module Day2.Puzzle2 (day2_2) where

import Data.List (sort, sortBy, inits, tails)
import Data.Ord

isSafe :: [Int] -> Bool
isSafe xs = (isAscending xs || isDescending xs) && maximum distances <= 3 && minimum distances >= 1
            where isAscending x  = x == sort x
                  isDescending x = x == sortBy (comparing Down) x
                  distances       = map abs $ zipWith (-) xs (drop 1 xs)

removeLevel :: [Int] -> [[Int]]
removeLevel xs = zipWith (++) ys zs
            where ys = map init $ drop 1 (inits xs)
                  zs = map (drop 1) $ init (tails xs)

day2_2 :: IO ()
day2_2 = do
    contents <- lines <$> readFile "input/day2.txt"
    let reports = map read . words <$> contents
    putStrLn $ "Day 2, Puzzle 2 solution: "
        ++ show (length . filter (any isSafe) $ map removeLevel reports)