2015 Day 09 in Haskell

2015/day09/day09.hs

@@ -0,0 +1,28 @@
+import Aoc
+import Data.List ( nub )
+import Data.Tuple ( swap )
+import Control.Applicative ( (<|>) )
+import Data.Maybe ( fromMaybe )
+
+parseFile :: [String] -> [((String, String), Int)]
+parseFile = map (\line ->
+    let [a, "to", b, "=", dist] = words line in
+    ((a, b), read dist))
+
+symLookup :: Eq a => (a, a) -> [((a,a), b)] -> Maybe b
+symLookup a ls = lookup a ls <|> lookup (swap a) ls
+
+process :: [((String, String), Int)] -> [Int]
+process connections = map (totalLength . adjacents) (permutations cities)
+    where 
+        cities = nub $ concatMap (unpair . fst) connections
+        totalLength = sum . map (fromMaybe undefined . (`symLookup` connections))
+
+
+part1 :: [Int] -> Int
+part1 = minimum
+
+part2 :: [Int] -> Int
+part2 = maximum
+
+main = aocMain (process . parseFile) part1 part2

aoc.hs

@@ -19,7 +19,7 @@ const' :: (a -> b) -> a -> c -> b
 const' f a _ = f a
 
 enumerate :: [a] -> [(Int, a)]
-enumerate xs = zip [0..length xs] xs
+enumerate = zip [0..]
 
 unimplemented :: Show p => p -> Int
 unimplemented = error . show
@@ -27,6 +27,9 @@ unimplemented = error . show
 pair :: [a] -> (a, a)
 pair as = let [a1, a2] = as in (a1, a2)
 
+unpair :: (a, a) -> [a]
+unpair (a, b) = [a, b]
+
 triplet :: [a] -> (a, a, a)
 triplet as = let [a1, a2, a3] = as in (a1, a2, a3)
 
@@ -37,4 +40,14 @@ both :: (a -> b) -> (a, a) -> (b, b)
 both f (a, b) = (f a, f b)
 
 lastSplit :: Int -> [a] -> ([a], [a])
-lastSplit n as = splitAt (length as - n) as
+lastSplit n as = splitAt (length as - n) as
+
+remove :: Int -> [a] -> (a, [a])
+remove n xs = let (front, a:rest) = splitAt n xs in (a, front ++ rest)
+
+permutations :: [a] -> [[a]]
+permutations [] = [[]]
+permutations xs = concatMap (\n -> let (a, rest) = remove n xs in map (a:) $ permutations rest) [0..length xs-1]
+
+adjacents :: [a] -> [(a, a)]
+adjacents as = zip as $ tail as