advent of code day 6

daysix.hs

{-# LANGUAGE OverloadedStrings #-}

module DaySix
    ( signalStart
    ) where
import qualified Data.Maybe as M
import qualified Data.List as L
import Data.Set as S

countDistinct :: (Ord a) => [a] -> Int
countDistinct = S.size . S.fromList

hasNDistinct :: (Ord a) => Int -> [a] -> Bool
hasNDistinct required candidate = (countDistinct candidate) == required

-- stolen from https://github.com/MarkDBlackwell/ruby-methods-haskell/blob/master/RubyMethods/EachCons.hs
batches digits items = L.transpose shortenedLists
  where shortenedLists = L.map dropFromItems [0..digits-1]
        dropFromItems n = L.drop n items

findIndexDangerous predicate items = M.fromJust $ L.findIndex predicate items

signalStart :: (Ord a) => [a] -> Int -> Int
signalStart candidate requiredDigits = requiredDigits + (findIndexDangerous (hasNDistinct requiredDigits) $ batches requiredDigits candidate)