difficulty of Sudoku is less determined by the number of empty fields but rather by the complexity of logical decisions

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Expert Sudoku solvers know this: It's simply not true that a Sudoku gains difficulty when reducing the number of clues. A Sudoko with 64 empty fields can look very hard at the first glance, but it inevitably isn't. It's rather the number and the complexity of logical decisions at player has to make that increase a Sudoku's difficulty.

So there should be an (additional) option which tells the generator to output only Sudokus that fit the required complexity level – and an evaluation function that determines the Sudoku's complexity.