1 files changed, 60 insertions, 0 deletions
diff --git a/2020/02-Password_Philosophy/second.hs b/2020/02-Password_Philosophy/second.hs
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+++ b/2020/02-Password_Philosophy/second.hs
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+-- requires cabal install --lib megaparsec
+module Main (main) where
+
+import Control.Monad (void, when)
+import Data.Void
+import Text.Megaparsec
+import Text.Megaparsec.Char
+import System.Exit (die)
+
+exampleExpectedOutput = 1
+
+data Rule = Rule { lower  :: Int
+                 , higher :: Int
+                 , elt    :: Char
+                 , pass   :: String
+                 } deriving (Show)
+
+type Parser = Parsec Void String
+
+parseRule :: Parser Rule
+parseRule = do
+  l <- try (some digitChar)
+  void (char '-')
+  h <- try (some digitChar)
+  void (char ' ')
+  e <- anySingle
+  void (string ": ")
+  pass <- (many letterChar)
+  void (char '\n')
+  return Rule { lower = (read l) - 1, higher = (read h) - 1, elt = e, pass = pass }
+
+parseRules :: Parser [Rule]
+parseRules = do
+  rules <- many parseRule
+  eof
+  return rules
+
+parseInput :: String -> IO [Rule]
+parseInput filename = do
+  input <- readFile filename
+  case runParser parseRules filename input of
+    Left bundle -> die $ errorBundlePretty bundle
+    Right rules -> return rules
+
+validateRule :: Rule -> Int
+validateRule Rule{lower=l, higher=h, elt=e, pass=pass} = if (a /= b) then 1 else 0
+  where
+    a = e == pass !! l
+    b = e == pass !! h
+
+compute :: [Rule] -> Int
+compute = sum . map validateRule
+
+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