Day 17 - it's a lot faster in Javascript...

.gitignore

@@ -1,4 +1,5 @@
 *
 !.gitignore
 !*.hs
+!*.js
 !*.txt

17.hs

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+{-# LANGUAGE BangPatterns #-}
+import Data.List (foldl')
+(%) = mod
+
+ith :: Int -> Int -> Int
+ith i steps =
+    let (position, list) = foldl' (\(currPos, currList) n -> 
+            let newPos  = (currPos + steps % n + 1) % n
+            in  (newPos, take newPos currList ++ [n] ++ drop newPos currList))
+            (0, [0]) [1..i]
+    in  list !! (position + 1)
+
+oneth :: Int -> Int -> Int
+oneth i steps =
+    snd $ foldl' (\(currPos, currOneth) n -> 
+        let newPos = (currPos + steps % n + 1) % n
+        in  (newPos, if newPos == 0 then n else currOneth)) 
+        (0, 1) [1..i]
+
+main :: IO ()
+main = do
+    print $ ith   2017     312
+    print $ oneth 50000000 312 

17.js

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+p = 0;
+arr = [0];
+for (n = 1; n <= 2017; n++) {
+    p = (p + 312 % n + 1) % n;
+    arr.splice(p, 0, n);
+}
+console.log(arr[arr.indexOf(2017) + 1]);
+
+p = 0;
+oneth = 1;
+for (n = 1; n <= 50000000; n++) {
+    p = (p + 312 % n + 1) % n;
+    if (p == 0) oneth = n;
+}
+console.log(oneth);