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Subject: [std-proposals] sample view (with/without repetition)
From: Wolfgang Brehm (wolfgang.brehm_at_[hidden])
Date: 2019-11-07 11:11:14

Dear std C++ community

The new ranges and views will change the way we write algorithms. I
recently found std::sample and thought it would be nice to have to have an
analogous sample view which samples from a population without repetition -
with repetition is so trivial that we can ignore it for now.

What do you think about a sample view that accepts a range and a random
generator and returns a range which can be iterated over to access the
elements of the input range in a random order evaluating a bijective
(perfect and minimal) hash function to generate random indices without
repetition? This range would could still be relatively lightweight, because
it would only store a state encoded in an integer and the randomized
bijective hash function, similar to a random number generator. Sadly this
could only work well for random access iterators, not for forward
iterators, there reservoir sampling is the non-lightweight solution.

To preempt one objection, yes a randomized bijective hash for n consecutive
integers can be generated in small constant time. Multiplication with a
coprime integer and addition modulo n is bijective and randomizes the
order, for example. There are better hashes however.

I'd be interested what you think and volunteer to write a sample
implementation if there is interest - I am using similar ideas in my own
code, but you know how it is, it's not pretty.

Best Regards,
Wolfgang Brehm

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