Today’s Advent of Code puzzle is the simplest code of the year to date. And I’m making it longer for posting than what I solved with.

It’s as literate Haskell as usual, but it really won’t take long. First two imports.

import Control.Arrow ((&&&))
import Data.List (findIndex,nub,tails)

We’re looking for a start-of-message marker, defined as w distinct characters. Here’s a function to identify whether we’re looking at that. It works by extracting the w first characters of its input, and checking whether removing duplicates is a no-op.

isMessageStart :: Int -> String -> Bool
isMessageStart w (take w -> pfx) = nub pfx == pfx

Finding the start-of-message end is now a simple matter of iterating, and adjusting the resulting index for prefix length.

locate :: Int -> String -> Maybe Int
locate w = fmap (+ w) . findIndex (isMessageStart w) . tails

A simple wrapper to perform both parts in a single sitting, and we’re done!

main :: IO ()
main = interact $ show . (locate 4 &&& locate 14)

This concludes today’s puzzle. See you tomorrow!