day 3

Signed-off-by: Amneesh Singh <natto@weirdnatto.in>

src/Day3.hs

@@ -0,0 +1,52 @@
+module Main where
+
+import qualified AoC as A (extract)
+import Data.Maybe (catMaybes)
+import Text.Parsec (anyChar, char, digit, many1, parse, string, try, (<|>))
+import Text.Parsec.String (Parser)
+
+type Pair = (Int, Int)
+
+data Instruction = Multiply Pair | Do | Dont deriving (Show, Eq)
+
+parseMul :: Parser Pair
+parseMul =
+  string "mul("
+    *> ( (,)
+           <$> (read <$> many1 digit)
+           <*> (char ',' *> (read <$> many1 digit) <* char ')')
+       )
+
+parseIns :: Parser [Maybe Instruction]
+parseIns =
+  many1
+    ( try (Just . Multiply <$> parseMul)
+        <|> (try (string "do()") >> return (Just Do))
+        <|> (try (string "don't()") >> return (Just Dont))
+        <|> (anyChar >> return Nothing)
+    )
+
+part1 :: [Instruction] -> Int
+part1 = sum . map (\case Multiply (a, b) -> a * b; _ -> 0)
+
+part2 :: [Instruction] -> Int
+part2 =
+  fst
+    . foldl
+      ( \(s, e) ins ->
+          case ins of
+            Multiply (a, b) | e -> (s + a * b, e)
+            Do -> (s, True)
+            Dont -> (s, False)
+            _ -> (s, e)
+      )
+      (0, True) -- (sum, enabled)
+
+main :: IO ()
+main =
+  do
+    raw <- readFile "./inputs/day3.in"
+    -- parse the input
+    let instructions = catMaybes . A.extract $ parse parseIns "" raw
+    putStr "Part 1: " >> print (part1 instructions)
+    putStr "Part 2: " >> print (part2 instructions)