{-# LANGUAGE CPP               #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE ViewPatterns      #-}
-- |
-- Module      :  Documentation.Haddock.Parser
-- Copyright   :  (c) Mateusz Kowalczyk 2013-2014,
--                    Simon Hengel      2013
-- License     :  BSD-like
--
-- Maintainer  :  haddock@projects.haskell.org
-- Stability   :  experimental
-- Portability :  portable
--
-- Parser used for Haddock comments. For external users of this
-- library, the most commonly used combination of functions is going
-- to be
--
-- @'toRegular' . '_doc' . 'parseParas'@
module Documentation.Haddock.Parser ( parseString, parseParas
                                    , overIdentifier, toRegular, Identifier
                                    ) where

import           Control.Applicative
import           Control.Arrow (first)
import           Control.Monad
import qualified Data.ByteString.Char8 as BS
import           Data.Char (chr, isUpper, isAlpha, isAlphaNum)
import           Data.List (stripPrefix, intercalate, unfoldr, elemIndex)
import           Data.Maybe (fromMaybe, mapMaybe)
import           Data.Monoid
import qualified Data.Set as Set
import           Documentation.Haddock.Doc
import           Documentation.Haddock.Parser.Monad hiding (take, endOfLine)
import           Documentation.Haddock.Parser.Util
import           Documentation.Haddock.Types
import           Documentation.Haddock.Utf8
import           Prelude hiding (takeWhile)
import qualified Prelude as P

#if MIN_VERSION_base(4,9,0)
import           Text.Read.Lex                      (isSymbolChar)
#else
import           Data.Char                          (GeneralCategory (..),
                                                     generalCategory)
#endif

-- $setup
-- >>> :set -XOverloadedStrings

#if !MIN_VERSION_base(4,9,0)
-- inlined from base-4.10.0.0
isSymbolChar :: Char -> Bool
isSymbolChar c = not (isPuncChar c) && case generalCategory c of
    MathSymbol           -> True
    CurrencySymbol       -> True
    ModifierSymbol       -> True
    OtherSymbol          -> True
    DashPunctuation      -> True
    OtherPunctuation     -> not (c `elem` ("'\"" :: String))
    ConnectorPunctuation -> c /= '_'
    _                    -> False
  where
    -- | The @special@ character class as defined in the Haskell Report.
    isPuncChar :: Char -> Bool
    isPuncChar = (`elem` (",;()[]{}`" :: String))
#endif

-- | Identifier string surrounded with opening and closing quotes/backticks.
type Identifier = (Char, String, Char)

-- | Drops the quotes/backticks around all identifiers, as if they
-- were valid but still 'String's.
toRegular :: DocH mod Identifier -> DocH mod String
toRegular = fmap (\(_, x, _) -> x)

-- | Maps over 'DocIdentifier's over 'String' with potentially failing
-- conversion using user-supplied function. If the conversion fails,
-- the identifier is deemed to not be valid and is treated as a
-- regular string.
overIdentifier :: (String -> Maybe a)
               -> DocH mod Identifier
               -> DocH mod a
overIdentifier f d = g d
  where
    g (DocIdentifier (o, x, e)) = case f x of
      Nothing -> DocString $ o : x ++ [e]
      Just x' -> DocIdentifier x'
    g DocEmpty = DocEmpty
    g (DocAppend x x') = DocAppend (g x) (g x')
    g (DocString x) = DocString x
    g (DocParagraph x) = DocParagraph $ g x
    g (DocIdentifierUnchecked x) = DocIdentifierUnchecked x
    g (DocModule x) = DocModule x
    g (DocWarning x) = DocWarning $ g x
    g (DocEmphasis x) = DocEmphasis $ g x
    g (DocMonospaced x) = DocMonospaced $ g x
    g (DocBold x) = DocBold $ g x
    g (DocUnorderedList x) = DocUnorderedList $ fmap g x
    g (DocOrderedList x) = DocOrderedList $ fmap g x
    g (DocDefList x) = DocDefList $ fmap (\(y, z) -> (g y, g z)) x
    g (DocCodeBlock x) = DocCodeBlock $ g x
    g (DocHyperlink x) = DocHyperlink x
    g (DocPic x) = DocPic x
    g (DocMathInline x) = DocMathInline x
    g (DocMathDisplay x) = DocMathDisplay x
    g (DocAName x) = DocAName x
    g (DocProperty x) = DocProperty x
    g (DocExamples x) = DocExamples x
    g (DocHeader (Header l x)) = DocHeader . Header l $ g x
    g (DocTable (Table h b)) = DocTable (Table (map (fmap g) h) (map (fmap g) b))

