2022/06/Main.idr

import Data.Vect
import Data.List
import Data.Fin
import System.File.ReadWrite

-- Get windows of a specific size over the data
windows : {n : Nat} -> List a -> List (Vect n a)
windows [] = []
windows {n} xs =
  let (head, _) = splitAt n xs
      tail = drop 1 xs
  in case toVect n head of
      Nothing => []
      (Just x) => x :: windows tail

findStart : Nat -> List Char -> Maybe Nat
findStart n cs =
  let segments = windows {n} cs
  in case findIndex (\x => let ( p ** _ ) = nub x in p == n) segments of
       Nothing => Nothing
       (Just x) => Just $ finToNat x + n

main : IO ()
main =
  do file <- readFile "input"
     case file of
       Right contents =>
         do putStr "Part 1: "
            putStrLn $ case findStart 4 (unpack contents) of
                         Just start => show start
                         Nothing => "Failed to find start"
            putStr "Part 2: "
            putStrLn $ case findStart 14 (unpack contents) of
                         Just start => show start
                         Nothing => "Failed to find start"
       Left err => printLn err