3 files changed, 68 insertions, 0 deletions
diff --git a/2020/23-Crab_Cups/example b/2020/23-Crab_Cups/example
new file mode 100644
index 0000000..ab40847
--- /dev/null
+++ b/2020/23-Crab_Cups/example
@@ -0,0 +1 @@
+389125467
diff --git a/2020/23-Crab_Cups/first.hs b/2020/23-Crab_Cups/first.hs
new file mode 100644
index 0000000..1b3e900
--- /dev/null
+++ b/2020/23-Crab_Cups/first.hs
@@ -0,0 +1,66 @@
+-- requires cabal install --lib megaparsec parser-combinators
+module Main (main) where
+import Control.Monad (void, when)
+import Data.List qualified as L
+import Data.Map qualified as M
+import Data.Maybe (fromJust)
+import Data.Set qualified as S
+import Data.Void (Void)
+import Text.Megaparsec
+import Text.Megaparsec.Char
+import System.Exit (die)
+
+exampleExpectedOutput = 67384529
+
+type Cup = Int
+type Input = [Cup]
+type History = [Input]
+
+type Parser = Parsec Void String
+
+parseCup :: Parser Cup
+parseCup = do
+  n <- digitChar
+  return $ read [n] - 1 -- so that there is a 0 in there and then we can use modulo
+
+parseInput' :: Parser Input
+parseInput' = some parseCup <* optional (char '\n') <* eof
+
+parseInput :: String -> IO Input
+parseInput filename = do
+  input <- readFile filename
+  case runParser parseInput' filename input of
+    Left bundle -> die $ errorBundlePretty bundle
+    Right input' -> return input'
+
+score :: Input -> Int
+score (0:xs) = read . concat $ map (\i -> show (i+1)) xs
+score (x:xs) = score $ xs ++ [x]
+
+compute' :: Input -> Int -> Int
+compute' input 0 = score input
+compute' (current:one:two:three:xs) remainingMoves = compute' step (remainingMoves - 1)
+  where
+    l = length xs + 4
+    destinationIndex :: Int
+    destinationIndex = case L.elemIndex ((current - 1) `mod` l) xs of
+      Just i -> i
+      Nothing -> case L.elemIndex ((current - 2) `mod` l) xs of
+        Just i -> i
+        Nothing -> case L.elemIndex ((current - 3) `mod` l) xs of
+          Just i -> i
+          Nothing -> fromJust $ L.elemIndex ((current - 4) `mod` l) xs
+    destinationIndex' = destinationIndex + 1
+    step :: Input
+    step = concat [take destinationIndex' xs, [one, two, three], drop destinationIndex' xs, [current]]
+
+compute :: Input -> Int
+compute input = compute' input 100
+
+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
diff --git a/2020/23-Crab_Cups/input b/2020/23-Crab_Cups/input
new file mode 100644
index 0000000..f69e63f
--- /dev/null
+++ b/2020/23-Crab_Cups/input
@@ -0,0 +1 @@
+963275481