From 7b2828487e722abee9cb15e65eeed9043fa53a2b Mon Sep 17 00:00:00 2001
From: Eric Mertens <emertens@gmail.com>
Date: Sun, 15 Dec 2019 22:00:58 -0800
Subject: [PATCH] Day 16

---
 advent2019.cabal   |  6 ++++++
 execs/Day16.hs     | 54 ++++++++++++++++++++++++++++++++++++++++++++++
 inputs/input16.txt |  1 +
 3 files changed, 61 insertions(+)
 create mode 100644 execs/Day16.hs
 create mode 100644 inputs/input16.txt

diff --git a/advent2019.cabal b/advent2019.cabal
index 4813668..e8a5216 100644
--- a/advent2019.cabal
+++ b/advent2019.cabal
@@ -145,3 +145,9 @@ executable Day15
   build-depends:       advent2019, base, containers
   hs-source-dirs:      execs
   default-language:    Haskell2010
+
+executable Day16
+  main-is:             Day16.hs
+  build-depends:       advent2019, base, containers, vector
+  hs-source-dirs:      execs
+  default-language:    Haskell2010
diff --git a/execs/Day16.hs b/execs/Day16.hs
new file mode 100644
index 0000000..b26aa98
--- /dev/null
+++ b/execs/Day16.hs
@@ -0,0 +1,54 @@
+{-|
+Module      : Main
+Description : Day 16 solution
+Copyright   : (c) Eric Mertens, 2019
+License     : ISC
+Maintainer  : emertens@gmail.com
+
+<https://adventofcode.com/2019/day/16>
+
+This code is not fast but if you let it run it will finish.
+I still want to figure out a more efficient solution.
+
+-}
+module Main (main) where
+
+import           Advent
+import           Control.Applicative
+import qualified Data.Vector.Unboxed as V
+
+main :: IO ()
+main =
+  do [inp] <- getParsedLines 16 (many anySingle)
+     let ns = digits inp
+
+     putStrLn $ concatMap show $ V.toList $ V.take 8 $ iterate fft ns !! 100
+
+     let offset = read (take 7 inp)
+
+     let ns' = V.concat (replicate 10000 ns)
+     putStrLn $ concatMap show $ V.toList $ V.take 8 $ V.drop offset $ iterate fft ns' !! 100
+
+digits :: String -> V.Vector Int
+digits = V.fromList . map (read . pure)
+
+fft :: V.Vector Int -> V.Vector Int
+fft xs = V.generate (V.length xs) one
+  where
+    n = V.length xs
+    ps = V.scanl (+) 0 xs
+    factors i = takeWhile (\(Region _ a _) -> a < n) (regions i)
+
+    one i = abs $ sum [ m * (ps V.! min n end - ps V.! start)     | Region m start end <- factors i]
+               `rem` 10
+
+data Region = Region !Int !Int !Int
+  deriving Show
+
+regions :: Int -> [Region]
+regions i = go 0
+  where
+    n = i + 1
+    go offset = Region 1 (offset + i) (offset + i+n)
+              : Region (-1) (offset + i + n*2) (offset + i+n*2+ n)
+              : go (offset + 4 * n)
diff --git a/inputs/input16.txt b/inputs/input16.txt
new file mode 100644
index 0000000..11bbe52
--- /dev/null
+++ b/inputs/input16.txt
@@ -0,0 +1 @@
+59782619540402316074783022180346847593683757122943307667976220344797950034514416918778776585040527955353805734321825495534399127207245390950629733658814914072657145711801385002282630494752854444244301169223921275844497892361271504096167480707096198155369207586705067956112600088460634830206233130995298022405587358756907593027694240400890003211841796487770173357003673931768403098808243977129249867076581200289745279553289300165042557391962340424462139799923966162395369050372874851854914571896058891964384077773019120993386024960845623120768409036628948085303152029722788889436708810209513982988162590896085150414396795104755977641352501522955134675