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    1 {-# LANGUAGE DeriveAnyClass #-}
    2 {-# LANGUAGE DeriveGeneric #-}
    3 {-|
    4 Module:      Y2021.D17
    5 Description: Advent of Code 2021 Day 17 Solutions.
    6 License:     MIT
    7 Maintainer:  @tylerjl
    8 
    9 Solutions to the 2021 day 17 set of problems for <adventofcode.com>.
   10 -}
   11 module Y2021.D17
   12   ( parse17
   13   , part17A
   14   , part17B
   15   , solve17A
   16   ) where
   17 
   18 import Control.Applicative
   19 import Data.Attoparsec.Text hiding (take, takeWhile)
   20 import Data.Either.Utils    (fromRight)
   21 import Data.Text            (Text)
   22 import Control.DeepSeq (NFData)
   23 import GHC.Generics (Generic)
   24 
   25 data LandingZone = LZ (Int, Int) (Int, Int)
   26   deriving (Generic, NFData, Show)
   27 
   28 -- |Solution to part A
   29 part17A :: Text -> Int
   30 part17A = solve17A . parse17
   31 
   32 -- |Solution to part A
   33 solve17A :: LandingZone -> Int
   34 solve17A (LZ _ (abs -> y1, abs -> y2)) = (y * (y - 1)) `div` 2
   35   where
   36     y = max y1 y2
   37 
   38 -- |Solution to part B
   39 part17B :: Text -> Int
   40 part17B (parse17 -> LZ (x1, x2) (y1, y2)) = length $
   41   [ a | dx <- [1 .. x2]
   42   , dy <- [y1 .. negate y1]
   43   , let a = arc (dx, dy)
   44   , isValid a
   45   ]
   46   where
   47     isValid = any (\(x, y) -> x1 <= x && x <= x2 && y1 <= y && y <= y2)
   48     arc = takeWhile (\(_, y) -> y >= y1) . steps (0, 0)
   49       where
   50         steps (x, y) (dx, dy) =
   51           (x, y) : steps (x + dx, y + dy) (dx - signum dx, dy - 1)
   52 
   53 -- |Parse.
   54 parse17 :: Text -> LandingZone
   55 parse17 = fromRight . parseOnly parser
   56   where
   57     parser = LZ <$> ("target area: " *> span') <*> (", " *> span')
   58     span' = sortTuple <$> range'
   59     range' = (,) <$> (axis *> "=" *> signed decimal) <*> (".." *> signed decimal)
   60     axis = char 'x' <|> char 'y'
   61 
   62 sortTuple :: Ord a => (a, a) -> (a, a)
   63 sortTuple t@(a, b) | a <= b = t
   64                    | otherwise = (b, a)