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+module Main where
+
+import System.Environment (getArgs)
+import Text.Printf
+import qualified Data.IntMap.Strict as Map
+
+-- This does the same thing as 15a, it's just done in a more memory-optimised way.
+
+-- Parse a string, with one number per line
+parseInput :: String -> [Int]
+parseInput = map read . lines
+
+-- Parse a file given the path
+-- Returns list of instructions
+parseFromFile :: String -> IO [Int]
+parseFromFile s = do 
+                   contents <- readFile s;
+                   return $ parseInput contents;
+
+-- Generate a map from the list of initial numbers
+mapFromInitial :: [Int] -> Map.IntMap Int
+mapFromInitial xs = foldr (\(i,x) m -> Map.insert x i m) Map.empty (zip [0..] xs)
+
+-- Run till the given turn, this time using a map to store last occurences.
+-- This means we don't get the full 'log', just the final turn.
+-- map, prev, current index, stopping len
+runTillTurn :: Map.IntMap Int -> Int -> Int -> Int -> Int
+runTillTurn occ prev i n | i >= n = prev
+                           | otherwise = runTillTurn occ' curr (i + 1) n
+                            where curr = case Map.lookup prev occ of 
+                                    Nothing -> 0
+                                    Just x -> (i - 1) - x
+                                  occ' = Map.insert prev (i - 1) occ
+
+-- runghc 15b.hs inputs/day15
+main :: IO ()
+main = do 
+        args <- getArgs;
+        ns <- parseFromFile (head args);
+        let x = runTillTurn (mapFromInitial ns) (last ns) (length ns) 30000000;
+
+        printf "Answer = %d\n" x :: IO ();
+
+        return ();