Day 8, part 1

8.hs

@@ -0,0 +1,56 @@
+import Data.HashMap (Map, insert, alter, empty, toList)
+import qualified Data.HashMap as M (lookup)
+import Debug.Trace
+
+data Instruction = I {
+    reg :: String,
+    inc :: Int,
+    src :: String,
+    fnc :: Int -> Bool
+}
+
+getFnc :: String -> (Int -> Int -> Bool)
+getFnc s = case s of 
+    "<"  -> (<)
+    "<=" -> (<=)
+    ">"  -> (>)
+    ">=" -> (>=)
+    "==" -> (==)
+    "!=" -> (/=)
+
+getOrSetZero :: String -> Map String Int -> (Int, Map String Int)
+getOrSetZero s m =
+    case M.lookup s m of
+        Just value -> (value, m)
+        Nothing    -> (0,     insert s 0 m)
+
+addToMaybe :: Int -> Maybe Int -> Maybe Int
+addToMaybe i (Just x) = Just $ i + x
+addToMaybe i Nothing  = Just i
+
+parseLine :: String -> Instruction
+parseLine s =
+    let (register : command : value : _ : source : function : argument : _) = words s
+    in I {
+        reg = register,
+        inc = case command of
+            "inc" ->    read value
+            "dec" -> (- read value),
+        src = source,
+        fnc = flip (getFnc function) $ read argument
+    }
+
+executeInstruction :: Map String Int -> Instruction -> Map String Int
+executeInstruction m (I r i s f) =
+    let (value, newMap) = getOrSetZero s m
+    in  if f value
+        then alter (addToMaybe i) r newMap
+        else newMap
+
+main :: IO ()
+main = do
+    input <- readFile "8.txt"
+    let instructions = map parseLine $ lines input
+        finalMap = foldl executeInstruction empty instructions
+        maxValue = maximum . snd . unzip . toList $ finalMap
+    print $ maxValue