Hardcore Sudoku fans, rejoice! Sudoku 4ever Plus (4/5 stars, all ages, FREE today only) is today’s Amazon Free App of the Day. This is Sudoku for true connoisseurs of the game From Amazon:
Advanced and Full-Featured: Sudoku 4ever Plus
Sudoku 4ever Plus 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.
Sudoku: A Brief History
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.
The Puzzles
Every puzzle included in Sudoku 4ever Plus 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.
Eight Levels of Difficulty
Sudoku 4ever Plus 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
Hint and Check Functionality
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.
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.
Pencil Marks
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.
Undo and Redo
Sudoku 4ever Plus 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.
Coloring Mode
Sudoku 4ever Plus 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.
Sudoku 4ever Plus: get it today, while it’s the Free App of the Day!
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