Advent of Code F# - Day 4 | theburningmonk.com

Yan Cui

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Day 4

Part 2

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

Day 4

See details of the challenge here.

The input for today’s challenge looks something like this:

bkwzkqsxq-tovvilokx-nozvyiwoxd-172[fstek]
wifilzof-wbiwifuny-yhachyylcha-526[qrazx]
jvyyvzpcl-jhukf-shivyhavyf-487[zhtsi]
kwvacumz-ozilm-kivlg-kwvbiqvumvb-694[gknyw]
mvhkvbdib-kmjezxodgz-mvwwdo-omvdidib-837[dmvbi]
nzydfxpc-rclop-qwzhpc-lnbftdtetzy-171[cptzd]
vhehkyne-unggr-inkvatlbgz-813[gnehk]
tcorcikpi-hnqygt-octmgvkpi-570[nzewo]
xmtjbzidx-wvnfzo-jkzmvodjin-447[uyzlp]
willimcpy-mwupyhayl-bohn-mufym-734[stjoc]
sbejpbdujwf-cvooz-xpsltipq-961[azfnd]

…

So our first task is to turn these into a more usable format, eg. a tuple of (encrypted name, sector ID, checksum).

	open System
	open System.IO

	let input =
	File.ReadAllLines(__SOURCE_DIRECTORY__ + "/Day04Input.txt")
	\|> Array.map (fun line ->
	let [\| rest; checksum \|] =
	line.Split('[', ']')
	\|> Array.filter (not << String.IsNullOrWhiteSpace)

	let lastDash = rest.LastIndexOf('-')
	let sectorId = rest.Substring(lastDash+1) \|> int
	let encryptedName = rest.Substring(0, lastDash)

	encryptedName, sectorId, checksum)

view raw Day04_01.fsx hosted with

by GitHub

To find the real room names, we need to calculate the checksum value from the encrypted name.

	let isReal (encryptedName : string) checksum =
	let checksum' =
	encryptedName
	\|> Seq.filter ((<>) '-')
	\|> Seq.groupBy id
	\|> Seq.sortBy (fun (c, cs) -> -(Seq.length cs), c)
	\|> Seq.map fst
	\|> Seq.take 5
	\|> Seq.toArray
	\|> fun chars -> new String(chars)

	checksum' = checksum

	let realRooms =
	input \|> Array.filter (fun (name, _, checksum) -> isReal name checksum)

	let part1 = realRooms \|> Array.sumBy (fun (_, sectorId, _) -> sectorId)

view raw Day04_02.fsx hosted with

by GitHub

To produce the desired sorting in LINQ, I’d have used OrderByDescending and ThenBy. There is no ThenBy in any of the F# modules, but F# tuples are ordered the way you expect – by first item, then second item, and so on.

So, I can achieve the same result as OrderByDescending + ThenBy with one line instead:

Seq.sortBy (fun (c, cs) -> –(Seq.length cs), c)

pretty neat, right?

Part 2

With all the decoy data out of the way, it’s time to decrypt this list and get
moving.

The room names are encrypted by a state-of-the-art shift cipher, which is nearly
unbreakable without the right software. However, the information kiosk designers
at Easter Bunny HQ were not expecting to deal with a master cryptographer like
yourself.

To decrypt a room name, rotate each letter forward through the alphabet a number
of times equal to the room’s sector ID. A becomes B, B becomes C, Z becomes A,
and so on. Dashes become spaces.

For example, the real name for qzmt-zixmtkozy-ivhz-343 is very encrypted name.

What is the sector ID of the room where North Pole objects are stored?

First, let’s add a decrypt function:

	let alphabets = [\|'a'..'z'\|]

	let shift c sectorId =
	let oldIdx = Array.findIndex ((=) c) alphabets
	let newIdx = (oldIdx + sectorId) % 26
	alphabets.[newIdx]

	let decrypt (encryptedName : string) sectorId =
	encryptedName
	\|> Seq.map (function
	\| '-' -> ' '
	\| c -> shift c sectorId)
	\|> fun chars -> new String(chars \|> Seq.toArray)

view raw Day04_03.fsx hosted with

by GitHub

I found the question “where North Pole objects are stored” ambigious. After a few failed attempts to guess what the room name should be I ended up just printing them all out and did a quick search for “north“. I’m not sure if the author of the challenge expected you to do that, or maybe I missed a clue somewhere..

Anyhow, once I figured out what the room name is, getting the sector ID was easy.

	let part2 =
	realRooms
	\|> Seq.map (fun (encryptedName, sectorId, _) ->
	decrypt encryptedName sectorId, sectorId)
	\|> Seq.filter (fst >> (=) "northpole object storage")
	\|> Seq.head
	\|> snd

view raw Day04_04.fsx hosted with

by GitHub

Advent of Code F# – Day 4

Day 4

Part 2

Links

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