day 3

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

day03.hs

@@ -0,0 +1,86 @@
+import Control.Monad (guard)
+import Data.Char (digitToInt, isDigit)
+import Data.Text qualified as T
+import Lib (readFile')
+
+-- i hate this problem, let's just do an ugly recursion and be done with it
+type Symbol = Int
+
+data Number = Number
+  { value :: Int,
+    start :: Int,
+    end :: Int
+  }
+  deriving (Show)
+
+parseLines :: [T.Text] -> ([[Symbol]], [[Number]])
+parseLines lines =
+  foldl
+    ( \(symbols, numbers) line ->
+        let (symbols', numbers') = parseLine line
+         in (symbols' : symbols, numbers' : numbers)
+    )
+    ([], [])
+    lines
+
+-- ugly, i know but runs p fast with -O2
+parseLine :: T.Text -> ([Symbol], [Number])
+parseLine line =
+  let (num, start, end, isNum, symbols, numbers) = T.foldl fn (0, 0, 0, False, [], []) line
+   in (symbols, if isNum then Number num start (end - 1) : numbers else numbers)
+  where
+    fn :: (Int, Int, Int, Bool, [Symbol], [Number]) -> Char -> (Int, Int, Int, Bool, [Symbol], [Number])
+    fn (num, start, end, isNum, symbols, numbers) char =
+      let isNum' = isDigit char
+          num' = if isNum' then num * 10 + digitToInt char else 0
+          start' = if not isNum' then end + 1 else start
+          symbols' =
+            if char /= '.' && not isNum'
+              then end : symbols
+              else symbols
+          numbers' =
+            if isNum && not isNum'
+              then Number num start (end - 1) : numbers
+              else numbers
+       in (num', start', end + 1, isNum', symbols', numbers')
+
+groups3 :: [a] -> [[a]]
+groups3 xs = take 3 xs : groups3 (tail xs)
+
+first :: [[Symbol]] -> [[Number]] -> [[Int]]
+first symbols numbers =
+  let zipped = zip numbers (init $ groups3 ([] : symbols))
+   in do
+        (numbersRow, neighbours) <- zipped
+        return $ do
+          Number value start end <- numbersRow
+          guard $ or $ do
+            neighboursRow <- neighbours
+            return $ or $ do
+              symbol <- neighboursRow
+              return $ symbol >= start - 1 && symbol <= end + 1
+          return value
+
+second :: [[Symbol]] -> [[Number]] -> [Int]
+second symbols numbers =
+  let zipped = zip symbols (init $ groups3 ([] : numbers))
+   in do
+        (symbols, neighbours) <- zipped
+        symbol <- symbols
+        let values = take 2 $ do
+              neighboursRow <- neighbours
+              Number value start end <- neighboursRow
+              guard $ symbol >= start - 1
+              guard $ symbol <= end + 1
+              return value
+        guard $ length values == 2
+        return $ product values
+
+main :: IO ()
+main = do
+  input <- T.lines <$> readFile' "day03.in"
+  let (symbols, numbers) = parseLines input
+  putStr "Q1: "
+  print . sum . concat $ first symbols numbers
+  putStr "Q2: "
+  print . sum $ second symbols numbers