Welcome to my Sudoku page

Since I first studied Sudoku in May 2005 I think I've finally got a handle on this puzzle. My original intention was to prove to myself that a small number of simple strategies existed that could solve every sudoku. How wrong I was. Sudoku has enormous depth and while this solver has grown up enough to crack 99.9% of puzzles there are many weird and wonderful examples that defeat it. The main reason to keep this solver in development is to analyse these difficult ones. To that end I've added new graphical tools and features which I hope you'll enjoy.

I am continuing to document the various strategies that I and many other people have invented. These are available here.

I've tried to give credit where credit is due. Please update me if I have missed anyone or need to make a correction.

I am now working independently on puzzle creation, especially at www.str8ts.com.

All feedback, comments, arguments, bug reports and strategy ideas are welcome. There is a FEEDBACK form with a column displaying comments and questions. Many thanks to all the people who have done so and helped improve this solver.

New: Candidate and cell colouring

Many people have written to me to comment about multiple solutions for a given Sudoku. There are no logical tricks the solver can use to detect this other than to not complete correctly. The only way to check this is to perform a brute force analysis which tests every possible legal placement of a number. Computers are good at this and that's what the Solution Count button is for. Try this on any Sudoku to check whether if it is sane or not.

Strategy Overview

> The first six tests are the simplest and are required for any sudoku. After that you are allowed to choose which strategies the solver will use. Tick and untick the check boxes.

> The order of these advanced strategies - and my inclusion of them in categories 'tough', 'diabolical' and 'extreme' are my personal choice after close study and are roughly in order of complexity. While the logic is different for each, you should be aware that there is considerable overlap in their power to solve in certain situations. For example, X-Cycles are a subset of Alternating Inference Chains. If you turn off X-Cycles the same elimination might turn up under AICs. Since March 2010 I have reworked most chaining strategies to find the best and shortest chains - not necessarily the first one it happens to find. More about what is 'best' will be posted as a document.

> All strategies in the list have links to documentation, but it's worth describing what the first tests do:

• Show Possibles (Naked Singles): For each unknown square we eliminate all possibles where those numbers are known in each row, column and box. This may reveal a single candidate, in which case we have a solution for that cell.
• Hidden Singles: If a candidate occurs once only in a row or column or box we can make it the solution to the cell.
• Test 2: In this test we check for 'naked' Pairs and Triples. For example, if we have two pairs, eg 3-4 and 3-4 in the same row, column or box, then both 3 and 4 must occupy those squares (in what ever order). 3 and 4 can then be eliminated from the rest of the row, column or box.
• Test 3: This test is for Hidden Pairs, Hidden Triples
• Test 4: Naked and Hidden Quads - much rarer but simply extends Pairs and Triples.
• Test 5: See Pointing Pairs and Triples for a full explanation. This test helps us eliminate numbers in rows and columns outside the box.
• Test 6: Box/Line Reduction. We check the box against the rows and columns that intersect it for each number.