Day 06.

README.md

@@ -11,9 +11,9 @@ To arrive at [⟨ortho|normal⟩](https://hilb.ert.space) as December comes.
 | 01  | < 0.1       |
 | 02  | < 0.1       |
 | 03  | ~ 2         |
-| 04  |             |
-| 05  |             |
-| 06  |             |
+| 04  | ~ 1         |
+| 05  | ~ 1         |
+| 06  | ~ 3         |
 | 07  |             |
 | 08  |             |
 | 09  |             |

app/Main.hs

@@ -30,4 +30,4 @@ import qualified Day24
 import qualified Day25
 
 main :: IO ()
-main = Day05.main
+main = Day06.main

input/06.txt

@@ -0,0 +1,50 @@
+77, 279
+216, 187
+72, 301
+183, 82
+57, 170
+46, 335
+55, 89
+71, 114
+313, 358
+82, 88
+78, 136
+339, 314
+156, 281
+260, 288
+125, 249
+150, 130
+210, 271
+190, 258
+73, 287
+187, 332
+283, 353
+66, 158
+108, 97
+237, 278
+243, 160
+61, 52
+353, 107
+260, 184
+234, 321
+181, 270
+104, 84
+290, 109
+193, 342
+43, 294
+134, 211
+50, 129
+92, 112
+309, 130
+291, 170
+89, 204
+186, 177
+286, 302
+188, 145
+40, 52
+254, 292
+270, 287
+238, 216
+299, 184
+141, 264
+117, 129

src/Day06.hs

@@ -1,6 +1,42 @@
 module Day06 (main) where
 
+import Data.List (maximumBy, sortBy)
+import Data.Ord (comparing)
+import Data.Function (on)
+import Data.Map (fromListWith, toAscList)
+import Data.Set (fromList, toList)
+
+-- given a list of elements, return pairs of the element with its frequency
+-- if two elements have the same frequency, the elements are sorted in ascending order
+getElemFreqs :: Ord a => [a] -> [(a, Int)]
+getElemFreqs = sortBy (flip $ comparing snd) . toAscList . fromListWith (+) . (flip zip) (repeat 1)
+
+part1 :: [(Int, Int)] -> Int
+part1 coords =
+    let maxrow = fst $ maximumBy (comparing fst) coords
+        maxcol = snd $ maximumBy (comparing snd) coords
+        grid   = map closest [(r, c) | r <- [1..maxrow], c <- [1..maxcol]]
+        edges  = toList . fromList . map closest . concat $ [[(r, 1), (1, c), (r, maxcol), (maxrow, c)] | r <- [1..maxrow], c <- [1..maxcol]]
+    in  snd . head . filter ((`notElem` edges) . fst) . getElemFreqs $ grid
+    where
+        closest coord =
+            let (car:cadr:_) = sortBy (comparing (manhattan coord)) coords
+            in  if   ((==) `on` manhattan coord) car cadr
+                then ((-1), (-1))
+                else car
+        manhattan (r1, c1) (r2, c2) = abs (r1 - r2) + abs (c1 - c2)
+
+part2 :: [(Int, Int)] -> Int
+part2 coords =
+    let maxrow = fst $ maximumBy (comparing fst) coords
+        maxcol = snd $ maximumBy (comparing snd) coords
+    in  length . filter closeBy $ [(r, c) | r <- [1..maxrow], c <- [1..maxcol]]
+    where 
+        closeBy coord = (sum $ map (manhattan coord) coords) < 10000
+        manhattan (r1, c1) (r2, c2) = abs (r1 - r2) + abs (c1 - c2)
+
 main :: IO ()
 main = do
-    input <- readFile "input/06.txt"
-    print input
+    input <- map (read . ('(':) . (++ [')'])) . lines <$> readFile "input/06.txt"
+    print $ part1 input
+    print $ part2 input