feat: day 1 part one

src/Day2.hs

@@ -1,18 +1,37 @@
 module Day2 (partOne, partTwo) where
 
-data Error = ParseError String
+import qualified Data.Bifunctor as BF
+import Text.Read (readEither)
+
+newtype Error = ParseError String
   deriving (Show)
 
+type Level = Int
+
+type Report = [Level]
+
 partOne :: String -> Either Error Int
-partOne contents = calculateScore <$> parseInput contents
+partOne contents = length . filter isValidReport <$> parseInput contents
   where
-    calculateScore _ = 0
+    isValidReport xs = isValidReportWith isSafeIncrease xs || isValidReportWith isSafeDecrease xs
+
+    isSafeIncrease x y = (x - y) > 0 && (x - y) < 4
+    isSafeDecrease x y = isSafeIncrease y x
+
+    isValidReportWith validator (x : y : xs) = validator x y && isValidReportWith validator (y : xs)
+    isValidReportWith _ [_] = True
+    isValidReportWith _ [] = False
 
 partTwo :: String -> Either Error Int
 partTwo contents = calculateScore <$> parseInput contents
   where
     calculateScore _ = 0
 
-parseInput :: String -> Either Error [Int]
-parseInput _ = Right []
+parseInput :: String -> Either Error [Report]
+parseInput contents = mapM parseReport (lines contents)
+  where
+    parseReport :: String -> Either Error Report
+    parseReport line = mapM parseLevel (words line)
 
+    parseLevel :: String -> Either Error Level
+    parseLevel word = BF.first (const $ ParseError word) (readEither word :: Either String Level)