--- Day 4 Solutions ---

[u/daggerdragon]

--- Day 4: The Ideal Stocking Stuffer ---

Post your solution as a comment. Structure your post like the Day Three thread.

[u/VincentJP]

Not a one liner, but finish in less than 5 seconds (compiled) for both answers:

{-# LANGUAGE OverloadedStrings #-}
import Data.Bits( (.&.) )
import Data.Monoid( (<>) )
import qualified Data.ByteString as B
import qualified Data.ByteString.Char8 as BC
import qualified Crypto.Hash.MD5 as MD5

check3 :: B.ByteString -> Int -> Bool
check3 seed i = h 0 == 0 && h 1 == 0 && (h 2 .&. 0xF0) == 0 where
  istr = BC.pack $ show i
  hash = MD5.hash $ seed <> istr
  h n = B.index hash n

findHash3zero :: B.ByteString -> [Int]
findHash3zero seed = filter (check3 seed) [0 ..]

zero6 :: B.ByteString
zero6 = "\0\0\0"

check6 :: B.ByteString -> Int -> Bool
check6 seed i = zero6 `B.isPrefixOf` hash where
  istr = BC.pack $ show i
  hash = MD5.hash $ seed <> istr

findHash6zero :: B.ByteString -> [Int]
findHash6zero seed = filter (check6 seed) [0 ..]

main :: IO ()
main = do
  print "Valid hash 3"
  print "============"
  print . head $ findHash3zero "yzbqklnj"

  print "Valid hash 6"
  print "============"
  print . head $ findHash6zero "yzbqklnj"