{-# LANGUAGE NumericUnderscores #-}
module Roman (solution) where

solution :: String -> Int
solution = sum . mergeParts . map fromRoman

mergeParts :: [Int] -> [Int]
mergeParts [] = []
mergeParts [n1] = [n1]
mergeParts (n1:n2:rest)
  | n1 < n2 = n2 - n1 : mergeParts rest
  | otherwise = n1 : mergeParts (n2:rest)

fromRoman :: Char -> Int
fromRoman c = case c of
  'I' -> 1
  'V' -> 5
  'X' -> 10
  'L' -> 50
  'C' -> 100
  'D' -> 500
  'M' -> 1_000
  _   -> 0