1 files changed, 80 insertions, 0 deletions
diff --git a/2020/07-Handy_Haversacks/second.hs b/2020/07-Handy_Haversacks/second.hs
new file mode 100644
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--- /dev/null
+++ b/2020/07-Handy_Haversacks/second.hs
@@ -0,0 +1,80 @@
+-- requires cabal install --lib megaparsec parser-combinators
+module Main (main) where
+import Control.Monad (void, when)
+import Data.List (foldl')
+import qualified Data.Map as M
+import Data.Void (Void)
+import Text.Megaparsec
+import Text.Megaparsec.Char
+import System.Exit (die)
+
+exampleExpectedOutput = 32
+exampleExpectedOutput2 = 126
+
+data Bag = Bag { name :: String
+               , contents :: [(Int, String)]
+               } deriving (Show)
+
+type Parser = Parsec Void String
+
+parseName :: Parser String
+parseName = do
+  adjective <- some letterChar
+  void (char ' ')
+  color <- some letterChar
+  return $ adjective ++ " " ++ color
+
+parseInt :: Parser Int
+parseInt = do
+  i <- some digitChar
+  return $ read i
+
+parseContent :: Parser (Int, String)
+parseContent = do
+  i <- parseInt
+  void (char ' ')
+  name <- parseName
+  void (string " bag")
+  void (optional $ char 's')
+  void (string ", " <|> string ".")
+  return (i, name)
+
+parseContents :: Parser [(Int, String)]
+parseContents = some parseContent <|> (string "no other bags." *> return [])
+
+parseBag :: Parser Bag
+parseBag = do
+  name <- parseName
+  void $ string " bags contain "
+  contents <- parseContents
+  void (char '\n')
+  return $ Bag { name=name, contents=contents }
+
+parseBags :: Parser [Bag]
+parseBags = some parseBag <* eof
+
+parseInput :: String -> IO [Bag]
+parseInput filename = do
+  input <- readFile filename
+  case runParser parseBags filename input of
+    Left bundle -> die $ errorBundlePretty bundle
+    Right bags -> return bags
+
+compute :: [Bag]-> Int
+compute bags = (compute' "shiny gold") - 1
+  where
+    bagsMap :: M.Map String Bag
+    bagsMap = foldl' (\acc b -> M.insert (name b) b acc) M.empty bags
+    compute' :: String -> Int
+    compute' s = foldl' (\acc (i, s') -> acc + i * (compute' s')) 1 (contents $ bagsMap M.! s)
+
+main :: IO ()
+main = do
+  example <- parseInput "example"
+  let exampleOutput = compute example
+  when  (exampleOutput /= exampleExpectedOutput)  (die $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
+  example2 <- parseInput "example2"
+  let exampleOutput2 = compute example2
+  when  (exampleOutput2 /= exampleExpectedOutput2)  (die $ "example2 failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
+  input <- parseInput "input"
+  print $ compute input