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-- requires cabal install --lib megaparsec parser-combinators heap vector text
module Main (main) where
import Control.Applicative.Permutations
import Control.Monad (void, when)
import Data.Bifunctor (bimap)
import qualified Data.Char as C
import Data.Either
import Data.Functor
import qualified Data.Heap as H
import qualified Data.List as L
import qualified Data.Map as M
import Data.Maybe
import qualified Data.Set as S
import qualified Data.Text as T
import qualified Data.Text.Read as T
import qualified Data.Vector as V
import qualified Data.Vector.Unboxed as VU
import Data.Void (Void)
import Text.Megaparsec
import Text.Megaparsec.Char
import Debug.Trace
exampleExpectedOutput = 952408144115
data Heading = U | D | L | R deriving (Eq, Show)
data Line = Line Int Heading
type Input = [Line]
type Parser = Parsec Void String
parseHeading :: Parser Heading
parseHeading = char '3' $> U
<|> char '1' $> D
<|> char '2' $> L
<|> char '0' $> R
parseLine :: Parser Line
parseLine = Line <$> (letterChar *> space *> some digitChar *> string " (#" *> (fst . fromRight (0, T.pack "") . T.hexadecimal . T.pack <$> count 5 alphaNumChar))
<*> (parseHeading <* char ')' <* eol)
parseInput' :: Parser Input
parseInput' = some parseLine <* 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'
pick_theorem :: [(Int, Int)] -> Int
pick_theorem vertices = 1 + perimeter `div` 2 + (abs . (`div` 2) . sum . zipWith crossProduct vertices $ tail vertices)
where crossProduct (x1, y1) (x2, y2) = y1 * x2 - x1 * y2
dist (x1, y1) (x2, y2) = abs (x1 - x2) + abs (y1 - y2)
perimeter = sum . zipWith dist vertices $ tail vertices
compute :: Input -> Int
compute input = pick_theorem $ (0, 0) : L.foldl' coords [(0, 0)] input
where
coords :: [(Int, Int)] -> Line -> [(Int, Int)]
coords acc@((x, y):_) (Line l h) | h == U = (x, y - l):acc
| h == D = (x, y + l):acc
| h == L = (x - l, y):acc
| h == R = (x + l, y):acc
main :: IO ()
main = do
example <- parseInput "example"
let exampleOutput = compute example
when (exampleOutput /= exampleExpectedOutput) (error $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
input <- parseInput "input"
print $ compute input
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