Done day 13

[advent-of-code-20.git] / advent13 / src / advent13.hs

diff --git a/advent13/src/advent13.hs b/advent13/src/advent13.hs
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+-- import Debug.Trace
+
+import Data.Text (Text)
+-- import qualified Data.Text as T
+import qualified Data.Text.IO as TIO
+
+import Data.Attoparsec.Text
+-- import Data.Attoparsec.Combinator
+import Control.Applicative
+import Data.Maybe
+import Data.List
+import Math.NumberTheory.Moduli.Chinese
+
+
+
+main :: IO ()
+main = 
+  do  text <- TIO.readFile "data/advent13.txt"
+      let (timestamp, busses) = successfulParse text
+      print timestamp
+      print busses
+      print $ part1 timestamp busses
+      print $ part2 busses
+
+part1 timestamp maybeBusses = eBus * eTime
+  where busses = catMaybes maybeBusses
+        busDepartures = map (busAndTime timestamp) busses
+        (eBus, eTime) = head $ sortOn snd busDepartures
+
+part2 maybeBusses = b - a
+  where (a, b) = earliestGroup $ periodOffsets maybeBusses
+
+
+busAndTime timestamp period = (period, earliestDeparture timestamp period)
+
+earliestDeparture timestamp period = period - (timestamp `mod` period)
+
+periodOffsets :: [Maybe Integer] -> [(Integer, Integer)]
+periodOffsets maybeBusses = offsetBusses
+  where offsetMaybeBusses = zip [0..] maybeBusses
+        offsetJustBusses = filter (isJust . snd) offsetMaybeBusses
+        offsetBusses = map (\(o, b) -> (o, fromJust b)) offsetJustBusses
+
+
+earliestGroup :: [(Integer, Integer)] -> (Integer, Integer)
+earliestGroup offsetBusses = foldl1 chineseStep offsetBusses
+
+
+chineseStep (n1, m1) (n2, m2) = (n, m1 * m2)
+  where n = fromJust $ chinese (n1, m1) (n2, m2)
+
+
+-- Parse the input file
+
+timeBusP = (,) <$> decimal <* endOfLine <*> busPeriodsP
+
+busPeriodsP = busPeriodP `sepBy` (string ",")
+
+busPeriodP = (Just <$> decimal) <|> ("x" *> pure Nothing)
+
+
+successfulParse :: Text -> (Integer, [Maybe Integer])
+successfulParse input = 
+  case parseOnly timeBusP input of
+    Left  _err -> (0, []) -- TIO.putStr $ T.pack $ parseErrorPretty err
+    Right timeBus -> timeBus