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# Re: [xsl] How to Do Random "Shuffle"?

 Subject: Re: [xsl] How to Do Random "Shuffle"? From: "Dimitre Novatchev dnovatchev@xxxxxxxxx" Date: Sat, 13 Sep 2014 17:01:50 -0000

```On Sat, Sep 13, 2014 at 8:18 AM, Michael Kay mike@xxxxxxxxxxxx
<xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx> wrote:
> The challenge is to do it efficiently, which depends on the time complexity of the remove() function.
> If remove() is O(n) (i.e. if it involves copying all the items other than the one that's removed), then the shuffle is O(n^2).

There doesn't need to be any remove() operation.

The random permutation can be built as a new sequence, adding a new
item at a time. If the append() operation can be performed in constant
time (as I believe is the case with Saxon), then the total
computational complexity is O(n * O(generate-random-number)).

In case an implementation performs appending to a sequence in O(N),
then it can pre-pend to a sequence in constant time -- the result will
be the reversed random sequence that would have been produced by
appending -- that is also a random sequence.

--
Cheers,
Dimitre Novatchev
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```