parse :: Parser a -> BS.ByteString -> (ParserState, a)
parse p = either err id . parseOnly (p <* endOfInput)
  where
    err = error . ("Haddock.Parser.parse: " ++)

-- | Main entry point to the parser. Appends the newline character
-- to the input string.
parseParas :: String -- ^ String to parse
           -> MetaDoc mod Identifier
parseParas input = case parseParasState input of
  (state, a) -> MetaDoc { _meta = Meta { _version = parserStateSince state }
                        , _doc = a
                        }

parseParasState :: String -> (ParserState, DocH mod Identifier)
parseParasState =
    parse (p <* skipSpace) . encodeUtf8 . (++ "\n") . filter (/= '\r')
  where
    p :: Parser (DocH mod Identifier)
    p = docConcat <$> paragraph `sepBy` many (skipHorizontalSpace *> "\n")

parseParagraphs :: String -> Parser (DocH mod Identifier)
parseParagraphs input = case parseParasState input of
  (state, a) -> setParserState state >> return a

-- | Parse a text paragraph. Actually just a wrapper over 'parseStringBS' which
-- drops leading whitespace and encodes the string to UTF8 first.
parseString :: String -> DocH mod Identifier
parseString = parseStringBS . encodeUtf8 . dropWhile isSpace . filter (/= '\r')

parseStringBS :: BS.ByteString -> DocH mod Identifier
parseStringBS = snd . parse p
  where
    p :: Parser (DocH mod Identifier)
    p = docConcat <$> many (monospace <|> anchor <|> identifier <|> moduleName
                            <|> picture <|> mathDisplay <|> mathInline
                            <|> markdownImage
                            <|> hyperlink <|> bold
                            <|> emphasis <|> encodedChar <|> string'
                            <|> skipSpecialChar)

-- | Parses and processes
-- <https://en.wikipedia.org/wiki/Numeric_character_reference Numeric character references>
--
-- >>> parseString "&#65;"
-- DocString "A"
encodedChar :: Parser (DocH mod a)
encodedChar = "&#" *> c <* ";"
  where
    c = DocString . return . chr <$> num
    num = hex <|> decimal
    hex = ("x" <|> "X") *> hexadecimal

-- | List of characters that we use to delimit any special markup.
-- Once we have checked for any of these and tried to parse the
-- relevant markup, we can assume they are used as regular text.
specialChar :: [Char]
specialChar = "_/<@\"&'`# "

-- | Plain, regular parser for text. Called as one of the last parsers
-- to ensure that we have already given a chance to more meaningful parsers
-- before capturing their characers.
string' :: Parser (DocH mod a)
string' = DocString . unescape . decodeUtf8 <$> takeWhile1_ (notInClass specialChar)
  where
    unescape "" = ""
    unescape ('\\':x:xs) = x : unescape xs
    unescape (x:xs) = x : unescape xs

-- | Skips a single special character and treats it as a plain string.
-- This is done to skip over any special characters belonging to other
-- elements but which were not deemed meaningful at their positions.
skipSpecialChar :: Parser (DocH mod a)
skipSpecialChar = DocString . return <$> satisfy (inClass specialChar)

