# SUDOKU RULES

A classic sudoku puzzle consists of 9 x 9 cells grid partially filled with numbers from 1 to 9. The cells are grouped in 3 x 3 blocks of 3 x 3 cells. The puzzle is solved when all empty cells are filled in such a way that every row, column and block contains every number from 1 to 9 exactly once.

A correct puzzle has only one solution and the minimum number of already filled cells needed is 17. There are 6670903752021072936960 possible puzzles including symmetries. The game will pick a random one for you and there is almost no chance you will get the same puzzle twice. If you want to challenge a friend with a particular sudoku that you liked or considered interesting you can use the option on the bottom right and get a link to the puzzle. Use that link by pasting into the address field of the browser and it will automatically bring up that puzzle.

More information about sudoku can be found on wikipedia.

# CONTROLS

In general you want to be quick when solving the puzzle. This implementation of the game is designed with that in mind, even if in some cases the controls could seem counter intuitive.

## INSERTING

There is no need for selecting the desired input for every location. Once a number is entered in the puzzle it becomes the current selected number and clicking or tapping an empty cell will insert it there. Another way to enter a digit is by pressing the corresponding key while the mouse is over the desired position.

## REMOVING

Pressing 0 or space while hovering a cell will remove the digit inside. When using a mouse or a touch device dragging a number anywhere outside its cell will remove it.

Pressing the 'clear' option in the menu will remove everything previously entered.

## HIGHLIGHTING

When a number is selected all its positions in the grid will be highlighted. A digit can be selected in several ways: by clicking or tapping a not empty cell, by clicking or tapping it in the select area or by pressing it on the keyboard. Clicking inside the page but not on a tile will remove the selection and highlight the fully completed numbers. There is a point to make here. The game doesn't check if you made mistakes. A completed number is just one that appears on every row, column and block. That doesn't mean it was entered in the right positions.

## PENCIL MARKS

Pencil marks can be used and they will be updated automatically. Also, they are available only for hard and expert puzzles. If the left mouse button is hold down while hovering a cell, pencil marks can be toggled using the keyboard. Be aware that removing a digit from a location will update the pencil marks that are affected regardless of their manually changed state.

## CHANGING DIFFICULTY

Hovering the 'new' option in the menu will let you choose a different difficulty level. Some devices or browsers will always show the submenu options. If you want to start a new game with the same difficulty level just click on 'new'.

## STATISTICS

When you successfully complete a puzzle you will be presented with a statistics page that will show your completion time, the number of games played, your best and your average time. The page will also render a graphical representation of all completion times of all players and will show you what percent of puzzles with the same difficulty were solved faster.

## SIGNING IN

If you want to keep your statistics across devices or browsers you can sign in with a well known provider like google, yahoo or facebook. The game will not query your name or other profile information from the provider. It will only use your unique id so it will recognize you when you sign in again.

# SUDOKU SOLVING TIPS

Several strategies have been developed by sudoku enthusiasts and even if you are not required to know them in order to solve puzzles, they may come in handy. The easy puzzles can be solved using only the first two techniques and the same is true for the medium ones but for those the patterns are harder to spot. For hard puzzles is better to know the more advanced techniques: naked and hidden subsets, pointing pairs or triples and box line reduction. Other techniques are needed only for expert level.

## NAKED SINGLES

That's a fancy name for saying that just one digit can go in a particular cell. In other words, inspecting the row, column and block to which that cell belongs, will reveal that all the other 8 numbers are present.

The easy sudokus can be solved using only this technique. Spotting the naked cells is easier than it might seem. The best way is to pick a number that appears in 2 rows within a band of 3 blocks. (the same goes for columns) The remaining row needs to have that digit in the third block that doesn't have it yet. So there are 3 cells where it can go. If 2 are already occupied or the number is in their column, the only place it can go is the third cell.

## HIDDEN SINGLES

This one is also directly derived from the rules. First of, pick a row, column or block usually having most of the locations filled. If one of the missing digits can only go in one of the cells, that's where it should go.

This technique is necessary for solving medium puzzles which cannot be completed using only the naked singles strategy.

The next techniques are needed for hard sudokus and can be applied using pencil marks.

## NAKED SUBSETS

The simplest subset is a pair. A naked pair is a set of exactly two candidates. If the naked pair is also present somewhere in the same row it means that those two candidates cannot be somewhere else in that row. So the candidates in the other cells can be erased. The same goes for columns and blocks. See controls section for how to toggle candidates.

A naked triple is the same. It just involves 3 candidates and 3 cells. It should be pointed out that not all candidates need to be present in every of the 3 cell. Larger subsets can sometimes be found and the same logic applies.

## HIDDEN SUBSETS

A hidden subset is a set of candidates that are only present in the same cells within a row, column or block, and the number of cells is equal to the number of candidates. This implies that all other candidates within those cells can be erased. Erasing them will not reveal a naked single or an immediate step in solving, but it may lead to other techniques.

## POINTING PAIRS AND TRIPLES

If within one block a candidate is only found in one of the rows then it is certain that candidate cannot be present on the same row in another block, so it can be removed from those positions. The same logic can be applied to columns

## BOX LINE REDUCTION

If one row or column has a candidate that is present only inside a block than all other cells of that block cannot have that candidate.

# ABOUT

Being a sudoku fan myself I always felt there was something missing from all sudoku sites out there. It was either functionality, either nice graphics. Some of the sites had just one new sudoku puzzle every day. (maybe that's not true anymore)

I am a programmer, but didn't have experience with web coding or design, so it was a challenge for me to try and create my own sudoku site. I hope I did a good job. Please tell me what you think.

## CONTACT INFORMATION

contact@fansudoku.com

## PRIVACY POLICY

Fansudoku does not share any information with third parties. You can only login using a well known identity provider like google, yahoo or facebook. After the login step we don't query personal information about you, we only retrieve and store a unique id that can identify you when you come back to our site.

Through the use of cookies that are stored on your device by the browser we are able to remember what difficulty you last played, whether you want to use pencil marks for hard and expert sudokus and a unique id that will let you track your progress.

This information may change in the future but it's highly unlikely.