Is java.util.Random really that random? How can I generate 52! (factorial) possible sequences?

Question

I\'ve been using Random (java.util.Random) to shuffle a deck of 52 cards. There are 52! (8.0658175e+67) possibilities. Yet, I\'ve found out that the seed for

Answer 1

I'm going to take a bit of a different tack on this. You're right on your assumptions - your PRNG isn't going to be able to hit all 52! possibilities.

The question is: what's the scale of your card game?

If you're making a simple klondike-style game? Then you definitely don't need all 52! possibilities. Instead, look at it like this: a player will have 18 quintillion distinct games. Even accounting for the 'Birthday Problem', they'd have to play billions of hands before they'd run into the first duplicate game.

If you're making a monte-carlo simulation? Then you're probably okay. You might have to deal with artifacts due to the 'P' in PRNG, but you're probably not going to run into problems simply due to a low seed space (again, you're looking at quintillions of unique possibilities.) On the flip side, if you're working with large iteration count, then, yeah, your low seed space might be a deal-breaker.

If you're making a multiplayer card game, particularly if there's money on the line? Then you're going to need to do some googling on how the online poker sites handled the same problem you're asking about. Because while the low seed space issue isn't noticeable to the average player, it is exploitable if it's worth the time investment. (The poker sites all went through a phase where their PRNGs were 'hacked', letting someone see the hole cards of all the other players, simply by deducing the seed from exposed cards.) If this is the situation you're in, don't simply find a better PRNG - you'll need to treat it as seriously as a Crypto problem.