Add 2020 day 23 part 1

haskellAoC/inputs/2020/23

@@ -0,0 +1 @@
+712643589

haskellAoC/src/Y2020/Day23.hs

@@ -1,6 +1,42 @@
 module Y2020.Day23 (y20day23) where
 
+import Data.Char
+
+type Setup = [Int]
+
+getDestination :: Int -> [Int] -> [Int] -> Int
+getDestination dest pickup available
+  | dest `elem` pickup    = getDestination (dest - 1) pickup available
+  | dest == 0             = maximum available
+  | dest `elem` available = dest
+  -- | otherwise             = getDestination (dest - 1) pickup available
+
+buildNewSetup :: Int -> [Int] -> Int -> [Int] -> Setup
+buildNewSetup cur pickup dest (h:t)
+  | h == dest  = (h:pickup) ++ t ++ [cur]
+  | otherwise  = h:(buildNewSetup cur pickup dest t)
+buildNewSetup _ _ _ [] = error ("invalid call (empty rest)")
+
+playRound :: Setup -> Setup
+playRound (current:t) = newSetup
+  where pickup      = take 3 t
+        rest        = drop 3 t
+        destination = getDestination (current - 1) pickup rest
+        newSetup    = buildNewSetup current pickup destination rest
+playRound [] = error ("invalid setup (empty)")
+
+getOrderAfter1 :: Setup -> String
+getOrderAfter1 (1:r) = map intToDigit r
+getOrderAfter1 (c:r) = getOrderAfter1 (r ++ [c])
+getOrderAfter1 []    = error ("invalid setup (empty)")
+
+get2after1 :: Setup -> [Int]
+get2after1 (1:r) = take 2 r
+get2after1 (_:r) = get2after1 r
+get2after1 []    = error ("invalid setup (1 is missing ?!)")
+
 y20day23 :: [String] -> (String, String)
-y20day23 input = (part1, part2)
-  where part1  = "WIP"
+y20day23 (input:_) = (part1, part2)
+  where part1   = getOrderAfter1 $ head $ drop 100 $ iterate playRound p1setup
         part2   = "WIP"
+        p1setup = map digitToInt input