|Don't ask to ask, just ask|
I meant my alternative reading of the criteria. That of "No column will end up in the same place". Unless I am reading his code wrong, it is possible that a given column could be swapped twice and end up in it's original position.
Indeed, if his Fischer-Yates shuffle is correct, this has to be a possibility in order to meet the fairness criteria. All possible outcomes, including a resultant where the output is in the same ordering as the input have to have equal chance.
I realise that by his original reading, swapping the order of a column twice, so that it ends up back in its original position, meets that criteria. However, the phraseology of the criteria is such, and the nature of the F_Y shuffle such, that explicitely noting it as a criteria made me consider the possibility that my alternative reading might be the true intent.
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
Lingua non convalesco, consenesco et abolesco. -- Rule 1 has a caveat! -- Who broke the cabal?
"Science is about questioning the status quo. Questioning authority".
The "good enough" maybe good enough for the now, and perfection maybe unobtainable, but that should not preclude us from striving for perfection, when time, circumstance or desire allow.