aoc-25/app/Day5.hs

module Main where

import Control.Arrow ((***))
import Data.Ix (inRange)
import Data.List (foldl', sort)
import System.Environment (getArgs)

main :: IO ()
main = do
  input <- readFile . head =<< getArgs
  putStrLn $ unwords ["Part 1:", show $ part1 input]
  putStrLn $ unwords ["Part 2:", show $ part2 input]

part1 :: String -> Int
part1 = length . uncurry (filter . isFresh) . parse
  where
    isFresh :: [(Int, Int)] -> Int -> Bool
    isFresh fresh ing = any (`inRange` ing) fresh

-- Splits input on empty line and parses the first half with genRanges
-- and the second half into ints with read.
parse :: String -> ([(Int, Int)], [Int])
parse = (genRanges *** (map read . drop 1)) . span (/= "") . lines
  where
    genRanges :: [String] -> [(Int, Int)]
    genRanges = foldl' (mergeRanges) mempty . sort . map parseRange

    -- Parse "X-Y" -> (X, Y)
    parseRange :: String -> (Int, Int)
    parseRange s = case span (/= '-') s of
      (start, '-' : end) -> (read start, read end)
      _ -> error $ "parseRange: expected a string on the form \"NUM-NUM\", got \"" <> s <> "\""

    -- Generate ranges by combining adjacent ranges if they overlap.
    --
    -- Ignores ranges that are fully enclosed by previous range.
    -- N.B. Assumes ranges are sorted on first component.
    mergeRanges :: [(Int, Int)] -> (Int, Int) -> [(Int, Int)]
    mergeRanges [] i = [i]
    mergeRanges fresh@((a, b) : rest) (x, y)
      | inRange (a, b) x && y > b = (a, y) : rest
      | x < a && inRange (a, b) y = (x, b) : rest
      | inRange (a, b) x && inRange (a, b) y = fresh
      | otherwise = (x, y) : fresh

part2 :: String -> Int
part2 = sum . map ((+ 1) . uncurry (flip (-))) . fst . parse

testInput :: String
testInput =
  unlines
    [ "3-5",
      "10-14",
      "16-20",
      "12-18",
      "",
      "1",
      "5",
      "8",
      "11",
      "17",
      "32"
    ]