1 files changed, 76 insertions, 0 deletions
diff --git a/2024/17-Chronospatial_Computer/second.hs b/2024/17-Chronospatial_Computer/second.hs
new file mode 100644
index 0000000..fa6d0b9
--- /dev/null
+++ b/2024/17-Chronospatial_Computer/second.hs
@@ -0,0 +1,76 @@
+-- requires cabal install --lib megaparsec parser-combinators heap vector
+module Main (main) where
+
+import           Control.Monad        (void, when)
+import           Data.Bits
+import           Data.Functor
+import qualified Data.List            as L
+import           Data.Maybe
+import           Data.Void            (Void)
+import           Text.Megaparsec
+import           Text.Megaparsec.Char
+
+type Register = Int
+type Op = Int
+data Input = Input [Register] [Op] deriving Show
+
+type Parser = Parsec Void String
+
+parseNumber :: Parser Int
+parseNumber = read <$> some digitChar
+
+parseRegister :: Parser Register
+parseRegister = string "Register " *> (char 'A' <|> char 'B' <|> char 'C') *> string ": " *> parseNumber
+
+parseInput' :: Parser Input
+parseInput' = Input <$> some (parseRegister <* eol) <* eol
+                    <*> (string "Program: " *> some (parseNumber <* optional (char ',')) <* 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'
+
+type Pointer = Int
+type PState = (Pointer, [Register], [Int])
+
+compute :: Input -> Int
+compute (Input start ops) = minimum $ findA 0 (tail $ L.reverse $ L.tails ops)
+  where
+    findA :: Int -> [[Int]] -> [Int]
+    findA a [] = [a]
+    findA a (l:ls) = concatMap (\next -> findA next ls) nexts
+      where
+        candidates :: [Int]
+        candidates = [a*8..a*8+7]
+        steps :: [PState]
+        steps = map (\x -> step (0, [x, 0, 0], [])) candidates
+        outputs :: [[Int]]
+        outputs = map (\(_, [_, _, _], x') -> reverse x') steps
+        valids :: [Int]
+        valids = L.elemIndices l outputs
+        nexts :: [Int]
+        nexts = map (candidates L.!!) valids
+    end = length ops
+    step :: PState -> PState
+    step ps@(p, [a, b, c], _) | p == end = ps
+                              | otherwise = step $ i ps co
+      where
+        i = [adv, bxl, bst, jnz, bxc, out, bdv, cdv] L.!! (ops L.!! p)
+        co = [0, 1, 2, 3, a, b, c] L.!! (ops L.!! (p+1))
+    adv (p, [a, b, c], acc) n = (p+2, [a `div` (2 ^ n), b, c], acc)
+    bxl (p, [a, b, c], acc) n = (p+2, [a, xor b n, c], acc)
+    bst (p, [a, b, c], acc) n = (p+2, [a, n `mod` 8, c], acc)
+    jnz (p, [0, b, c], acc) n = (p+2, [0, b, c], acc)
+    jnz (p, [a, b, c], acc) n = (n, [a, b, c], acc)
+    bxc (p, [a, b, c], acc) _ = (p+2, [a, xor b c, c], acc)
+    out (p, [a, b, c], acc) n = (p+2, [a, b, c], (n `mod` 8) : acc)
+    bdv (p, [a, b, c], acc) n = (p+2, [a, a `div` (2 ^ n), c], acc)
+    cdv (p, [a, b, c], acc) n = (p+2, [a, b, a `div` (2 ^ n)], acc)
+
+main :: IO ()
+main = do
+  input <- parseInput "input"
+  print $ compute input