CODEWITHSUNDEEP
mysqlrandomprobability
James Simpson
James Simpson

Reputation: 13688

Is MySQL's RAND() good enough for "shuffling" card decks for card games?

I've got a poker and blackjack game that store a base card deck in a MySQL database. In order to shuffle the cards, I order the table at random using ORDER BY RAND() and insert the cards in that order into a different table. Will using RAND() result in realistic odds that would be found playing with real cards and physically shuffling, or is this function not random enough?

Upvotes: 2

Views: 1859

Answers (2)

Matthew
Matthew

Reputation: 10444

For your use: Probably.

From the gaming industry perspective: No.

The Nevada Gaming Commission oversees the odds and randomness of approved games here. For such a purpose, it is in the interest of the house to eliminate the possibility of RE the randomizer so the real gaming software uses expiring "true random" seeds which must be obtained by certified methods (usually using some naturally chaotic input). Banks will often employ similar tools.

For further reading, see LavaRand

http://www.lavarand.com/

Upvotes: 0

Mark Byers
Mark Byers

Reputation: 838376

Shuffling with real cards isn't really all that random unless you practise it a lot. It's easy to shuffle incorrectly so that either the top or the bottom of the pack isn't shuffled well.

Using ORDER BY RAND() to shuffle is a reasonable approach but there are some things to be aware of:

  • A slight bias is introduced if two exactly equal random numbers are generated by RAND() as it will not shuffle those two cards correctly with respect to each other.
  • RAND() is not cryptographically secure. By seeing some of the first cards in the deck it might be possible for a skilled attacker to deduce the internal state of the PRNG used to shuffle the deck and therefore predict the remaining cards.
  • ORDER BY RAND() requires O(n log(n)) operations. It will most likely have acceptable performance for shuffling 52 rows, but it is probably not something you want to use for shuffling millions of rows, for example.

For entertainment purposes your approach should be fine. If this is for serious money you might want to use a better shuffle algorithm such as the Fisher Yates shuffle and cryptographically secure random number generator.

Upvotes: 5

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