Advent of Code 2021
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module Day8_1 where
import System.IO (openFile, IOMode (ReadMode), hClose, hGetContents)
import Data.Text (splitOn, unpack, pack)
import Prelude hiding (splitAt)
import Data.List (sort, (\\))
splitAt :: String -> String -> [String]
splitAt delimiter str = map unpack $ splitOn (pack delimiter) $ pack str
decode :: [String] -> [(String, Int)]
decode nums = zip (head <$> [zero, one, two, three, four, five, six, seven, eight, nine]) [0..9]
where
one = withLength 2
two = filter (not . all (`elem` head nine)) $ withLength 5
three = withLength 5 \\ [head two, head five]
four = withLength 4
five = filter (all (`elem` head six)) $ withLength 5
six = filter (\x -> not $ all (`elem` x) (head seven)) $ withLength 6
seven = withLength 3
eight = withLength 7
nine = filter (\x -> all (`elem` x) (head four)) $ withLength 6
zero = withLength 6 \\ [head nine, head six]
--
withLength y = filter ((== y) . length) nums
decodeDigit :: [(String, Int)] -> String -> Int
decodeDigit [] d = error "oh no"
decodeDigit (x:xs) d
| fst x == d = snd x
| otherwise = decodeDigit xs d
applyDecode :: [(String, Int)] -> [String] -> String
applyDecode codes = concatMap (show . decodeDigit codes)
main :: IO ()
main = do
input <- openFile "inputs/8.txt" ReadMode
contents <- hGetContents input
let patterns = map (map sort . splitAt " ") . splitAt " | " <$> lines contents
let pairs = zip (head <$> patterns) ((!! 1) <$> patterns)
print $ sum (read <$> map (\x -> applyDecode (decode $ fst x) (snd x)) pairs :: [Int])
hClose input