1 files changed, 85 insertions, 0 deletions
diff --git a/2020/16-Ticket_Translation/first.hs b/2020/16-Ticket_Translation/first.hs
new file mode 100644
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--- /dev/null
+++ b/2020/16-Ticket_Translation/first.hs
@@ -0,0 +1,85 @@
+-- requires cabal install --lib megaparsec parser-combinators
+module Main (main) where
+import Control.Monad (void, when)
+import Data.List (foldl')
+import Data.Map qualified as M
+import Data.Void (Void)
+import Text.Megaparsec
+import Text.Megaparsec.Char
+import System.Exit (die)
+
+exampleExpectedOutput = 71
+
+data Range = Range { field :: String
+                   , ll :: Int
+                   , lh :: Int
+                   , hl :: Int
+                   , hh :: Int
+                   } deriving (Show)
+type Ticket = [Int]
+data Input = Input { ranges :: [Range]
+                   , myTicket :: Ticket
+                   , nearbyTickets :: [Ticket]
+                   } deriving (Show)
+
+type Parser = Parsec Void String
+
+parseInt :: Parser Int
+parseInt = do
+  n <- some digitChar
+  void $ optional (char ',')
+  return $ read n
+
+parseRange :: Parser Range
+parseRange = do
+  f <- some (letterChar <|> char ' ')
+  void $ string ": "
+  ll <- parseInt
+  void $ char '-'
+  lh <- parseInt
+  void $ string " or "
+  hl <- parseInt
+  void $ char '-'
+  hh <- parseInt
+  void $ char '\n'
+  return $ Range f ll lh hl hh
+
+parseTicket :: Parser Ticket
+parseTicket = some parseInt <* char '\n'
+
+parseInput' :: Parser Input
+parseInput' = do
+  r <- some parseRange
+  void $ string "\nyour ticket:\n"
+  t <- parseTicket
+  void $ string "\nnearby tickets:\n"
+  nts <- some parseTicket
+  void eof
+  return $ Input r t nts
+
+parseInput :: String -> IO Input
+parseInput filename = do
+  input <- readFile filename
+  case runParser parseInput' filename input of
+    Left bundle -> die $ errorBundlePretty bundle
+    Right ops -> return ops
+
+compute :: Input -> Int
+compute (Input ranges ticket nearbyTickets) = foldl' compute' 0 nearbyTickets
+  where
+    compute' :: Int -> Ticket -> Int
+    compute' acc nt = acc + (sum $ map isValid nt)
+    isValid :: Int -> Int
+    isValid n = minimum $ map (isValid' n) ranges
+    isValid' :: Int -> Range -> Int
+    isValid' n (Range{ll, lh, hl, hh})
+      | (n >= ll && n <= lh) || (n >= hl && n <= hh) = 0
+      | otherwise = n
+
+main :: IO ()
+main = do
+  example <- parseInput "example"
+  let exampleOutput = compute example
+  when  (exampleOutput /= exampleExpectedOutput)  (die $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
+  input <- parseInput "input"
+  print $ compute input