-- | Emphasis parser.
--
-- >>> parseString "/Hello world/"
-- DocEmphasis (DocString "Hello world")
emphasis :: Parser (DocH mod Identifier)
emphasis = DocEmphasis . parseStringBS <$>
  mfilter ('\n' `BS.notElem`) ("/" *> takeWhile1_ (/= '/') <* "/")

-- | Bold parser.
--
-- >>> parseString "__Hello world__"
-- DocBold (DocString "Hello world")
bold :: Parser (DocH mod Identifier)
bold = DocBold . parseStringBS <$> disallowNewline ("__" *> takeUntil "__")

disallowNewline :: Parser BS.ByteString -> Parser BS.ByteString
disallowNewline = mfilter ('\n' `BS.notElem`)

-- | Like `takeWhile`, but unconditionally take escaped characters.
takeWhile_ :: (Char -> Bool) -> Parser BS.ByteString
takeWhile_ p = scan False p_
  where
    p_ escaped c
      | escaped = Just False
      | not $ p c = Nothing
      | otherwise = Just (c == '\\')

-- | Like `takeWhile1`, but unconditionally take escaped characters.
takeWhile1_ :: (Char -> Bool) -> Parser BS.ByteString
takeWhile1_ = mfilter (not . BS.null) . takeWhile_

-- | Text anchors to allow for jumping around the generated documentation.
--
-- >>> parseString "#Hello world#"
-- DocAName "Hello world"
anchor :: Parser (DocH mod a)
anchor = DocAName . decodeUtf8 <$>
         disallowNewline ("#" *> takeWhile1_ (/= '#') <* "#")

-- | Monospaced strings.
--
-- >>> parseString "@cruel@"
-- DocMonospaced (DocString "cruel")
monospace :: Parser (DocH mod Identifier)
monospace = DocMonospaced . parseStringBS
            <$> ("@" *> takeWhile1_ (/= '@') <* "@")

-- | Module names.
--
-- Note that we allow '#' and '\' to support anchors (old style anchors are of
-- the form "SomeModule\#anchor").
moduleName :: Parser (DocH mod a)
moduleName = DocModule <$> (char '"' *> modid <* char '"')
  where
    modid = intercalate "." <$> conid `sepBy1` "."
    conid = (:)
      <$> satisfyUnicode (\c -> isAlpha c && isUpper c)
      <*> many (satisfyUnicode conChar <|> char '\\' <|> char '#')

    conChar c = isAlphaNum c || c == '_'

-- | Picture parser, surrounded by \<\< and \>\>. It's possible to specify
-- a title for the picture.
--
-- >>> parseString "<<hello.png>>"
-- DocPic (Picture {pictureUri = "hello.png", pictureTitle = Nothing})
-- >>> parseString "<<hello.png world>>"
-- DocPic (Picture {pictureUri = "hello.png", pictureTitle = Just "world"})
picture :: Parser (DocH mod a)
picture = DocPic . makeLabeled Picture . decodeUtf8
          <$> disallowNewline ("<<" *> takeUntil ">>")

-- | Inline math parser, surrounded by \\( and \\).
--
-- >>> parseString "\\(\\int_{-\\infty}^{\\infty} e^{-x^2/2} = \\sqrt{2\\pi}\\)"
-- DocMathInline "\\int_{-\\infty}^{\\infty} e^{-x^2/2} = \\sqrt{2\\pi}"
mathInline :: Parser (DocH mod a)
mathInline = DocMathInline . decodeUtf8
             <$> disallowNewline  ("\\(" *> takeUntil "\\)")

