advent20.hs

   1 import Text.Parsec
   2 import Text.ParserCombinators.Parsec.Number
   3 import Control.Applicative ((<$), (<*), (*>), (<*>), liftA)
   4 import Data.List (foldl')
   5
   6 data Interval = Interval Int Int deriving (Show, Eq)
   7
   8 low :: Interval -> Int
   9 low (Interval l _) = l
  10
  11 high :: Interval -> Int
  12 high (Interval _ h) = h
  13
  14 main :: IO ()
  15 main = do
  16     text <- readFile "advent20.txt"
  17     let intervals = successfulParse $ parseIfile text
  18     part1 intervals
  19     part2 intervals
  20
  21 part1 :: [Interval] -> IO ()
  22 part1 intervals = print $ (+1) $ high $ head $ foldl' (mergeAdjacent) [] $ foldl' (merge) [] intervals
  23
  24 part2 :: [Interval] -> IO ()
  25 part2 intervals = do
  26     let ints = foldl' (mergeAdjacent) [] $ foldl' (merge) [] intervals
  27     let gapCount = gaps ints
  28     let lowGap = low $ head ints
  29     let highGap = 4294967295 - (high $ last ints)
  30     print (lowGap + gapCount + highGap)
  31
  32 disjoint :: Interval -> Interval -> Bool
  33 disjoint (Interval a b) (Interval c d)
  34     | b < c = True
  35     | d < a = True
  36     | a > d = True
  37     | c > b = True
  38     | otherwise = False
  39
  40 intersect :: Interval -> Interval -> Bool
  41 intersect a b = not $ disjoint a b
  42
  43 merge :: [Interval] -> Interval -> [Interval]
  44 merge [] i0 = [i0]
  45 merge (i1:intervals) i0
  46     | (high i0) < (low i1) = i0:i1:intervals
  47     | intersect i0 i1 = merge intervals (Interval a' b')
  48     | otherwise = i1:(merge intervals i0)
  49         where a' = minimum [low i0, low i1]
  50               b' = maximum [high i0, high i1]
  51
  52 mergeAdjacent :: [Interval] -> Interval -> [Interval]
  53 mergeAdjacent [] i0 = [i0]
  54 mergeAdjacent (i1:intervals) i0
  55     | high i0 + 1 == low i1 = (Interval (low i0) (high i1)):intervals
  56     | low i0 == high i1 + 1 = (Interval (low i1) (high i0)):intervals
  57     | otherwise = i1:(mergeAdjacent intervals i0)
  58
  59 gaps :: [Interval] -> Int
  60 gaps [] = 0
  61 gaps [_] = 0
  62 gaps ((Interval _ b):(Interval c d):intervals) =
  63     (c - b - 1) + gaps ((Interval c d):intervals)
  64
  65 intervalFile = intervalLine `endBy` newline
  66 intervalLine = Interval <$> int <*> (string "-" *> int)
  67
  68 parseIfile :: String -> Either ParseError [Interval]
  69 parseIfile input = parse intervalFile "(unknown)" input
  70
  71 successfulParse :: Either ParseError [a] -> [a]
  72 successfulParse (Left _) = []
  73 successfulParse (Right a) = a