Sudoko

I've now solved over 70 of the Project Euler site i mentioned a couple months ago. One of the problems i just solved was to generate solutions to 50 Sudoku puzzles. People solve these sorts of puzzles by hand all the time. I could have done it that way. I started by solving the first one by hand. It was pretty easy. I really only used one rule to make it happen. I thought about the rule for a bit, and decided it would be easy enough to code into a program, and that might be quicker than doing 50 by hand. And this first version solved some 29 of the 50. My program showed how far it had gotten. I used that to tackle the next unsolved puzzle by hand. And, indeed, i found an easy rule to code, and that version managed 39. From there, it might have been quicker to solve the last 11 by hand. Eventually, there were five ideas, and 46 problems were solved. One of the problems could be easily solved by hand from there if a guess was made of one of the two remaining possibilities for the cell. I decided to code in guesses. After making a guess, it would use it's previous system to see if it could get a solution. It continued to make a single guess until it either solved the puzzle, or it ran out of single guesses. All fifty puzzles were solved.

Total run time was difficult to get. What you'd like is the amount of time it takes to solve each puzzle. So, if you had a thousand puzzles to solve, you could multiply your time by a thousand and get a good estimate for how long that would take. But the total time was much less than a second. It's unlikely that the time would scale. It's quite possible that a thousand puzzles would also take less than a second.

Of notable interest is that nearly five thousand people have solved this problem. There are tons of programs that have been published that also solve this problem. So at least one person in a million, world wide, has written a Sudoku solver. That's a good deal more than i'd have expected.