advent21.hs

   1 import Text.Parsec hiding (State)
   2 import Text.ParserCombinators.Parsec.Number
   3 import Data.Maybe (fromJust)
   4 import Data.List (elemIndex)
   5
   6 import Control.Monad.Identity
   7 import Control.Monad.State
   8 import Control.Monad.Writer
   9
  10 data Instruction =    SwapPosition Int Int
  11                     | SwapLetter Char Char
  12                     | RotateSteps Int
  13                     | RotateLetter Char
  14                     | Reverse Int Int
  15                     | Move Int Int
  16                     deriving (Show, Eq)
  17
  18 data Log = Log {
  19     action :: String
  20     } deriving (Show)
  21
  22 data Password = Password {
  23     password :: String
  24     } deriving (Show)
  25
  26
  27 type App = WriterT [Log] (StateT Password Identity)
  28
  29 infixl 9 ??
  30
  31 (??) :: Eq a => [a] -> a -> Int
  32 (??) items item = fromJust $ elemIndex item items
  33
  34
  35 initial = "abcdefgh"
  36 final   = "fbgdceah"
  37
  38 testInstructions = "\
  39 \swap position 4 with position 0\n\
  40 \swap letter d with letter b\n\
  41 \reverse positions 0 through 4\n\
  42 \rotate left 1 step\n\
  43 \move position 1 to position 4\n\
  44 \move position 3 to position 0\n\
  45 \rotate based on position of letter b\n\
  46 \rotate based on position of letter d\n"
  47
  48 main :: IO ()
  49 main = do
  50     -- let ti = successfulParse $ parseIfile testInstructions
  51     -- part1 ti "abcde"
  52     -- part2 (reverse ti) "decab"
  53     text <- readFile "advent21.txt"
  54     let instructions = successfulParse $ parseIfile text
  55     part1 instructions initial
  56     part2 (reverse instructions) final
  57
  58 part1 :: [Instruction] -> String -> IO ()
  59 part1 instructions start =
  60     let st = Password {password = start}
  61         ((_, log), st') = runIdentity (runStateT (runWriterT (apply instructions)) st)
  62     in do
  63         -- putStrLn $ unlines $ map (action) log
  64         putStrLn $ password st'
  65
  66 part2 :: [Instruction] -> String -> IO ()
  67 part2 instructions end =
  68     let st = Password {password = end}
  69         ((_, log), st') = runIdentity (runStateT (runWriterT (unApply instructions)) st)
  70     in do
  71         -- putStrLn $ unlines $ map (action) log
  72         putStrLn $ password st'
  73
  74
  75 apply :: [Instruction] -> App ()
  76 apply [] = return ()
  77 apply (i:is) =
  78     do  st <- get
  79         let p0 = password st
  80         let p1 = applyInstruction i p0
  81         put st {password = p1}
  82         tell [Log (p0 ++ " -> " ++ p1 ++ " : " ++ (show i))]
  83         apply is
  84
  85
  86 applyInstruction :: Instruction -> String -> String
  87 applyInstruction (SwapPosition from to) p0
  88     | from == to = p0
  89     | otherwise = prefix ++ [p0!!end] ++ midfix ++ [p0!!start] ++ suffix
  90         where start = minimum [from, to]
  91               end = maximum [from, to]
  92               prefix = take start p0
  93               midfix = take (end-start-1) $ drop (start+1) p0
  94               suffix = drop (end+1) p0
  95
  96 applyInstruction (SwapLetter l0 l1) p0 = applyInstruction (SwapPosition (p0??l0) (p0??l1)) p0
  97
  98 applyInstruction (RotateSteps n) p0 = (drop n' p0) ++ (take n' p0)
  99     where n' = if n < 0
 100                 then (-1 * n)
 101                 else (length p0) - n
 102
 103 applyInstruction (RotateLetter l) p0 = applyInstruction (RotateSteps n) p0
 104     where n = (1 + (p0??l) + if (p0??l) >= 4 then 1 else 0) `mod` (length p0)
 105
 106 applyInstruction (Reverse from to) p0
 107     | from == to = p0
 108     | otherwise = prefix ++ (reverse midfix) ++ suffix
