Advent of Code F# – Day 7

Yan Cui

I help clients go faster for less using serverless technologies.

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ps. look out for all my other solutions for Advent of Code challenges here.


Sorry for the delay in getting this post out today, I enjoyed some really fine Ramen with some of the lovely people I worked with at Yubl and only managed to find time to do the challenge and write this post late in the evening.

(ps. here are some of the best people I have ever had the pleasure to work with )


anyways, let’s get back to the main topic…


Day 7

See details of the challenge here.

The input for today’s challenge looks like this:


Looking at the input, they all follow a similar pattern and we can separate them out with String.Split(‘[‘, ‘]’) and end up with an array where the odd number elements are the hypernet sequences.

We still need a way to look for ABBAs, which fortunately can be achieved easily with Seq.windowed and a bit of pattern matching (see the hasABBA function below).


Part 2

You would also like to know which IPs support SSL (super-secret listening).

An IP supports SSL if it has an Area-Broadcast Accessor, or ABA, anywhere in the
supernet sequences (outside any square bracketed sections), and a corresponding
Byte Allocation Block, or BAB, anywhere in the hypernet sequences. An ABA is any
three-character sequence which consists of the same character twice with a
different character between them, such as xyx or aba. A corresponding BAB is the
same characters but in reversed positions: yxy and bab, respectively.

For example:

aba[bab]xyz supports SSL (aba outside square brackets with corresponding bab
within square brackets).
xyx[xyx]xyx does not support SSL (xyx, but no corresponding yxy).
aaa[kek]eke supports SSL (eke in supernet with corresponding kek in hypernet;
the aaa sequence is not related, because the interior character must be different).
zazbz[bzb]cdb supports SSL (zaz has no corresponding aza, but zbz has a
corresponding bzb, even though zaz and zbz overlap).
How many IPs in your puzzle input support SSL?

Part 2 is slightly more challenging as it requires to track the ABAs from the sequences that are not part of the hypernet sequences, and look for corresponding BABs in the hypernet sequences (seriously, who thought of these acronym…).

We can use the same technique of + pattern matching to find all the ABAs, but we also need a hasBAB function to check if any of the corresponding BABs exists in a hypernet sequence.

Whilst I was at it, I also threw in a ToBABs active pattern to convert an array of ABAs into their corresponding BABs.

The question was slightly ambiguous – I wasn’t sure if all the corresponding BABs need to exist in the hypernet sequences or just one, I went with the later and that got me the right answer so I guess I got lucky there.



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