Advent of Code 2017 - Day 1

README.md

The captcha requires you to review a sequence of digits (your puzzle input) and find the sum of all digits that match the next digit in the list. The list is circular, so the digit after the last digit is the first digit in the list.

For example:

1122 produces a sum of 3 (1 + 2) because the first digit (1) matches the second digit and the third digit (2) matches the fourth digit.
1111 produces 4 because each digit (all 1) matches the next.
1234 produces 0 because no digit matches the next.
91212129 produces 9 because the only digit that matches the next one is the last digit, 9.

day1.hs

import Data.Char

main :: IO ()
main = do
    putStrLn "Introduce el captcha:"
    captcha <- getLine :: IO String
    let digits = sumDigits captcha
    putStrLn ("Suma: " ++ (show digits))

sumDigits :: [Char] -> Int
sumDigits [] = 0
sumDigits xs = foldl (+) 0 $ map digitToInt $ repeatedDigits xs

repeatedDigits :: [Char] -> [Char]
repeatedDigits xs = if (head xs) == (last xs) then (head xs):(repeatedDigits' xs) else repeatedDigits' xs
                 
repeatedDigits' :: [Char] -> [Char]
repeatedDigits' [] = []
repeatedDigits' (x:xs) = 
    case length xs of 
        0 -> []
        _ -> if x == (xs !! 0) then
                x:(repeatedDigits' xs)
            else
                repeatedDigits' xs

input.txt

34997744892914653296827871613388552993634935173733597474997393431324121718942484674492133736486619515246829248477836544451943938832848157199224116563715646126431493563772112714741546635764665586452858349326658345524573681224829221829772728531278893357146638772291782796744812479595172578555931968285326741191558735491923682586844185476584124677856856612582263263124715916498254659761312225295947328671873729594182695425852559718922816832816341259695766322357565252335851264933471555351536363944572763621761489944217787785564355131756948331413652646811626742168857634856234347432698931371757454156396432993421795675147273229642441888776517165375965288923515378871773449714189311167849788519479274172617334378412661574885156988171532483385528342851358599792154331889342985168528186562873736117113242271863318873917355428393173152783223727362282169982597123525671895452937118687191281382949335937173323862618172284254741935865963877359477126188879481911148827453781546789437317581568931445259912541273353345254171252588344612386649134562638758915336976347291218848744548755462493981871543949697331735577243658722111371552363179584543521149944247848176793571855164329415143753479297879926959141597695174674386467854776481689314612324534729187335368471697738925271618243312864656442299938886755679996568297498965651652337961837876468596749433454633975722561971935459554979713344313292511447288939379369279487299557326137798219646395436241742751581363752896833892713543627966633788455384129347637693559713174477262914916598991823983686226378396341554219544683439536933338185723832743964258335163993324191589246399535845434167819135413916443764931668386817282279877264296262823999224943974974489892778799656723453849139194948368998995531261224669478559359689167934624681622834931223728318247832134758581882736415334187562342375144693398771223127132562692525629392889723242374746911936313136382354858767169452656224519128287899264831463597663461857119132312578648894815417348364532372836621644176295776978942783714778954864719541832176633892147845693752248565147794357864859961462918847471158244516279178346514129117328285132341339595664283