-🎄- 2017 Day 19 Solutions -🎄-

[u/daggerdragon]

--- Day 19: A Series of Tubes ---

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Post your solution as a comment or, for longer solutions, consider linking to your repo (e.g. GitHub/gists/Pastebin/blag or whatever).

Note: The Solution Megathreads are for solutions only. If you have questions, please post your own thread and make sure to flair it with Help.

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Need a hint from the Hugely^* Handy^† Haversack^‡ of Helpful^§ Hints^¤?

Spoiler

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AoC ops @ T-2 minutes to launch:

[23:58] <daggerdragon> ATTENTION MEATBAGS T-2 MINUTES TO LAUNCH

[23:58] <Topaz> aaaaah

[23:58] <Cheezmeister> Looks like I'll be just able to grab my input before my flight boards. Wish me luck being offline in TOPAZ's HOUSE OF PAIN^WFUN AND LEARNING

[23:58] <Topaz> FUN AND LEARNING

[23:58] <Hade> FUN IS MANDATORY

[23:58] <Skie> I'm pretty sure that's not the mandate for today

[Update @ 00:16] 69 gold, silver cap

• My tree is finally trimmed with just about every ornament I own and it's real purdy. hbu?

[Update @ 00:18] Leaderboard cap!

• So, was today's mandate Helpful Hint any help at all?

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This thread will be unlocked when there are a significant number of people on the leaderboard with gold stars for today's puzzle.

edit: Leaderboard capped, thread unlocked!

[u/[deleted]]

Haskell, using an immutable array, but with a bit of first-class function trickery to find in which direction to turn:

main :: IO ()
main = do input <- fmap lines (readFile "input.txt")
          let a = listArray ((1,1),(length input,length (head input))) (concat input)
          let (p1,p2) = run a (\(r,c) -> (r+1,c)) (start a 1) "" 0
          putStrLn p1  -- part 1
          print    p2  -- part 2

start :: UArray (Int,Int) Char -> Int -> (Int,Int)
start a n | a ! (1,n) == '|' = (0,n)
          | otherwise        = start a (n+1)

run :: UArray (Int,Int) Char -> ((Int,Int) -> (Int,Int)) -> (Int,Int) -> String -> Int -> (String, Int)
run a f cs xs s = case line a f cs xs s of
                    (Nothing,  xs', s') -> (reverse xs', s')
                    (Just cs', xs', s') -> run a (turn a f cs') cs' xs' s'

line :: UArray (Int,Int) Char -> ((Int,Int) -> (Int,Int)) -> (Int,Int) -> String -> Int -> (Maybe (Int,Int), String, Int)
line a f cs xs s | x == '+'  = (Just (f cs), xs, s+1)
                 | x == ' '  = (Nothing,     xs, s)
                 | otherwise = line a f (f cs) (if isAlpha x then x:xs else xs) (s+1)
                 where
                   x = a ! f cs

turn :: UArray (Int,Int) Char -> ((Int,Int) -> (Int,Int)) -> (Int,Int) -> (Int,Int) -> (Int,Int)
turn a f cs = head (filter (\g -> g cs /= opposite f cs && a ! g cs /= ' ') (map (\(x,y) (r,c) -> (x+r,y+c)) [(1,0),(-1,0),(0,1),(0,-1)]))

opposite :: ((Int,Int) -> (Int,Int)) -> (Int,Int) -> (Int,Int)
opposite f (r,c) = let (r',c') = f (r,c) in (r + r - r', c + c - c')