1 files changed, 55 insertions, 0 deletions
diff --git a/2024/02-Red-Nosed_Reports/second.hs b/2024/02-Red-Nosed_Reports/second.hs
new file mode 100644
index 0000000..a9dfd4e
--- /dev/null
+++ b/2024/02-Red-Nosed_Reports/second.hs
@@ -0,0 +1,55 @@
+-- requires cabal install --lib megaparsec parser-combinators heap vector
+module Main (main) where
+
+import           Control.Monad        (void, when)
+import qualified Data.List            as L
+import           Data.Void            (Void)
+import           Text.Megaparsec
+import           Text.Megaparsec.Char
+
+import           Debug.Trace
+
+exampleExpectedOutput = 4
+
+type Report = [Int]
+type Input = [Report]
+
+type Parser = Parsec Void String
+
+parseNumber :: Parser Int
+parseNumber = read <$> some digitChar <* optional hspace
+
+parseReport :: Parser Report
+parseReport = some parseNumber
+
+parseInput' :: Parser Input
+parseInput' = some (parseReport <* eol) <* eof
+
+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 = length . filter id $ map (any (valid . differences) . candidates) input
+  where
+    candidates :: Report -> [Report]
+    candidates report = report : [splitThis i|i<-[1..length report]]
+      where
+        splitThis i = let (l, r) = L.splitAt i report
+                      in init l ++ r
+    differences report = zipWith (-) (init report) (tail report)
+    valid :: Report -> Bool
+    valid rs@(r:_) | r > 0 = all (\v -> v > 0 && v <= 3) rs
+                   | r < 0 = all (\v -> v < 0 && v >= (-3)) rs
+                   | otherwise = False
+
+main :: IO ()
+main = do
+  example <- parseInput "example"
+  let exampleOutput = compute example
+  when  (exampleOutput /= exampleExpectedOutput)  (error $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
+  input <- parseInput "input"
+  print $ compute input