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-- requires cabal install --lib megaparsec parser-combinators heap vector
module Main (main) where
import Control.Monad (void, when)
import qualified Data.List as L
import Data.Void (Void)
import Text.Megaparsec
import Text.Megaparsec.Char
import Debug.Trace
exampleExpectedOutput = 4
type Report = [Int]
type Input = [Report]
type Parser = Parsec Void String
parseNumber :: Parser Int
parseNumber = read <$> some digitChar <* optional hspace
parseReport :: Parser Report
parseReport = some parseNumber
parseInput' :: Parser Input
parseInput' = some (parseReport <* eol) <* 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'
compute :: Input -> Int
compute input = length . filter id $ map (any (valid . differences) . candidates) input
where
candidates :: Report -> [Report]
candidates report = report : [splitThis i|i<-[1..length report]]
where
splitThis i = let (l, r) = L.splitAt i report
in init l ++ r
differences report = zipWith (-) (init report) (tail report)
valid :: Report -> Bool
valid rs@(r:_) | r > 0 = all (\v -> v > 0 && v <= 3) rs
| r < 0 = all (\v -> v < 0 && v >= (-3)) rs
| otherwise = False
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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