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# Sudoku 4ever Plus

Sudoku4ever by 1gravity offers an unlimited number of puzzles, eight difficulty levels from Very Easy to Nightmare, a world-class hint engine, game statistics and global high scores.

A standard Sudoku puzzle consists of a 9×9 grid, with a total of 81 cells. Each position in the grid is called a cell. A cell can be empty, or filled with a digit. This digit can be any digit between one and nine. You must place digits into the grid in such a way that every row, every column, and every 3×3 box contains each of the digits one through nine.

A number of digits have been placed into the grid by the maker. They are called givens, clues, or fixed digits. They are placed in such a way that you can use logical reasoning to find the solution. A well-formed Sudoku has a single solution that can be found by reasoning, regardless of the complexity of that reasoning.

A game called “Number Place” was designed by Howard Garns, and first published by Dell Magazines in 1979. The puzzle was introduced in Japan by Nikoli in 1984, who gave it the name “Sudoku,” which is an abbreviation for the original Japanese name meaning “The Numbers must be Unique.” The whole world adopted this new name, but in Japan, Nikoli owns the trademark, so other publishers in Japan call it Nanpure, the Japanese translation of Number Place.

By the end of 2004, Sudoku appeared in The Times in Britain, brought to Europe by Wayne Gould, the founder of Pappocom. Many newspapers worldwide followed. Sudoku magazines can be found everywhere. Sudokus are published on the Internet on a daily basis. And many people have become addicted to it.

Every puzzle included in Sudoku4ever has a unique solution and can be solved using logic only (no guessing), although there’s a controversial discussion going on in the Sudoku community about where logic ends and guessing starts, and which solving technique belongs to which category. If you doubt that a puzzle is solvable, use the solve function (the solver also checks for multiple solutions) or the hint function to get a step-by-step solution path.

For aesthetic reasons, a large part of Sudoku puzzle makers think they look nicer by making them symmetric one way or the other. From a mathematical point of view, the symmetry has no effect, other than that the chances for redundant givens in the grid increases with a higher level of symmetry. Sudoku4ever generates symmetrical puzzles only.

Sudoku4ever offers eight difficulty levels. Each level contains only puzzles with difficulty ratings within a well-defined range:

- Very Easy: rating between 1.0 and 1.2
- Easy: rating between 1.3 and 1.5
- Moderate: rating between 1.6 and 2.3
- Advanced: rating between 2.4 and 2.8
- Hard: rating between 2.9 and 3.4
- Very Hard: rating between 3.5 and 4.4
- Fiendish: rating between 4.5 and 6.2
- Nightmare: rating between 6.3 and 10.0

The hint functionality is helpful should you get stuck in a puzzle. Each hint with its three detail levels (small clue, big clue, and hint) has been designed to present exactly the right amount of help: not too much to spoil the fun but enough to be able to move a step forward. A lot of effort has been put into the visualization of hints. Even complex hints should be comprehensible when combining the text explanations and the visualization on the board, the latter sometimes using several views, e.g. for multiple forcing chains. Some hints are too complex to be visualized because there’s just too much information to be shown. You shouldn’t run into that particular problem for puzzles with difficulty level below Nightmare.

The hint engine doesn’t just produce hints; it is also able to check for incorrect digits, incorrect pencil marks and illegal puzzles (no solution, multiple solutions, too few clues or givens etc.). If incorrect digits or pencil marks exist, the first hint will be a warning hint that can be used to correct the faulty digits on the board.

Pencil marks are incorrect if a value is either illogical (digit already set in the same row, column or block) or if the correct value is missing (check against solution).

Once no full houses and hidden singles are present, a hint will be produced to compute all missing pencil marks because all but the most basic solving techniques require pencil marks. Of course naked singles and other patterns can also be found without pencil marks, but it’s just a bit more difficult.

It can be frustrating to work on a puzzle for 10, 20 or more minutes just to realize that you made a mistake and need to backtrack. That’s where the check functionality comes in handy.

Depending on the selected input method, the digit buttons work differently.

- Digit First: first press a digit button, then select the cell to enter or clear the digit
- Cell First: first select a cell then press a digit button to enter or clear one digit
- Andoku Cell First: first select a cell then press one or several digit buttons to enter or clear one or more digits
- Andoku Digit First: basically a Digit First input method that allows entering or clearing one or more digits at once (multiple digit buttons can be selected before selecting a cell)
- Mixed: Digit First for values, Andoku Cell First for pencil marks
- Popup: Select a cell and a popup appears to pick a digit to be entered

To clear a digit either enter the same digit again or use the erase button.

Keyboard: you can use the delete key and the digit-key zero to delete digits and pencil marks. Pressing these keys is identical to pressing the erase button. The digit keys one through nine are used to set digits and pencil marks. Pressing the digit keys is identical to pressing the digit buttons.

Pencil marks are used to remember the remaining candidates for a cell and thus to find patterns that help in solving the puzzle. While it’s possible to solve easy puzzles without using pencil marks, harder puzzles can’t be solved without them.

If you select the pencil mark button, you can alter pencil marks. Entering and clearing of pencil marks works the same as entering and clearing digits. If auto pencil mode is set to Auto Set, pencil marks can’t be altered, but the pencil button is still enabled to support the different highlighting options.

Sudoku4ever supports unlimited undo and redo. The undo and redo history is saved together with the puzzle and restored whenever the game is resumed. There are separate undo and redo histories for altering a puzzle’s digits (values and pencil marks) and its colors in Coloring Mode.

Sudoku4ever is one of the few Sudoku apps on Android offering a special coloring mode to support color-solving techniques. Coloring mode can be enabled either by selecting the associated item in the options menu or by long-pressing an enabled button or the Sudoku board if Toggle Coloring Mode is enabled.

Color buttons: the six color buttons are used to pick one or two of four different colors. The bi-color buttons are used for multi-coloring techniques.

- Color erase button: the color eraser is used to erase the colors of one cell at a time.
- Erase button: the eraser removes all colors at once (undo possible).
- Color mode button: if the color mode button is checked, colors can be altered; if it’s unchecked, digits (values and pencil marks) can be altered.
- Color pencil button: if the color pencil button is checked, pencil marks can be colored instead of cells. This is for advanced coloring techniques as well as for finding forcing chains.

If the pause button is pressed, the game is paused; the game timer stops and the board will show question marks instead of digits. The game is also paused if you navigate away from the application. Pressing the space key is another way to toggle the pause button.

There’s no need to directly save a game, as this happens automatically, e.g. when the game is paused or if you navigate away from the play screen. You can play as many games as you like in parallel, and resume them whenever you want.

If you enable OpenFeint, and you are logged in, your played time will be submitted to the global leaderboard (one for each difficulty).

Every puzzle is eligible for being submitted, but only the first time it’s played and if none of the two options, Check Puzzle, or Highlight Errors were set to show incorrect digits (even temporarily). You can re-play a puzzle, but the time won’t be submitted to the global leaderboard.

Setting a bookmark allows you to save the current board position and to return to that same position. This is useful for exploring different solving paths, as some might lead to board positions that are practically unsolvable.

The analysis function computes a list of solving techniques needed to solve a puzzle. It also computes the puzzle’s difficulty rating, a decimal value between 1.0 (easiest) and 10.0 (hardest). Harder puzzles can take a long time to analyze so please be patient when using this function on Nightmare puzzles.

If you’ve downloaded and played other Sudoku apps, you’ll soon find them lacking features listed here for Sudoku4ever. If you’re a serious Sudoku player, graduate to the higher levels of play and functionality. Try Sudoku4ever and see the difference.

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