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

---
 2020/19b.hs | 95 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 95 insertions(+)
 create mode 100644 2020/19b.hs

(limited to '2020/19b.hs')

diff --git a/2020/19b.hs b/2020/19b.hs
new file mode 100644
index 0000000..d1fa763
--- /dev/null
+++ b/2020/19b.hs
@@ -0,0 +1,95 @@
+module Day19B where
+
+import System.Environment (getArgs)
+import Text.Printf
+import Data.List.Split (splitOn)
+import Data.Char (isDigit)
+import qualified Data.Set as S
+import qualified Data.Map.Strict as M
+
+
+-- Rules parser
+
+-- Types
+data Rule = Literal Char | Reference Int | Concat Rule Rule | Union Rule Rule | End
+        deriving (Eq, Show);
+
+-- Convert a list of rules to the union of those rules
+toUnion :: [Rule] -> Rule
+toUnion [x] = x
+toUnion (x:ys) = Union x (toUnion ys)
+
+-- Parse the given line seperated list of rules
+-- Returns a map from id to rule
+parseRules :: String -> M.Map Int Rule
+parseRules = M.fromList . map parseRule . lines
+
+-- Parse the given rule, in format `0: 1 2 | 3 4`
+parseRule :: String -> (Int, Rule)
+parseRule s = (read idx, toUnion $ map parseNoUnion $ splitOn " | " def)
+            where [idx, def] = splitOn ": " s
+
+
+-- Parse the lower precedence parts (anything that isn't a union)
+parseNoUnion :: String -> Rule
+parseNoUnion "" = End;
+parseNoUnion (' ':xs) = parseNoUnion xs
+parseNoUnion ('"':xs) = Literal (head xs)
+parseNoUnion xs@(x:_) | not (null re)= Concat (Reference (read n)) (parseNoUnion (drop (length n + 1) xs))
+                      | otherwise = Reference (read n)
+                        where (n:re) = splitOn " " xs
+
+-- Parse the input into a list of rules and list of messages
+parseInput :: String -> (M.Map Int Rule, [String])
+parseInput s = (parseRules rs, lines ms)
+        where [rs, ms] = splitOn "\n\n" s
+
+-- Parse the file with the given name
+parseFile :: String -> IO (M.Map Int Rule, [String])
+parseFile f = do 
+                contents <- readFile f
+                return $ parseInput contents;
+
+-- Parser
+
+-- Returns remaining if valid
+attemptConsume :: M.Map Int Rule -> Rule -> String -> [Maybe String]
+attemptConsume m End xs = [Just xs]
+attemptConsume m r [] = [Nothing]
+attemptConsume m (Literal t) (x:xs) | t == x = [Just xs]
+                                    | otherwise = [Nothing]
+attemptConsume m (Concat a b) xs = concat [attemptConsume m b re | Just re <- attemptConsume m a xs]
+attemptConsume m (Union a b) xs = attemptConsume m b xs ++ attemptConsume m a xs
+attemptConsume m (Reference n) xs = attemptConsume m (unwrap $ M.lookup n m) xs
+
+
+matchedBy :: M.Map Int Rule -> String -> Bool
+matchedBy m s = or [x == "" | Just x <- ps]
+              where ps = attemptConsume m (unwrap $ M.lookup 0 m) s
+
+-- runghc --ghc-arg='-package split' 19b.hs inputs/day19
+main :: IO ()
+main = do 
+        args <- getArgs;
+        (rs, ms) <- parseFile (head args);
+
+        let ps = filter (matchedBy rs) ms;
+
+        print (length ps);
+
+        return ();
+
+-- Helpers
+
+-- Unwrap a maybe value, throwing an error if wrong
+unwrap :: Maybe a -> a
+unwrap (Just x) = x
+unwrap _ = error "unwrap on null value"
+
+-- First of a 3tuple
+fst3 :: (a, b, c) -> a
+fst3 (a, _, _) = a
+
+-- Last of a 3 tuple
+lst3 :: (a, b, c) -> c
+lst3 (_, _, c) = c
\ No newline at end of file
-- 
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