From f52cd41da133a41de5095acb1f83bb4ff60d1ea8 Mon Sep 17 00:00:00 2001
From: eriedaberrie <eriedaberrie@gmail.com>
Date: Fri, 15 Dec 2023 23:11:22 -0800
Subject: [PATCH] 2023 day 13

Star 2 is the most scuffed and probably incorrect thing ever but it produced the
right answer for my input and that's good enough for me.
---
 2023/13/Main1.hs | 33 +++++++++++++++++++++++++++++++++
 2023/13/Main2.hs | 40 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 73 insertions(+)
 create mode 100644 2023/13/Main1.hs
 create mode 100644 2023/13/Main2.hs

diff --git a/2023/13/Main1.hs b/2023/13/Main1.hs
new file mode 100644
index 0000000..3ef8eee
--- /dev/null
+++ b/2023/13/Main1.hs
@@ -0,0 +1,33 @@
+import           Data.List.Split (splitOn)
+import           Data.List (foldl', transpose)
+import qualified Data.Vector.Unboxed as VU
+import           Data.Vector.Unboxed (Vector)
+
+hashChar :: Char -> Int
+hashChar '#' = 1
+hashChar '.' = 0
+
+lineToInt :: String -> Int
+lineToInt = foldl' (\x y -> 2 * x + hashChar y) 0
+
+matchesAt :: Vector Int -> Int -> Bool
+matchesAt v i = VU.reverse rightSide == leftSide
+  where
+    nMirrored = min i $ VU.length v - i
+
+    leftSide = VU.slice (i - nMirrored) nMirrored v
+
+    rightSide = VU.slice i nMirrored v
+
+findMatches :: Vector Int -> [Int]
+findMatches v = filter (matchesAt v) [1 .. VU.length v - 1]
+
+solveDirection :: [String] -> Int
+solveDirection = sum . findMatches . VU.fromList . map lineToInt
+
+solve :: [String] -> Int
+solve l = 100 * solveDirection l + (solveDirection . transpose) l
+
+main :: IO ()
+main = print . sum . map (solve . lines) . splitOn "\n\n"
+  =<< readFile "data.txt"
diff --git a/2023/13/Main2.hs b/2023/13/Main2.hs
new file mode 100644
index 0000000..9c17421
--- /dev/null
+++ b/2023/13/Main2.hs
@@ -0,0 +1,40 @@
+import           Data.Bits ((.&.), countTrailingZeros, xor)
+import           Data.List.Split (splitOn)
+import           Data.List (foldl', transpose)
+import qualified Data.Vector.Unboxed as VU
+import           Data.Vector.Unboxed (Vector)
+
+isPowTwo :: Int -> Bool
+isPowTwo n = n .&. (n - 1) == 0
+
+hashChar :: Char -> Int
+hashChar '#' = 1
+hashChar '.' = 0
+
+lineToInt :: String -> Int
+lineToInt = foldl' (\x y -> 2 * x + hashChar y) 0
+
+matchesAt :: Vector Int -> Int -> Bool
+matchesAt v i = VU.length differences == 1 && isPowTwo (VU.head differences)
+  where
+    nMirrored = min i $ VU.length v - i
+
+    leftSide = VU.slice (i - nMirrored) nMirrored v
+
+    rightSide = VU.slice i nMirrored v
+
+    differences = VU.filter (/= 0)
+      $ VU.zipWith xor (VU.reverse rightSide) leftSide
+
+findMatches :: Vector Int -> [Int]
+findMatches v = filter (matchesAt v) [1 .. VU.length v - 1]
+
+solveDirection :: [String] -> Int
+solveDirection = last . (0:) . findMatches . VU.fromList . map lineToInt
+
+solve :: [String] -> Int
+solve l = 100 * solveDirection l + (solveDirection . transpose) l
+
+main :: IO ()
+main = print . sum . map (solve . lines) . splitOn "\n\n"
+  =<< readFile "data.txt"