Comment 9 for bug 552315

The new puzzle generator introduced in KDE SC 4.8 does not have this problem, except as you have observed in Unlimited difficulty mode. It encounters "road blocks" unpredictably during the generation process, instead of churning out and checking candidate puzzles regularly.

There is already a limit of 20 attempts to generate a puzzle with the type, size, difficulty and symmetry as ordered, after which KSudoku picks the best attempt so far and the user has the option to go on trying. Unlimited mode is just not getting that far on large-size puzzles. This is not right, but I cannot decide if it is worth investigating.

The point of Unlimited mode is for possible savants out there who have no problem with how many guessing and backtracking points there may be nor how soon the guessing has to start. The levels up to and including Hard normally require no guessing. These and the Diabolical level, which usually does require guessing, have been tested and calibrated against published puzzles in newspapers etc. My wife, who regularly does Hard and Diabolical puzzles, found Unlimited puzzles "inhuman" (i.e. tedious, very difficult and no fun) and my own experience was the same. That is why I introduced limits on guessing in the other levels.