ..3,.2.,6..;9..,3.5,..1;..1,8.6,4..;..8,1.2,9..;7..,...,..8;..6,7.8,2..;..2,6.9,5..;8..,2.3,..9;..5,.1.,3.. 003,020,600;900,305,001;001,806,400.008,102,900!700,000,008;006,708,200!002,609,500;800,203,009;005,010,300

003020600 900305001 001806400 008102900 700000008 006708200 002609500 800203009 005010300