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The math of shuffling cards almost brought down an online poker empire

(www.scientificamerican.com)
97 points indigodaddy | 1 comments | 05 Sep 25 22:50 UTC | HN request time: 0.22s | source
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pmarreck ◴[07 Sep 25 03:54 UTC] No.45155251[source]▶
Why not just simulate a real shuffle?

Just "cut" in a random location (rotate the deck a random amount), then split the deck roughly in half (add a bit of random variation), then "flip them together" back to front by alternately taking 1 or 2 (randomly, add a small chance of 3, so maybe 50% 1, 40% 2 and 10% 3) from each side till there are no cards left to shuffle. Then repeat 8 times or so (there's a certain minimum number of times that ensures good randomness)

replies(3): >>45155382 #>>45155506 #>>45156321 #
1. amluto ◴[07 Sep 25 04:57 UTC] No.45155506[source]▶
You can:

(a) Use Fischer-Yates, which is fast and, if correctly implemented, unbiased, or

(b) Come up with an ad-hoc scheme like this, do some fancy math to bound its total variation distance (or, worse, some weaker concept of distance), convince yourself that the distance to the uniform distribution is acceptable, and end up with a slower algorithm when you’re done.