2023-03 in haskell

2023/03-Gear_Ratios/second.hs

@@ -0,0 +1,79 @@
+-- very slow with runghc, use ghc -O3 -o second second.hs
+-- requires cabal install --lib megaparsec parser-combinators
+module Main (main) where
+
+import Control.Applicative.Permutations
+import Control.Monad (void, when)
+import Data.Char qualified as C
+import Data.Either
+import Data.List qualified as L
+import Data.Map qualified as M
+import Data.Maybe
+import Data.Set qualified as S
+import Data.Void (Void)
+import Text.Megaparsec
+import Text.Megaparsec.Char
+
+exampleExpectedOutput = 467835
+
+data Number = Number Int Int Int Int deriving Show -- Value x1 x2 y1
+data Symbol = Symbol Int Int deriving (Eq, Ord, Show) -- x y
+data Input = Input [Number] [Symbol] deriving Show
+
+type Parser = Parsec Void String
+
+parseNumber :: Parser Number
+parseNumber = do
+  (SourcePos _ y' x1') <- getSourcePos
+  value <- read <$> some digitChar
+  (SourcePos _ _ x2') <- getSourcePos
+  let (x1, x2, y) = (unPos x1', unPos x2' - 1, unPos y')
+  return $ Number value x1 x2 y
+
+parseSymbol :: Parser Symbol
+parseSymbol = do
+  (SourcePos _ y' x') <- getSourcePos
+  void $ char '*'
+  let (x, y) = (unPos x', unPos y')
+  return $ Symbol x y
+
+parseNumbersAndSymbols :: Parser (Either Number Symbol)
+parseNumbersAndSymbols = do
+  many $ void (char '.' <|> symbolChar <|> char '#' <|> char '%' <|> char '@' <|> char '-' <|> char '/' <|> char '&') <|> void eol
+  something <- (Left <$> parseNumber) <|> (Right <$> parseSymbol)
+  many $ void (char '.') <|> void eol
+  return something
+
+parseInput' :: Parser Input
+parseInput' = do
+  things <- some parseNumbersAndSymbols <* eof
+  return $ Input (lefts things) (rights things)
+
+parseInput :: String -> IO Input
+parseInput filename = do
+  input <- readFile filename
+  case runParser parseInput' filename input of
+    Left bundle -> error $ errorBundlePretty bundle
+    Right input' -> return input'
+
+compute :: Input -> Int
+compute (Input nums syms) = sum $ map process syms
+  where
+    process :: Symbol -> Int
+    process sym = case L.foldl' process' [] nums of
+      [a, b] -> a * b
+      _ -> 0
+      where
+        process' :: [Int] -> Number -> [Int]
+        process' acc (Number v x1 x2 y') | not . S.null $ S.fromList [Symbol x y|x<-[x1-1..x2+1],y<-[y'-1..y'+1]] `S.intersection` S.singleton sym = v:acc
+                                         | otherwise = acc
+
+main :: IO ()
+main = do
+  example <- parseInput "example"
+  print example
+  let exampleOutput = compute example
+  when  (exampleOutput /= exampleExpectedOutput)  (error $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
+  input <- parseInput "input"
+  print $ compute input
+