From 72a9d740bf9d06b00b50c43a2e875b04c6d98444 Mon Sep 17 00:00:00 2001
From: Julien Dessaux
Date: Thu, 21 Nov 2024 00:23:25 +0100
Subject: 2023-25 in haskell

---
 2023/25-Snowverload/first.hs | 76 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 76 insertions(+)
 create mode 100644 2023/25-Snowverload/first.hs

(limited to '2023/25-Snowverload/first.hs')

diff --git a/2023/25-Snowverload/first.hs b/2023/25-Snowverload/first.hs
new file mode 100644
index 0000000..18063f2
--- /dev/null
+++ b/2023/25-Snowverload/first.hs
@@ -0,0 +1,76 @@
+-- requires cabal install --lib megaparsec parser-combinators heap vector fgl
+-- very slow with runghc, use ghc -O3 -o first first.hs
+module Main (main) where
+
+import           Control.Applicative.Permutations
+import           Control.Monad                    (void, when)
+import qualified Data.Char                        as C
+import           Data.Either
+import           Data.Function
+import           Data.Functor
+import           Data.Graph.Inductive
+import qualified Data.Heap                        as H
+import qualified Data.List                        as L
+import qualified Data.Map                         as M
+import           Data.Maybe
+import           Data.Ratio
+import qualified Data.Set                         as S
+import           Data.Tuple
+import qualified Data.Vector                      as V
+import qualified Data.Vector.Unboxed              as VU
+import           Data.Void                        (Void)
+import           Text.Megaparsec
+import           Text.Megaparsec.Char
+
+import           Debug.Trace
+
+exampleExpectedOutput = 54
+
+type Input = M.Map String (S.Set String)
+
+type Parser = Parsec Void String
+
+parseLabel :: Parser String
+parseLabel = some letterChar <* optional (char ':') <* optional hspace
+
+parseLine :: Parser (String, S.Set String)
+parseLine = (,) <$> parseLabel
+                <*> (S.fromList <$> some parseLabel)
+
+parseInput' :: Parser Input
+parseInput' = M.fromList <$> (some (parseLine <* eol) <* eof)
+
+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'
+
+compute :: Input -> Int
+compute input = size1 * (order g - size1)
+  where
+    n = L.nub $ M.keys input ++ (S.elems . S.unions $ M.elems input)
+    e = [(from, to) | (from, tos) <- M.assocs input, to <- S.elems tos]
+    n' = zip n [1..]
+    e' = map (\(from, to) -> (fromJust $ L.lookup from n', fromJust $ L.lookup to n', from ++ "-" ++ to)) e
+    g :: Gr String String
+    g = undir $ mkGraph (map swap n') e'
+    size1 = length group1
+    group1 = compute' (nodes g)
+    -- Karger's algorithm to compute a minimum cut
+    -- implementation from https://github.com/jrp2014/AoC2023/blob/main/Day25/Day25.hs#L60
+    compute' :: [Node] -> [Node]
+    compute' s | sum (map count s) == 3 = s
+               | otherwise = compute' (L.delete (L.maximumBy (compare `on` count) s) s)
+      where
+        count :: Node -> Int
+        count ns = length $ suc g ns L.\\ s
+
+main :: IO ()
+main = do
+  example <- parseInput "example"
+  let exampleOutput = compute example
+  when  (exampleOutput /= exampleExpectedOutput)  (error $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
+  input <- parseInput "input"
+  print $ compute input
-- 
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