adventofcode-2020/day2/main.hs

#! /usr/bin/env -S"ANSWER=42" nix-shell
#! nix-shell -p ghcid
#! nix-shell -p "haskellPackages.ghcWithPackages (p: with p; [attoparsec])"
#! nix-shell -i "ghcid -c 'ghci -Wall' -T main"

{-# LANGUAGE OverloadedStrings #-}

import Data.Attoparsec.Text
import Data.Text (pack)
import Data.Maybe (catMaybes)
import Data.Bits (xor)

testData :: [String]
testData = [ "1-3 a: abcde"
           , "1-3 b: cdefg"
           , "2-9 c: ccccccccc"
           ]

data Input = Input {
  atLeast :: Int
, notMore :: Int
, character :: Char
, password :: String
} deriving Show

inputParser :: Parser Input
inputParser = do
  atLeast'    <- decimal
  _           <- char '-'
  notMore'    <- decimal
  _           <- space
  character'  <- letter
  _           <- char ':'
  _           <- space
  password'   <- many1 letter
  return $ Input atLeast' notMore' character' password'

parseInputLine :: String -> Maybe Input
parseInputLine s = eitherToMaybe $ parseOnly inputParser (pack s)

eitherToMaybe :: Either a b -> Maybe b
eitherToMaybe (Right e) = Just e
eitherToMaybe (Left _) = Nothing

match' :: Char -> Char -> Int
match' c1 c2 | c1 == c2 = 1
match' _ _ = 0

-- How many passwords are valid according to their policies?

validatePassword1 :: Input -> Bool
validatePassword1 (Input min' max' char' pass) =
  c >= min' && c <= max'
  where
    c = foldl (\acc c' -> acc + match' char' c') 0 pass

-- How many passwords are valid according to the new interpretation of the
-- policies?

validatePassword2 :: Input -> Bool
validatePassword2 (Input pos1 pos2 char' pass) =
  (pass !! (pos1-1) == char') `xor` (pass !! (pos2-1) == char')

validateLine :: (Input -> Bool) -> String -> Maybe Bool
validateLine f s = do
  i <- parseInputLine s
  pure $ f i

countTrue :: [Bool] -> Int
countTrue = length . filter (== True)

solveDay2Part1 :: [String] -> Int
solveDay2Part1 = countTrue . catMaybes . (map (validateLine validatePassword1))

solveDay2Part2 :: [String] -> Int
solveDay2Part2 = countTrue . catMaybes . (map (validateLine validatePassword2))

main :: IO ()
main = do
  putStrLn "Day 2 - Part 1"
  inputData <- readFile "./day2/input"
  print $ solveDay2Part1 testData
  print $ solveDay2Part1 (lines inputData)
  print $ solveDay2Part2 testData
  print $ solveDay2Part2 (lines inputData)