I re-wrote my code early on to take in jigsaw sudokus and treat a normal sudoku as a special jigsaw 'shape'. One reason was to start treating units generically and smooth out the code. I was wondering if anyone else has jigsaws in mind?

Two properties I've noticed. In order to grade jigsaws I've found that to match the same distribution of grades the scoring bands must be about twice as big, ie jigsaws are almost exactly twice as hard in terms of what strategies are required and the collapse rate (number of solvable cells in any one turn). This of course depends on the shapes. If it's close to a normal Sudoku then the effect if less. If the jigsaw is highly irregular (lots of snaky 'boxes' its also easier.

The other property of jigsaws makes for a conjecture. The minimum number of clues in a normal Sudoku is 17 (well, not proved I know, but there are good libraries out there with thousands of 17s but I've never seen a 16). I've noticed that low-clue jigsaws are much easier to create. I suspect there exist sub-17 jigsaw sudokus. When I've got a spare week of processing time I'll have a hunt.