A "perfect shuffle" according to the article:

>The riffle shuffle has to follow a realistic but strict model where cards are randomly interleaved from the left or right pile one by one. (Each card gets dropped from either the left or the right pile with a probability that’s proportional to the number of cards remaining in that pile. This means that the cards don’t simply alternate between left and right, which would result in a predictable structure; instead, the order might go “left, right, right, left, right, left, left.”)

You misunderstood because the title is ambiguous.

This talks about seven consecutive riffle shuffles ("cut the deck and interleave the piles"): Those are not a "perfect shuffle" (i.e. same probability for every permutation) by themselves, only after doing them several times consecutively (which is kinda suprising by itself).

It's modelled with randomness, each card is taken from left or right with a probability, it's not a deterministic model.

I don’t know on perfect shuffles but for the sloppy shuffles, the deck is cut at a random location between each shuffle.

See the paragraph beginning "Yet terms and conditions also apply."

Yeah, a "perfect" shuffle is known as a faro shuffle and it's the basis of a lot of magic tricks, but it's a weird looking shuffle and it sort of ruins the tricks once you can recognize it.

shouldn't a perfect hackernews rtfa?