1 files changed, 34 insertions, 0 deletions
diff --git a/2020/10b.hs b/2020/10b.hs
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+++ b/2020/10b.hs
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+module Main where
+
+import System.Environment (getArgs)
+import Data.List (sort)
+import qualified Data.IntMap.Strict as M
+
+-- Delete elements from a list until element t is encountered
+deleteUntil :: Eq a => [a] -> a -> [a]
+deleteUntil [] _ = []
+deleteUntil (x:xs) t | x == t = xs
+                     | otherwise = deleteUntil xs t
+
+-- Get the number of posisble valid permutations of a list of adapters
+possiblePerms :: [Int] -> M.IntMap Int
+possiblePerms ss = foldl addToMap (M.fromList [(0, 1)]) (sort ss ++ [end])
+            where getChildren x m = [k | k <- [x-3..x-1], k `M.member` m]
+                  end = maximum ss + 3
+                  addToMap m x = M.insert x (sum $ map (m M.!) (getChildren x m)) m
+
+
+-- Read a line-seperated file of numbers
+numsFromFile :: String -> IO [Int]
+numsFromFile p = do
+                    c <- readFile p;
+                    return $ map read $ lines c; 
+
+
+main :: IO ()
+main = do 
+        args <- getArgs;
+        xs <- numsFromFile $ head args;
+
+        print $ possiblePerms xs;
+        return ();