From 45cb3b76ba231e0bc98e0e1358f3ab6cbf5674e8 Mon Sep 17 00:00:00 2001
From: Julien Dessaux <julien.dessaux@adyxax.org>
Date: Mon, 16 Jun 2025 00:17:43 +0200
Subject: [PATCH] 2024-20 part 2 in haskell

---
 2024/20-Race_Condition/first.hs  | 18 ++++---
 2024/20-Race_Condition/second.hs | 86 ++++++++++++++++++++++++++++++++
 2 files changed, 96 insertions(+), 8 deletions(-)
 create mode 100644 2024/20-Race_Condition/second.hs

diff --git a/2024/20-Race_Condition/first.hs b/2024/20-Race_Condition/first.hs
index b767165..8b2327a 100644
--- a/2024/20-Race_Condition/first.hs
+++ b/2024/20-Race_Condition/first.hs
@@ -49,15 +49,17 @@ type Visited = M.Map Coord Int
 type Cost = Int
 
 compute :: Input -> Int -> Int
-compute input minGains = trace (show findAllShortcutCosts) $ length $ filter (>= minGains) findAllShortcutCosts
+compute input minGain = findAllShortcut
   where
-    findAllShortcutCosts :: [Int]
-    findAllShortcutCosts = M.foldrWithKey findShortcutCostsFrom [] costs
-    findShortcutCostsFrom :: Coord -> Int -> [Int] -> [Int]
-    findShortcutCostsFrom (x, y) c acc = acc ++ catMaybes [ if and [input V.! y V.! (x+1) == Wall, M.member (x+2, y) costs] then Just (costs M.! (x+2, y) - c - 2) else Nothing
-                                                          , if and [input V.! y V.! (x-1) == Wall, M.member (x-2, y) costs] then Just (costs M.! (x-2, y) - c - 2) else Nothing
-                                                          , if and [input V.! (y+1) V.! x == Wall, M.member (x, y+2) costs] then Just (costs M.! (x, y+2) - c - 2) else Nothing
-                                                          , if and [input V.! (y-1) V.! x == Wall, M.member (x, y-2) costs] then Just (costs M.! (x, y-2) - c - 2) else Nothing]
+    findAllShortcut :: Int
+    findAllShortcut = M.foldrWithKey findShortcutFrom 0 costs
+    findShortcutFrom :: Coord -> Int -> Int -> Int
+    findShortcutFrom (x, y) c acc = M.foldrWithKey validShortcut acc candidates
+      where
+        candidates = M.filterWithKey (\(x', y') _ -> (abs $ x - x') + (abs $ y - y') <= 2) costs
+        validShortcut :: Coord -> Int -> Int -> Int
+        validShortcut (x', y') c' acc' = let gain = c' - c - (abs $ x - x') - (abs $ y - y')
+                                         in if gain >= minGain then acc' + 1 else acc'
     (costs, len) = walk M.empty 0 (sx, sy)
       where
         walk :: Visited -> Int -> Coord -> (Visited, Int)
diff --git a/2024/20-Race_Condition/second.hs b/2024/20-Race_Condition/second.hs
new file mode 100644
index 0000000..34634d2
--- /dev/null
+++ b/2024/20-Race_Condition/second.hs
@@ -0,0 +1,86 @@
+-- requires cabal install --lib megaparsec parser-combinators heap vector
+module Main (main) where
+
+import           Control.Monad        (void, when)
+import           Data.Functor
+import qualified Data.List            as L
+import qualified Data.Map             as M
+import           Data.Maybe
+import qualified Data.Vector          as V
+import           Data.Void            (Void)
+import           Text.Megaparsec
+import           Text.Megaparsec.Char
+
+import           Debug.Trace
+
+exampleExpectedOutput = 3
+
+data Tile = Wall | Floor | Start | End deriving (Eq, Show)
+type Line = V.Vector Tile
+type Input = V.Vector Line
+
+type Parser = Parsec Void String
+
+parseTile :: Parser Tile
+parseTile = char '#' $> Wall
+        <|> char '.' $> Floor
+        <|> char 'E' $> End
+        <|> char 'S' $> Start
+
+parseLine :: Parser Line
+parseLine = do
+  line <- some parseTile <* eol
+  return $ V.generate (length line) (line !!)
+
+parseInput' :: Parser Input
+parseInput' = do
+  line <- some parseLine <* eof
+  return $ V.generate (length line) (line !!)
+
+parseInput :: String -> IO Input
+parseInput filename = do
+  input <- readFile filename
+  case runParser parseInput' filename input of
+    Left bundle  -> error $ errorBundlePretty bundle
+    Right input' -> return input'
+
+type Coord = (Int, Int)
+type Visited = M.Map Coord Int
+type Cost = Int
+
+compute :: Input -> Int -> Int
+compute input minGain = findAllShortcut
+  where
+    findAllShortcut :: Int
+    findAllShortcut = M.foldrWithKey findShortcutFrom 0 costs
+    findShortcutFrom :: Coord -> Int -> Int -> Int
+    findShortcutFrom (x, y) c acc = M.foldrWithKey validShortcut acc candidates
+      where
+        candidates = M.filterWithKey (\(x', y') _ -> (abs $ x - x') + (abs $ y - y') <= 20) costs
+        validShortcut :: Coord -> Int -> Int -> Int
+        validShortcut (x', y') c' acc' = let gain = c' - c - (abs $ x - x') - (abs $ y - y')
+                                         in if gain >= minGain then acc' + 1 else acc'
+    (costs, len) = walk M.empty 0 (sx, sy)
+      where
+        walk :: Visited -> Int -> Coord -> (Visited, Int)
+        walk m c (x, y) | t == End = (m', c)
+                        | otherwise = walk m' (c+1) next
+          where
+            [next] = filter (\p@(i, j) -> and [input V.! j V.! i /= Wall, not $ M.member p m']) candidates
+            candidates = [(x-1, y), (x+1, y), (x, y-1), (x, y+1)]
+            m' = M.insert (x, y) c m
+            t = input V.! y V.! x
+    height = V.length input
+    width = V.length (input V.! 0)
+    sx = fst $ (V.filter (\(_, t) -> t == Start) $ V.indexed $ input V.! sy) V.! 0
+    sy = fst $ (V.filter (\(_, v) -> V.length (V.filter (== Start) v) == 1) $ V.indexed input) V.! 0
+
+main :: IO ()
+main = do
+  example <- parseInput "example"
+  let exampleOutput = compute example 76
+  when  (exampleOutput /= exampleExpectedOutput)  (error $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
+  let exampleOutput2 = compute example 74
+  when  (exampleOutput2 /= 7)  (error $ "example2 failed: got " ++ show exampleOutput2 ++ " instead of " ++ show 7)
+  input <- parseInput "input"
+  print $ compute input 100