-- | Display math parser, surrounded by \\[ and \\].
--
-- >>> parseString "\\[\\int_{-\\infty}^{\\infty} e^{-x^2/2} = \\sqrt{2\\pi}\\]"
-- DocMathDisplay "\\int_{-\\infty}^{\\infty} e^{-x^2/2} = \\sqrt{2\\pi}"
mathDisplay :: Parser (DocH mod a)
mathDisplay = DocMathDisplay . decodeUtf8
              <$> ("\\[" *> takeUntil "\\]")

markdownImage :: Parser (DocH mod a)
markdownImage = fromHyperlink <$> ("!" *> linkParser)
  where
    fromHyperlink (Hyperlink url label) = DocPic (Picture url label)

-- | Paragraph parser, called by 'parseParas'.
paragraph :: Parser (DocH mod Identifier)
paragraph = examples <|> table <|> do
  indent <- takeIndent
  choice
    [ since
    , unorderedList indent
    , orderedList indent
    , birdtracks
    , codeblock
    , property
    , header
    , textParagraphThatStartsWithMarkdownLink
    , definitionList indent
    , docParagraph <$> textParagraph
    ]

-- | Provides support for grid tables.
--
-- Tables are composed by an optional header and body. The header is composed by
-- a single row. The body is composed by a non-empty list of rows.
--
-- Example table with header:
--
-- > +----------+----------+
-- > | /32bit/  |   64bit  |
-- > +==========+==========+
-- > |  0x0000  | @0x0000@ |
-- > +----------+----------+
--
-- Algorithms loosely follows ideas in
-- http://docutils.sourceforge.net/docutils/parsers/rst/tableparser.py
--
table :: Parser (DocH mod Identifier)
table = do
    -- first we parse the first row, which determines the width of the table
    firstRow <- parseFirstRow
    let len = BS.length firstRow

    -- then we parse all consequtive rows starting and ending with + or |,
    -- of the width `len`.
    restRows <- many (parseRestRows len)

    -- Now we gathered the table block, the next step is to split the block
    -- into cells.
    DocTable <$> tableStepTwo len (firstRow : restRows)
  where
    parseFirstRow :: Parser BS.ByteString
    parseFirstRow = do
        skipHorizontalSpace
        -- upper-left corner is +
        c <- char '+'
        cs <- many1 (char '-' <|> char '+')

        -- upper right corner is + too
        guard (last cs == '+')

        -- trailing space
        skipHorizontalSpace
        _ <- char '\n'

        return (BS.cons c $ BS.pack cs)

    parseRestRows :: Int -> Parser BS.ByteString
    parseRestRows l = do
        skipHorizontalSpace

        c <- char '|' <|> char '+'
        bs <- scan (l - 2) predicate
        c2 <- char '|' <|> char '+'

        -- trailing space
        skipHorizontalSpace
        _ <- char '\n'

        return (BS.cons c (BS.snoc bs c2))
      where
        predicate n c
            | n <= 0    = Nothing
            | c == '\n' = Nothing
            | otherwise = Just (n - 1)

-- Second step searchs for row of '+' and '=' characters, records it's index
-- and changes to '=' to '-'.
tableStepTwo
    :: Int              -- ^ width
    -> [BS.ByteString]  -- ^ rows
    -> Parser (Table (DocH mod Identifier))
tableStepTwo width = go 0 [] where
    go _ left [] = tableStepThree width (reverse left) Nothing
    go n left (r : rs)
        | BS.all (`elem` ['+', '=']) r =
            tableStepThree width (reverse left ++ r' : rs) (Just n)
        | otherwise =
            go (n + 1) (r :  left) rs
      where
        r' = BS.map (\c -> if c == '=' then '-' else c) r

-- Third step recognises cells in the table area, returning a list of TC, cells.
tableStepThree
    :: Int              -- ^ width
    -> [BS.ByteString]  -- ^ rows
    -> Maybe Int        -- ^ index of header separator
    -> Parser (Table (DocH mod Identifier))
tableStepThree width rs hdrIndex = do
    cells <- loop (Set.singleton (0, 0))
    tableStepFour rs hdrIndex cells
  where
    height = length rs

    loop :: Set.Set (Int, Int) -> Parser [TC]
    loop queue = case Set.minView queue of
        Nothing -> return []
        Just ((y, x), queue')
            | y + 1 >= height || x + 1 >= width -> loop queue'
            | otherwise -> case scanRight x y of
                Nothing -> loop queue'
                Just (x2, y2) -> do
                    let tc = TC y x y2 x2
                    fmap (tc :) $ loop $ queue' `Set.union` Set.fromList
                        [(y, x2), (y2, x), (y2, x2)]

