From 5eb58ad076f2cd435b11b140820da224b60b73d5 Mon Sep 17 00:00:00 2001
From: Aria <me@aria.rip>
Date: Mon, 2 Jan 2023 21:58:56 +0000
Subject: initial commit

---
 2020/23a.hs | 54 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 54 insertions(+)
 create mode 100644 2020/23a.hs

(limited to '2020/23a.hs')

diff --git a/2020/23a.hs b/2020/23a.hs
new file mode 100644
index 0000000..ef515d4
--- /dev/null
+++ b/2020/23a.hs
@@ -0,0 +1,54 @@
+module Day23A where
+
+import System.Environment (getArgs)
+import Data.List (elemIndex)
+
+-- Types
+type State = [Int];
+
+-- Parsing input
+parseLine :: String -> [Int]
+parseLine = map (read . (: ""))
+
+parseFile :: String -> IO [Int]
+parseFile f = do 
+                c <- readFile f
+                return (parseLine c);
+
+-- Get the index of the destination cup, given (n - 1)
+getDestIdx :: Int -> [Int] -> Int
+getDestIdx x is | x `elem` is = unwrap $ elemIndex x is
+                | x < minimum is = unwrap $ elemIndex (maximum is) is
+                | otherwise = getDestIdx (x - 1) is
+
+-- n, current state, assumes current cup is cup 0
+performMoves :: Int -> State -> State
+performMoves 0 s = s
+performMoves n (c:cs) = performMoves (n - 1) s''
+        where re = c : drop 3 cs
+              as = take 3 cs
+              dI = getDestIdx (c - 1) re
+              s' = take (dI + 1) re ++ as ++ drop (dI + 1) re
+              s'' = moveToFront s' (head $ tail re)
+
+-- Usage: runghc 23a.hs inputs/day23
+main :: IO ()
+main = do 
+        args <- getArgs;
+        circ <- parseFile $ head args;
+
+        let final = performMoves 100 circ;
+        putStrLn $ tail $ concatMap show (moveToFront final 1);
+        return ();
+
+-- Utilities
+unwrap :: Maybe a -> a
+unwrap (Just x) = x
+unwrap _ = error "unwrap on null value"
+
+
+-- Move the given item to the front of the list, maintaining order.
+moveToFront :: Eq a => [a] -> a -> [a]
+moveToFront [] _ = []
+moveToFront (x:xs) t | t == x = x : xs
+                     | otherwise = moveToFront (xs ++ [x]) t
-- 
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