-- requires cabal install --lib megaparsec parser-combinators heap vector
module Main (main) where

import           Control.Monad        (void, when)
import qualified Data.List            as L
import           Data.Void            (Void)
import           Text.Megaparsec
import           Text.Megaparsec.Char

exampleExpectedOutput = 31

type Pair = (Int, Int)
type Input = [Pair]

type Parser = Parsec Void String

parseNumber :: Parser Int
parseNumber = read <$> some digitChar <* optional hspace

parsePair :: Parser Pair
parsePair = (,) <$> parseNumber
                <*> parseNumber

parseInput' :: Parser Input
parseInput' = some (parsePair <* eol) <* eof

parseInput :: String -> IO Input
parseInput filename = do
  input <- readFile filename
  case runParser parseInput' filename input of
    Left bundle  -> error $ errorBundlePretty bundle
    Right input' -> return input'

compute :: Input -> Int
compute input = L.foldl' similarity 0 l
  where
    (l, r) = unzip input
    similarity :: Int -> Int -> Int
    similarity acc i = acc + i * (length $ L.elemIndices i r)

main :: IO ()
main = do
  example <- parseInput "example"
  let exampleOutput = compute example
  when  (exampleOutput /= exampleExpectedOutput)  (error $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
  input <- parseInput "input"
  print $ compute input