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    1 {-# LANGUAGE FlexibleContexts #-}
    2 
    3 {-|
    4 Module:      Y2021.D02
    5 Description: Advent of Code 2021 Day 02 Solutions.
    6 License:     MIT
    7 Maintainer:  @tylerjl
    8 
    9 Solutions to the 2021 day 02 set of problems for <adventofcode.com>.
   10 -}
   11 module Y2021.D02 where
   12 
   13 import Data.Text (Text)
   14 import qualified Text.Parsec as P
   15 import Text.Parsec.Text (Parser)
   16 import Y2015.Util (regularParse', intParser')
   17 import Control.Applicative ((<|>))
   18 import qualified Data.Text as T
   19 
   20 data Navigation
   21   = Forward Int
   22   | Down Int
   23   | Up Int
   24 
   25 navParser :: Parser [Navigation]
   26 navParser = P.many (parseNav <* P.optional P.endOfLine)
   27 
   28 parseNav :: Parser Navigation
   29 parseNav = Forward <$> pInstr "forward"
   30        <|> Down    <$> pInstr "down"
   31        <|> Up      <$> pInstr "up"
   32 
   33 pInstr :: String -> Parser Int
   34 pInstr s = P.string s *> P.skipMany1 P.space *> intParser'
   35 
   36 part2A :: Text -> Int
   37 part2A (regularParse' navParser -> Right navs) = uncurry (*) $ go (0, 0) navs
   38   where go (x, y) (Forward n:xs) = go (x + n, y) xs
   39         go (x, y) (Down n:xs)    = go (x, y + n) xs
   40         go (x, y) (Up n:xs)      = go (x, y - n) xs
   41         go coords []             = coords
   42 part2A (regularParse' navParser -> Left err) = error (show err)
   43 
   44 part2B :: Text -> Int
   45 part2B (regularParse' navParser -> Right navs) = (\(x, y, _) -> x * y) $ go (0, 0, 0) navs
   46   where go (x, y, aim) (Forward n:xs) = go (x + n, y + (aim * n), aim) xs
   47         go (x, y, aim) (Down n:xs)    = go (x, y, aim + n) xs
   48         go (x, y, aim) (Up n:xs)      = go (x, y, aim - n) xs
   49         go coords []                  = coords
   50 part2B (regularParse' navParser -> Left err) = error (show err)
   51 
   52 d2sample :: Text
   53 d2sample = T.unlines
   54   [ "forward 5"
   55   , "down 5"
   56   , "forward 8"
   57   , "up 3"
   58   , "down 8"
   59   , "forward 2"
   60   ]