fun

import System.Environment

import Data.Time

import Text.Regex.PCRE -- Necessary for determining the type of String in the input

main = do

  args <- getArgs -- Gives list of given arguments

  fileContents <- readFile (head args) -- <- turns the IO String in a String

                  -- inside fileContents

  writeFile (last args) (correctFile fileContents)

  -- This is EVERYTHING that main is supposed to do, so it's FINISHED.

  -- From now the program works purely functional. Other lines on this

  -- indentation are for debugging.

-- | Entry function for the program. It receives the "raw" file content from

-- 'main', applies some necessary changes, and returns the corrected 'String',

-- ready for writing to a file.

correctFile :: String -> String

correctFile [] = []

correctFile input = unlines ("WEBVTT" : (editLines (lines input) (-60)))

    -- The "WEBVTT" in the front is necessary to comply with WebVTT specs

    -- The -60 initial seconds is necessary because the files start with an

    -- X-TIMESTAMP line

-- | Transforms the Strings from the file by inspecting the type and editing the

-- resulting list of Strings accordingly.

-- This function is applied in a recursive manner,

editLines :: [String] -- ^ The remaining lines of the original file

          -> Int      -- ^ The amount of seconds that will be added 

          -> [String] -- ^ The corrected lines, ready for writing

editLines [] _ = [] -- If the list of remaining Strings is depleted, return an empty list

editLines (current : remaining) seconds

  | current =~ "X-TIMESTAMP-MAP=MPEGTS:[0-9]+,LOCAL:[0-9:.]+" :: Bool = editLines remaining (seconds + 60)

  | current =~ "([0-9]{2}:)?[0-9]{2}:[0-9]{2}.[0-9]{3} --> ([0-9]{2}:)?[0-9]{2}:[0-9]{2}.[0-9]{3}" :: Bool = correctTimeString current seconds : editLines remaining seconds

  | current == "WEBVTT" = editLines remaining seconds

  | otherwise = current : editLines remaining seconds

correctTimeString :: String -> Int -> String

correctTimeString s 0 = s

correctTimeString s seconds =

    let start = vttTimeToUTCTime (head (words s))

        end = vttTimeToUTCTime (last (words s))

        delta = diffUTCTime end start

        correctStart = addOffset start seconds

    in uTCTimeToVttTime correctStart ++ " --> " ++ uTCTimeToVttTime (addUTCTime delta correctStart)

addOffset :: UTCTime -> Int -> UTCTime

addOffset time seconds = addUTCTime (fromInteger (toInteger (seconds)) :: NominalDiffTime) time

uTCTimeToVttTime :: UTCTime -> String

uTCTimeToVttTime t = formatTime defaultTimeLocale "%T%03Q" t

vttTimeToUTCTime :: String -> UTCTime

vttTimeToUTCTime timeString =

    parseTimeOrError False defaultTimeLocale "%T%Q" timeString :: UTCTime