Sudoku X is a great variant of normal Sudoku and this solver is an extension of my Sudoku Solver to help you discover the logical solutions for this puzzle. The difference is that in Sudoku X the two diagonals are known to contain the numbers 1 ro 9 uniquely. These extra constraints allow you the puzzle solver to dervice new conclusions about candidates to eliminate and find solutions to cells. You can look along the diagonals (marked with a darked X on th board) and make deductions. However, the extra constraints mean that the puzzle creator can leave less clues than normal sudoku.

For the easier Sudoku X puzzles you won't really find a necessary example of a deduction based on the diagonals although you will want to scan them in case you see an easy 'single'. For tough puzzles and above the diagonals must be checked. In this solver they are checked before rows, columns and boxes. All the normal rules and logical posibilities apply to Sudoku X with some exceptions. There are pitfalls, for example, with Unique Rectangles, which rely on a certain formations. I have documented these here. Please check this stratgy guide if you want to use the advanced strategies.

I am now working independently on puzzle creation. For further Sudoku strategy information I recommend the forums on sudoku.org.uk and sudoku.com.

All feedback, comments, arguements, bug reports and strategy ideas are welcome. There is a new 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 in version 1.64 (March 10th 2010)
Fixed a series of bugs with editing candidates in the large board.

New in version 1.63 (March 3rd 2010)
3D Medusa and much improved forcing chains. A new system of symbolising chains.

New in version 1.60 (June 13th 2009)

Version history here Original version 1.42 12th Jan 2008

Michael from Denmark has sent me the 'Unsolvable' - a great puzzle from a Sudoku magazine which I can't logically solve yet.

Use Clear to empty the board before entering your own puzzle. Save will remember the current state of the board so you can Re-load it again (even if you close your browser - you must allow cookies for this to work). Re-Start applies only to the example puzzles in the list. The current list contains an example puzzle that tests some of the strategies.

Take Step first displays the possibles or candidates for each unknown cell. These are the numbers that do not contradict any known or solved square. Once these are displayed Take Step will step through other tests and then loop until it can go no further. The first few tests are the most productive and the solver will often loop between them. If any are successful and the board is changed in any way it will go back to the start and "Check for Solved Squares". The reason for this step is to make it easier to spot what's changed. Many of the strategies have knock-on effects which mean that they can't be run back-to-back - it's essential that we return to the basic steps. We go back because we want the least hard solve route.

> The first seven 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 un-tick the check boxes. For example, you may not want to use any strategies that rely on a unique solution. Uncheck test 15.

> The order of these advanced strategies - and my inclusion of them in categories 'tough', 'diabolical' and 'evil' 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, 'Guardians' will never solve anything while 'Multi-colouring' is switched on since they both attack similar configurations.

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

• Show Possibles: 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.
  
• Test 1: If a possible number occurs once in a row or column we can eliminate other candidates and make this the solution to the square.
  
• Test 2: If a possible number occurs once in a diagonal or box (3 by 3 cell) we can eliminate other candidates and make this the solution to the square. Same test as Test 1 but for boxes.
  
• Test 3: 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 4: This test is for Hidden Pairs and Hidden Triples.
• Test 5: This test is for Naked Quads and Hidden Quads.
• Test 6: See pointing pairs and triples for a full explanation. This test help us eliminate numbers in rows and columns outside the box.
  
• Test 7: Box/Line Reduction. We check the box against the rows and columns that intersect it for each number.