What's a good shuffle percentage?

Question

I'm basically new to coding for random results, but did some reading and tested out the javascript version of the Fisher-Yates algorithm (as seen on wikipedia), with an ordered list.

I ended up adding code to make sure the array was shuffled differently than its initial order, and also calculated the percentage of how many objects were shuffled to a different position by the algorithm.

So I'm wondering what might be considered a good result. Kind of a generic question. If I shuffled a deck of cards, what would be the least acceptable amount of shuffle? Right now I have mine coded to repeat the algorithm if it comes out less than 25 percent shuffled.

What do you think?

Answer 1

Zero. You can make any number of checks you like to make it feel more random, but even the check for the same order makes your algorithm flawed.

Answer 2

If your shuffle algorithm is correctly implemented and produces a truly random shuffle (modulo your PRNG's randomness, or lack thereof), I wouldn't reshuffle at all. In particular, the fact that you don't accept random configurations that are 25+% similar to your original configuration tells an adversary that they can expect not to see any of those configurations after your shuffling completes.