    -- scan right looking for +, then try scan down
    --
    -- do we need to record + saw on the way left and down?
    scanRight :: Int -> Int -> Maybe (Int, Int)
    scanRight x y = go (x + 1) where
        bs = rs !! y
        go x' | x' >= width           = fail "overflow right "
              | BS.index bs x' == '+' = scanDown x y x' <|> go (x' + 1)
              | BS.index bs x' == '-' = go (x' + 1)
              | otherwise             = fail $ "not a border (right) " ++ show (x,y,x')

    -- scan down looking for +
    scanDown :: Int -> Int -> Int -> Maybe (Int, Int)
    scanDown x y x2 = go (y + 1) where
        go y' | y' >= height                  = fail "overflow down"
              | BS.index (rs !! y') x2 == '+' = scanLeft x y x2 y' <|> go (y' + 1)
              | BS.index (rs !! y') x2 == '|' = go (y' + 1)
              | otherwise                     = fail $ "not a border (down) " ++ show (x,y,x2,y')

    -- check that at y2 x..x2 characters are '+' or '-'
    scanLeft :: Int -> Int -> Int -> Int -> Maybe (Int, Int)
    scanLeft x y x2 y2
        | all (\x' -> BS.index bs x' `elem` ['+', '-']) [x..x2] = scanUp x y x2 y2
        | otherwise                                             = fail $ "not a border (left) " ++ show (x,y,x2,y2)
      where
        bs = rs !! y2

    -- check that at y2 x..x2 characters are '+' or '-'
    scanUp :: Int -> Int -> Int -> Int -> Maybe (Int, Int)
    scanUp x y x2 y2
        | all (\y' -> BS.index (rs !! y') x `elem` ['+', '|']) [y..y2] = return (x2, y2)
        | otherwise                                                    = fail $ "not a border (up) " ++ show (x,y,x2,y2)

-- | table cell: top left bottom right
data TC = TC !Int !Int !Int !Int
  deriving Show

tcXS :: TC -> [Int]
tcXS (TC _ x _ x2) = [x, x2]

tcYS :: TC -> [Int]
tcYS (TC y _ y2 _) = [y, y2]

-- | Fourth step. Given the locations of cells, forms 'Table' structure.
tableStepFour :: [BS.ByteString] -> Maybe Int -> [TC] -> Parser (Table (DocH mod Identifier))
tableStepFour rs hdrIndex cells =  case hdrIndex of
    Nothing -> return $ Table [] rowsDoc
    Just i  -> case elemIndex i yTabStops of
        Nothing -> return $ Table [] rowsDoc
        Just i' -> return $ uncurry Table $ splitAt i' rowsDoc
  where
    xTabStops = sortNub $ concatMap tcXS cells
    yTabStops = sortNub $ concatMap tcYS cells

    sortNub :: Ord a => [a] -> [a]
    sortNub = Set.toList . Set.fromList

    init' :: [a] -> [a]
    init' []       = []
    init' [_]      = []
    init' (x : xs) = x : init' xs

    rowsDoc = (fmap . fmap) parseStringBS rows

    rows = map makeRow (init' yTabStops)
      where
        makeRow y = TableRow $ mapMaybe (makeCell y) cells
        makeCell y (TC y' x y2 x2)
            | y /= y' = Nothing
            | otherwise = Just $ TableCell xts yts (extract (x + 1) (y + 1) (x2 - 1) (y2 - 1))
          where
            xts = length $ P.takeWhile (< x2) $ dropWhile (< x) xTabStops
            yts = length $ P.takeWhile (< y2) $ dropWhile (< y) yTabStops

