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Pick and Choose (Part 1½)

2018-03-17 01:00, Tags: combinatorics

Part 1½? What is this? Yes, I know I said I would tackle some permutations thingy, but I got side-tracked thinking about Iterators... so I wanted to make a quick post about those thoughts while they're still bubbling. If you don't care for such jibber jabber, you can head straight on over to Part 2 right now!

So what is an Iterator? Briefly... It is a Role that can be applied to a Class (typically a Sequence, but not necessarily) that adds additional behaviors to that Class. In my previous post, I demonstrated how the power of Iterators allowed me to quickly count the elements of a sequence without actually producing them. This is because Iterators can implement a count-only method that gets called when you do something like call .elems on it. By the way, I'm not going to go over Iterators in a lot of detail. For that, you should head over Zoffix Znet's blog Perl 6 Party, and check out... well, all the articles... but particularly the series on Sequences and Iterators: Seqs, Drugs, and Rock'n'Roll.

The only thing you need to tell an Iterator how to do is produce values, but there are other things you can tell an Iterator how to do. I've already mentioned count-only (which I should be able to implement for all combinatorial sequences), but the other special Iterator method that I'd like to see work it's way into my module is efficient skip-at-least implementations. You see, some combinatorial algorithms have efficient means to generate an element in the middle of the sequence. This ability could then be incorporated into the Iterator to allow skipping of elements in (possibly very large) sequences.

While talking about this in the Reddit comments, I threw together a quick gist to demonstrate an example of a permutations function that is capable of producing enormous sequences (that you'd never be able to iterate through in a lifetime) which allows huge numbers of sequences to be skipped. Here's a brief preview, but check out the gist for the full code.

my @l = 'A'..'Z';

say permute(@l).elems;
# OUTPUT: 403291461126605635584000000

say permute(@l).skip(268860974084403757046816342)[0];
# OUTPUT: [R I J Z Y X W V U T S Q P O N K E H B L M D F C A G]

say "Completed in { now - INIT now } seconds";
# OUTPUT: Completed in 0.0531508 seconds

Now, I obviously have a preference for algorithms that are fast, and produce results lexicographically. Typically these algorithms are iterative in nature, in that each element in the sequence is generated by modifying the previous element... which is usually what makes them so fast. I don't want to sacrifice too much speed to get efficient skipping, but it's something I've been thinking about. For now, though, I think I just need to focus on getting working implementations of as many combinatorial algorithms as I can. This is the part where I exclaim pull requests are welcome!

Anyways, in those same Reddit comments, I also mentioned a few issues with the built in permutations routine. One that always kinda irked me was that the permutations() function, and the .permutations method produce different results. The method accepts a List-y argument, produces a sequence of permutations of that List. The subroutine accepts an Int (or coerces it's argument to an Int) - let's call it n - and produces a sequence of permutations of the list 0 up to n.

> (<A B C>).permutations
((A B C) (A C B) (B A C) (B C A) (C A B) (C B A))
> permutations(<A B C>)
((0 1 2) (0 2 1) (1 0 2) (1 2 0) (2 0 1) (2 1 0))

I think it's a little silly, but I've made peace with it. I suspect the function was made that way to allow maths lovers to get the permutations of n just by typing permutations(n). That got me thinking about different things that could happen when giving my functions an Int instead of a list.

As you'd expect, when given a list, combinatorial functions in this module will produce the combinatorial sequence of that given list. What if, however, when called with an Int in place of the list, it would instead provide the number of elements in that sequence! Here's some imaginary - but totally do-able - example code

> permutations(3)
6
> permutations([0, 1, 2])
((0 1 2) (0 2 1) (1 0 2) (1 2 0) (2 0 1) (2 1 0))

Is that a stupid idea? Let me know... If you shame me enough you might convince me to abandon the idea altogether... but it's seems fairly sane to me.

Lastly, in the same Reddit comments mentioned above, Zoffix linked to a handy helper function that is used in the Rakudo core for testing iterators to ensure that the important Iterator methods are working as expected. I'll certainly be adding that into my test files.

Now on to the real Part 2.