Print the final position for reference

hs/src/Day14.hs

@@ -7,13 +7,15 @@ module Day14 (part1, part1', part2) where
 
 import Common (groupConsecutiveBy)
 import Control.Monad (join, liftM2)
+import Data.Char (intToDigit)
 import Data.Map qualified as Map (findWithDefault)
 import Data.Map.Strict qualified as Map (fromListWith)
 import Data.Ord (Down (Down))
 import Data.Set qualified as Set (fromList, toList)
 import Data.String (IsString)
 import Data.Text (Text)
 import Data.Void (Void)
+import Debug.Trace (traceM)
 import Text.Megaparsec (MonadParsec, ParseErrorBundle, Stream (Token, Tokens), parse, sepEndBy1)
 import Text.Megaparsec.Char (char, newline, string)
 import Text.Megaparsec.Char.Lexer qualified as L (decimal, signed)
@@ -44,16 +46,30 @@ part1' width height input = do
 part2 :: Text -> Either (ParseErrorBundle Text Void) Int
 part2 input = do
   robots <- parse parser "" input
-  pure . snd . minimum $
-    [ (Down $ maximum $ map length verticalLines, t)
-    | t <- [0 .. lcm width height - 1],
-      let verticalLines =
-            groupConsecutiveBy isLine . Set.toList . Set.fromList $
-              [ ((y0 + vy * t) `mod` height, (x0 + vx * t) `mod` width)
-              | ((x0, y0), (vx, vy)) <- robots
-              ]
-          isLine (y0, x0) (y1, x1) = y0 == y1 && x0 + 1 == x1
-    ]
+  let (_, bestTime) =
+        minimum
+          [ (Down $ maximum $ map length verticalLines, t)
+          | t <- [0 .. lcm width height - 1],
+            let verticalLines =
+                  groupConsecutiveBy isLine . Set.toList . Set.fromList $
+                    [ ((y0 + vy * t) `mod` height, (x0 + vx * t) `mod` width)
+                    | ((x0, y0), (vx, vy)) <- robots
+                    ]
+                isLine (y0, x0) (y1, x1) = y0 == y1 && x0 + 1 == x1
+          ]
+      positions =
+        Map.fromListWith (+) $
+          [ (((x0 + vx * bestTime) `mod` width, (y0 + vy * bestTime) `mod` height), 1)
+          | ((x0, y0), (vx, vy)) <- robots
+          ]
+      line y =
+        [ case Map.findWithDefault 0 (x, y) positions of
+            0 -> '.'
+            n -> if n < 10 then intToDigit n else '+'
+        | x <- [0 .. width - 1]
+        ]
+  mapM_ (traceM . line) [0 .. height - 1]
+  pure bestTime
   where
     width = 101
     height = 103