From a6189867047fde942000eab181af634493ac8ecf Mon Sep 17 00:00:00 2001
From: Amneesh Singh <natto@weirdnatto.in>
Date: Sat, 10 Dec 2022 00:18:59 +0530
Subject: [PATCH] day 9

very slow very swag

Signed-off-by: Amneesh Singh <natto@weirdnatto.in>
---
 day9.hs | 48 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 48 insertions(+)
 create mode 100644 day9.hs

diff --git a/day9.hs b/day9.hs
new file mode 100644
index 0000000..66be592
--- /dev/null
+++ b/day9.hs
@@ -0,0 +1,48 @@
+{-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE ViewPatterns #-}
+
+import Data.List (nub, scanl)
+import Data.Text (Text)
+import qualified Data.Text as T (lines, unpack, words)
+import qualified Data.Text.Read as T (decimal)
+import Lib (readFile')
+
+-- a rather slow program, takes 300 ms on my system (with `time`, but idc anymore 😭
+
+main :: IO ()
+main = do
+  input <-
+    concatMap
+      ((\[T.unpack -> [d], T.decimal -> Right (n, "")] -> replicate n d) . T.words)
+      . T.lines
+      <$> readFile' "day9.in"
+  putStr "Q1: "
+  print $ q 1 input
+  putStr "Q2: "
+  print $ q 9 input
+
+moveHead :: Char -> (Int, Int) -> (Int, Int)
+moveHead d (x, y) = case d of
+  'L' -> (x - 1, y)
+  'R' -> (x + 1, y)
+  'U' -> (x, y + 1)
+  'D' -> (x, y - 1)
+
+moveTail :: (Int, Int) -> (Int, Int) -> (Int, Int)
+moveTail (hx, hy) (tx, ty)
+  | abs dx <= 1 && abs dy <= 1 = (tx, ty) -- dont move
+  | abs dx > abs dy = (tx + signum dx, hy) -- move horizontally
+  | abs dy > abs dx = (hx, ty + signum dy) -- move vertically
+  | otherwise = (tx + signum dx, ty + signum dy ) -- move diagonally
+  where
+    dx = hx - tx
+    dy = hy - ty
+
+q :: Int -> [Char] -> Int
+q n =
+  length . nub . map last
+    . scanl
+      ( \(h : t) d ->
+          reverse $ foldl (\(h : xs) x -> moveTail h x : (h : xs)) [moveHead d h] t
+      )
+      (replicate (n + 1) (0, 0))