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Rating System
  Sudoku Snake uses a move-based puzzle rating system that uses passes or sweeps of solving techniques.

  When Sudoku Snake is analyzing, it first scans the puzzle for hidden singles by box - the easiest of all solving techniques. If none are found, it then moves on to the next harder technique, and so forth. Once a solving technique is spotted from among the 44 different ones used by Sudoku Snake, the skill score for that technique is added to the skill score for the puzzle, the solving technique is applied, and the process is repeated until the puzzle is solved.

  Note that sweeps of moves are used as opposed to individual moves. For instance, suppose that the easiest solving technique at a given point in the analysis is hidden singles. Now say there are 8 of them! How does Sudoku Snake know which one to use?

  This problem has been the biggest question mark in rating sudoku puzzles around the world. Perhaps one of the hidden singles unlocks a whole new set of hidden singles that make the puzzle easier to solve, and the other seven do not. The chances are only 1 in 8 that a human will find the right hidden single to go on to easily solve the puzzle. The other 7 times the human will spot a wrong one, making the puzzle appear more difficult.

  This is why one person sometimes says a puzzle is easy, while another person of equal skill will say it is hard.

  Some rating systems bypass this issue by saying that the only way to rate a puzzle is by using the hardest solving technique required to solve it. The harder that one technique, the harder the puzzle. This sounds fine, but it doesn't account for how many times a human has to use solving techniques. A puzzle where you need to use 5 naked singles, 3 x-wings, 6 subsets, and 2 swordfish is certainly more difficult than one where you only need 1 xy-wing, despite the fact than an xy-wing is slightly more difficult to spot than any of those other techniques.

  Other rating systems use more computational methods. For instance, how many bifurcations are required to solve a puzzle, ignoring any human-like solving techniques. Or what level of complexity of multi-value fishy patterns are required, despite the fact that no human can spot such patterns.

  Sudoku Snake overcomes all these problems using strictly human-like solving techniques, and performing them in sweeps. Going back to the example above of 8 hidden single techniques. Sudoku Snake doesn't choose one at random, there's no logic in that. It also doesn't choose the one that unlocks the puzzle, because that doesn't account for the ease in finding one of the others. Sudoku Snake chooses them all at once.

  We consider every move as if you had to sweep over the entire puzzle for it. If there are eight of them, then it should be roughly as easy to find all of them as it would be if there were only four to find all of those. The difficulty in spotting each group of techniques is a full-sweep of the entire sudoku board.

  This accounts for all of the variation in solving a sudoku puzzle. Some people will be lucky and spot all the key techniques quickly, others will happen to spot the wrong ones, but the general Sudoku Snake skill level of the puzzle should be accurate.

  Many of Sudoku Snake's customers have applauded our rating system as giving the most accurate assessment of the difficulty of sudoku puzzles they have ever seen.

  The following is a list of Skill Levels as they compare to the points involved.

Level   Score   Skill Points
Very Easy   (0.0-0.9)   0-4
Easy   (1.0-1.9)   5-9
Moderate   (2.0-2.9)   10-19
Hard   (3.0-3.9)   20-54
Very Hard   (4.0-4.9)   55-148
Expert   (5.0-5.9)   149-403
Fiendish   (6.0-6.9)   404-1096
Diabolical   (7.0-7.9)   1097-2980
Serpentine   (8.0-9.9)   2981-22026
Ludicrous   (10.0+)   22027+

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