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    1 {-|
    2 Module:      Y2016.D01
    3 Description: Advent of Code Day 01 Solutions.
    4 License:     MIT
    5 Maintainer:  @tylerjl
    6 
    7 Solutions to the day 01 set of problems for <adventofcode.com>.
    8 -}
    9 module Y2018.D01
   10   ( frequency
   11   , twiceFrequency
   12   ) where
   13 
   14 import Y2015.Util (regularParse, intParser)
   15 
   16 import Control.Applicative ((<|>))
   17 import Data.List (foldl')
   18 import Text.Parsec.String  (Parser)
   19 import Text.Parsec.Char    (endOfLine)
   20 import Text.Parsec
   21     ( ParseError
   22     , many
   23     , optional
   24     , string
   25     , try)
   26 
   27 data Change = Increase Int
   28             | Decrease Int
   29             deriving Eq
   30 
   31 parseFrequency :: String
   32                -> Either ParseError [Change]
   33 parseFrequency = regularParse freqParser
   34 
   35 freqParser :: Parser [Change]
   36 freqParser = many (parseChange <* optional endOfLine)
   37 
   38 parseChange :: Parser Change
   39 parseChange = try (Increase <$ string "+" <*> intParser)
   40           <|> try (Decrease <$ string "-" <*> intParser)
   41 
   42 frequency :: String -> Maybe Int
   43 frequency input = case parseFrequency input of
   44                     Left _ -> Nothing
   45                     Right f -> Just $ foldl' freqSum 0 f
   46 
   47 freqSum :: Int -> Change -> Int
   48 freqSum n (Increase i) = n + i
   49 freqSum n (Decrease i) = n - i
   50 
   51 twiceFrequency :: String -> Maybe Int
   52 twiceFrequency input = case parseFrequency input of
   53                     Left _ -> Nothing
   54                     Right f -> Just $ findRepeatedFrequency (cycle f) [] 0
   55 
   56 findRepeatedFrequency :: [Change] -> [Int] -> Int -> Int
   57 findRepeatedFrequency (change:changes) history current =
   58   if current' `elem` history then
   59     current'
   60   else
   61     findRepeatedFrequency changes history' current'
   62   where history' = history ++ [current]
   63         current' = freqSum current change
   64 findRepeatedFrequency [] _ current = current