2020 day16/haskell: add solution

2020/README.md

@@ -20,5 +20,6 @@ https://adventofcode.com/2020/
 |13 |        |  `**`   |      |
 |14 |        |  `**`   |      |
 |15 |  `**`  |         |      |
+|16 |        |  `**`   |      |
 
 `test.sh` can be used to run all solutions and automatically compares them to (my) puzzle inputs and the expected outputs.

2020/data/day16.expected

@@ -0,0 +1,2 @@
+25984
+1265347500049

2020/data/day16.input

@@ -0,0 +1,268 @@
+departure location: 25-568 or 594-957
+departure station: 33-447 or 466-952
+departure platform: 31-700 or 725-956
+departure track: 43-124 or 141-952
+departure date: 26-290 or 306-962
+departure time: 34-754 or 763-960
+arrival location: 29-208 or 217-958
+arrival station: 48-118 or 124-973
+arrival platform: 35-368 or 389-972
+arrival track: 47-91 or 106-970
+class: 35-521 or 528-960
+duration: 27-833 or 855-965
+price: 25-870 or 895-957
+route: 31-140 or 146-965
+row: 35-736 or 743-957
+seat: 33-227 or 249-961
+train: 27-763 or 788-961
+type: 34-167 or 193-950
+wagon: 47-437 or 443-952
+zone: 48-928 or 940-955
+
+your ticket:
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+
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2020/day16/day16.hs

@@ -0,0 +1,96 @@
+{-# LANGUAGE NamedFieldPuns #-}
+
+module Day16 where
+
+import AoC
+
+import Data.List
+import Data.Maybe
+import qualified Data.Set as S
+import Data.Set (Set)
+import Text.ParserCombinators.ReadP
+import Text.Read.Lex
+
+data Field = Field { name :: String
+                   , valids :: Set Int
+                   } deriving (Show)
+
+type Ticket = [Int]
+
+data Input = Input { fields :: [Field]
+                   , myTicket :: Ticket
+                   , otherTickets :: [Ticket]
+                   } deriving (Show)
+
+line :: ReadP a -> ReadP a
+line p = p <* char '\n'
+
+parseField :: ReadP Field
+parseField = do
+    name <- many1 $ satisfy (/=':')
+    string ": "
+    ranges <- parseRange `sepBy` string " or "
+    let valids = S.fromList . concat . map (uncurry enumFromTo) $ ranges
+    return $ Field { name, valids }
+    where parseRange = do
+            n1 <- readDecP
+            char '-'
+            n2 <- readDecP
+            return (n1, n2)
+
+parseTicket :: ReadP Ticket
+parseTicket = readDecP `sepBy` char ','
+
+parseInput :: ReadP Input
+parseInput = do
+    fields <- many1 $ line parseField
+    line $ string ""
+    line $ string "your ticket:"
+    myTicket <- line parseTicket
+    line $ string ""
+    line $ string "nearby tickets:"
+    otherTickets <- many1 $ line parseTicket
+    return $ Input {fields, myTicket, otherTickets }
+
+matchesAnyField :: [Field] -> Int -> Bool
+matchesAnyField fields n = any (n `S.member`) . map valids $ fields
+
+iterateUntilDone :: (a -> Maybe a) -> a -> a
+iterateUntilDone f x = case f x of
+                         Just x' -> iterateUntilDone f x'
+                         Nothing -> x
+
+data Resolve a = Resolved a | Choice (Set a) deriving (Show)
+
+resolve :: Ord a => [[a]] -> Maybe [a]
+resolve = sequence . map fromResolved . iterateUntilDone resolve' . map (Choice . S.fromList)
+    where fromResolved (Resolved x) = Just x
+          fromResolved _ = Nothing
+
+findSingletonChoice :: [Resolve a] -> Maybe (a, [Resolve a])
+findSingletonChoice (c@(Choice s):xs) | S.size s == 1 = let [x] = S.elems s
+                                                        in Just (x, Resolved x:xs)
+                                      | otherwise = (fmap.fmap) (c:) $ findSingletonChoice xs
+findSingletonChoice (x:xs) = (fmap.fmap) (x:) $ findSingletonChoice xs
+findSingletonChoice _ = Nothing
+
+resolve' :: Ord a => [Resolve a] -> Maybe [Resolve a]
+resolve' xs = do
+    (x, xs') <- findSingletonChoice xs
+    return $ map (dropChoice x) xs'
+    where dropChoice x (Choice s) = Choice (S.delete x s)
+          dropChoice _ r = r
+
+part1 :: Input -> Int
+part1 Input { fields, otherTickets } = sum . filter (not . matchesAnyField fields) . concat $ otherTickets
+
+part2 :: Input -> Int
+part2 Input { fields, myTicket, otherTickets } = product departureValues
+    where validTickets = filter (all $ matchesAnyField fields) otherTickets
+          columns = transpose validTickets
+          fieldNames = fromJust . resolve . map findPossibleFields $ columns
+          departureIndices = findIndices ("departure" `isPrefixOf`) fieldNames
+          departureValues = map (myTicket !!) departureIndices
+          findPossibleFields col = map name . filter (\field -> all (`S.member` valids field) col) $ fields
+
+main = runAoC (fromJust . oneCompleteResult parseInput) part1 part2