 109         where start = minimum [from, to]
 110               end = maximum [from, to]
 111               prefix = take start p0
 112               midfix = take (end-start+1) $ drop start p0
 113               suffix = drop (end+1) p0
 114
 115 applyInstruction (Move from to) p0
 116     | from == to = p0
 117     | otherwise = prefix ++ [p0!!from] ++ suffix
 118         where without = take from p0 ++ drop (from+1) p0
 119               prefix = take to without
 120               suffix = drop (to) without
 121
 122
 123 unApply :: [Instruction] -> App ()
 124 unApply [] = return ()
 125 unApply (i:is) =
 126     do  st <- get
 127         let p0 = password st
 128         let p1 = unApplyInstruction i p0
 129         put st {password = p1}
 130         tell [Log (p1 ++ " <- " ++ p0 ++ " : " ++ (show i))]
 131         unApply is
 132
 133 unApplyInstruction :: Instruction -> String -> String
 134 unApplyInstruction (SwapPosition from to) p0 = applyInstruction (SwapPosition from to) p0
 135 unApplyInstruction (SwapLetter l0 l1) p0 = applyInstruction (SwapLetter l0 l1) p0
 136 unApplyInstruction (RotateSteps n) p0 = applyInstruction (RotateSteps (-1 * n)) p0
 137 unApplyInstruction (Reverse from to) p0 = applyInstruction (Reverse from to) p0
 138 unApplyInstruction (Move from to) p0 = applyInstruction (Move to from) p0
 139 unApplyInstruction (RotateLetter l) p0 = applyInstruction (RotateSteps n) p0
 140     where n = case (p0??l) of
 141                 0 -> -1
 142                 1 -> -1
 143                 2 ->  2
 144                 3 -> -2
 145                 4 ->  1
 146                 5 -> -3
 147                 6 ->  0
 148                 7 -> -4
 149     -- where n = case (p0??l) of
 150     --             0 -> -1
 151     --             1 -> -1
 152     --             2 ->  1
 153     --             3 -> -2
 154     --             4 ->  1
 155
 156
 157 instructionFile = instructionLine `endBy` newline
 158 instructionLine = choice [ swapL
 159                          , rotateL
 160                          , reverseL
 161                          , moveL
 162                          ]
 163
 164 swapL = (try (string "swap ")) *> (swapPosL <|> swapLetterL)
 165
 166 swapPosL = SwapPosition <$> (string "position" *> spaces *> int)
 167                         <*> (spaces *> string "with position" *> spaces *> int)
 168
 169 swapLetterL = SwapLetter <$> (string "letter" *> spaces *> letter)
 170                          <*> (spaces *> string "with letter" *> spaces *> letter)
 171
 172 rotateL = (try (string "rotate ")) *> (rotateDirL <|> rotateLetterL)
 173
 174 rotateDirL = rotateStepify <$> ((string "left") <|> (string "right"))
 175                            <*> (spaces *> int <* spaces <* skipMany letter)
 176     where rotateStepify dir n = case dir of
 177                                      "left" -> (RotateSteps (-1 * n))
 178                                      "right" -> (RotateSteps n)
 179 rotateLetterL = RotateLetter <$> (string "based on position of letter " *> letter)
 180
 181 reverseL = Reverse <$> (string "reverse positions" *> spaces *> int)
 182                    <*> (spaces *> (string "through") *> spaces *> int)
 183
 184 moveL = Move <$> (string "move position" *> spaces *> int)
 185              <*> (spaces *> (string "to position") *> spaces *> int)
 186
 187
 188 parseIfile :: String -> Either ParseError [Instruction]
 189 parseIfile input = parse instructionFile "(unknown)" input
 190
 191 parseIline :: String -> Either ParseError Instruction
 192 parseIline input = parse instructionLine "(unknown)" input
 193
 194 successfulParse :: Either ParseError [a] -> [a]
 195 successfulParse (Left _) = []
 196 successfulParse (Right a) = a