d13.hs

import Text.Parsec
import Text.Parsec.String

data Direction = U | D deriving Show
data Layer = Layer Int Int Int Direction deriving Show

-- parse
input = [ Layer 0 3 0 U, Layer 1 2 0 U, Layer 4 4 0 U, Layer 6 4 0 U ]

p_layer :: Parser Layer
p_layer = do
    d <- read <$> many1 digit
    char ':'
    spaces
    r <- read <$> many1 digit
    return $ Layer d r 0 U

p_layers :: Parser [Layer]
p_layers = p_layer `sepBy` char '\n'

-- run
next :: Layer -> Layer
next l@(Layer _ 0 _ _) = l
next (Layer i l p U)
    | p == (l - 1)     = Layer i l (p-1) D
    | otherwise        = Layer i l (p+1) U
next (Layer i l p D)
    | p == 0           = Layer i l (p+1) U
    | otherwise        = Layer i l (p-1) D

after :: Int -> [Layer] -> [Layer]
after n l = (iterate (map next) l) !! n

run ((Layer d r p _):[])
    | p == 0 = d * r 
    | otherwise = 0

run ((Layer d r p _):l@((Layer d' _ _ _):xs))
    | p == 0    = d * r + (run . after (d' - d) $ l)
    | otherwise = run . after (d' - d) $ l

caught ((Layer d r p _):[])
    | p == 0 = 1
    | otherwise = 0

caught ((Layer d r p _):l@((Layer d' _ _ _):xs))
    | p == 0    = 1 + (run . after (d' - d) $ l)
    | otherwise = run . after (d' - d) $ l


p2 l = let r = caught l
        in if r == 0 then 0 else (1 + (p2 $ after 1 l))