day 6

day6/first.hs

@@ -0,0 +1,18 @@
+wordsWhen :: (a -> Bool) -> [a] -> [[a]]
+wordsWhen pred s =
+  case dropWhile pred s of
+    [] -> []
+    s' -> w : wordsWhen pred s''
+      where (w, s'') = break pred s'
+
+parseFish :: String -> [Int]
+parseFish = map read . wordsWhen (==',')
+
+simulate :: Int -> [Int] -> Int
+simulate 0 fish = length fish
+simulate days fish =
+  simulate (days-1) $ concatMap (\f -> if f == 0
+                                       then [6,8]
+                                       else [f-1]) fish
+
+main = interact $ show . simulate 80 . parseFish

day6/input

@@ -0,0 +1 @@
+5,1,4,1,5,1,1,5,4,4,4,4,5,1,2,2,1,3,4,1,1,5,1,5,2,2,2,2,1,4,2,4,3,3,3,3,1,1,1,4,3,4,3,1,2,1,5,1,1,4,3,3,1,5,3,4,1,1,3,5,2,4,1,5,3,3,5,4,2,2,3,2,1,1,4,1,2,4,4,2,1,4,3,3,4,4,5,3,4,5,1,1,3,2,5,1,5,1,1,5,2,1,1,4,3,2,5,2,1,1,4,1,5,5,3,4,1,5,4,5,3,1,1,1,4,5,3,1,1,1,5,3,3,5,1,4,1,1,3,2,4,1,3,1,4,5,5,1,4,4,4,2,2,5,5,5,5,5,1,2,3,1,1,2,2,2,2,4,4,1,5,4,5,2,1,2,5,4,4,3,2,1,5,1,4,5,1,4,3,4,1,3,1,5,5,3,1,1,5,1,1,1,2,1,2,2,1,4,3,2,4,4,4,3,1,1,1,5,5,5,3,2,5,2,1,1,5,4,1,2,1,1,1,1,1,2,1,1,4,2,1,3,4,2,3,1,2,2,3,3,4,3,5,4,1,3,1,1,1,2,5,2,4,5,2,3,3,2,1,2,1,1,2,5,3,1,5,2,2,5,1,3,3,2,5,1,3,1,1,3,1,1,2,2,2,3,1,1,4,2

day6/second.hs

@@ -0,0 +1,32 @@
+import Data.List
+
+wordsWhen :: (a -> Bool) -> [a] -> [[a]]
+wordsWhen pred s =
+  case dropWhile pred s of
+    [] -> []
+    s' -> w : wordsWhen pred s''
+      where (w, s'') = break pred s'
+
+replaceNth :: Int -> a -> [a] -> [a]
+replaceNth _ _ [] = error "too small index"
+replaceNth 0 y (x:xs) = y : xs
+replaceNth n y (x:xs) = x : replaceNth (n-1) y xs
+
+parseFish :: String -> [Integer]
+parseFish = flip (++) [0,0,0]
+  . (:) 0
+  . map (toInteger . length)
+  . group
+  . sort
+  . map (read :: String -> Integer)
+  . wordsWhen (==',')
+
+simulate :: Integer -> [Integer] -> Integer
+simulate 0 fish = foldr1 (+) fish
+simulate days fish =
+  simulate (days-1) newFish
+  where zeros = head fish
+        withEights = (tail fish) ++ [zeros]
+        newFish = replaceNth 6 (zeros + withEights !! 6) withEights
+
+main = interact $ show . simulate 256 . parseFish