    -- extract cell contents given boundaries
    extract :: Int -> Int -> Int -> Int -> BS.ByteString
    extract x y x2 y2 = BS.intercalate "\n"
        [ BS.take (x2 - x + 1) $ BS.drop x $ rs !! y'
        | y' <- [y .. y2]
        ]

-- | Parse \@since annotations.
since :: Parser (DocH mod a)
since = ("@since " *> version <* skipHorizontalSpace <* endOfLine) >>= setSince >> return DocEmpty
  where
    version = decimal `sepBy1'` "."

-- | Headers inside the comment denoted with @=@ signs, up to 6 levels
-- deep.
--
-- >>> snd <$> parseOnly header "= Hello"
-- Right (DocHeader (Header {headerLevel = 1, headerTitle = DocString "Hello"}))
-- >>> snd <$> parseOnly header "== World"
-- Right (DocHeader (Header {headerLevel = 2, headerTitle = DocString "World"}))
header :: Parser (DocH mod Identifier)
header = do
  let psers = map (string . encodeUtf8 . concat . flip replicate "=") [6, 5 .. 1]
      pser = foldl1 (<|>) psers
  delim <- decodeUtf8 <$> pser
  line <- skipHorizontalSpace *> nonEmptyLine >>= return . parseString
  rest <- paragraph <|> return DocEmpty
  return $ DocHeader (Header (length delim) line) `docAppend` rest

textParagraph :: Parser (DocH mod Identifier)
textParagraph = parseString . intercalate "\n" <$> many1 nonEmptyLine

textParagraphThatStartsWithMarkdownLink :: Parser (DocH mod Identifier)
textParagraphThatStartsWithMarkdownLink = docParagraph <$> (docAppend <$> markdownLink <*> optionalTextParagraph)
  where
    optionalTextParagraph :: Parser (DocH mod Identifier)
    optionalTextParagraph = (docAppend <$> whitespace <*> textParagraph) <|> pure DocEmpty

    whitespace :: Parser (DocH mod a)
    whitespace = DocString <$> (f <$> takeHorizontalSpace <*> optional "\n")
      where
        f :: BS.ByteString -> Maybe BS.ByteString -> String
        f xs (fromMaybe "" -> x)
          | BS.null (xs <> x) = ""
          | otherwise = " "

-- | Parses unordered (bullet) lists.
unorderedList :: BS.ByteString -> Parser (DocH mod Identifier)
unorderedList indent = DocUnorderedList <$> p
  where
    p = ("*" <|> "-") *> innerList indent p

-- | Parses ordered lists (numbered or dashed).
orderedList :: BS.ByteString -> Parser (DocH mod Identifier)
orderedList indent = DocOrderedList <$> p
  where
    p = (paren <|> dot) *> innerList indent p
    dot = (decimal :: Parser Int) <* "."
    paren = "(" *> decimal <* ")"

-- | Generic function collecting any further lines belonging to the
-- list entry and recursively collecting any further lists in the
-- same paragraph. Usually used as
--
-- > someListFunction = listBeginning *> innerList someListFunction
innerList :: BS.ByteString -> Parser [DocH mod Identifier]
          -> Parser [DocH mod Identifier]
innerList indent item = do
  c <- takeLine
  (cs, items) <- more indent item
  let contents = docParagraph . parseString . dropNLs . unlines $ c : cs
  return $ case items of
    Left p -> [contents `docAppend` p]
    Right i -> contents : i

-- | Parses definition lists.
definitionList :: BS.ByteString -> Parser (DocH mod Identifier)
definitionList indent = DocDefList <$> p
  where
    p = do
      label <- "[" *> (parseStringBS <$> takeWhile1_ (notInClass "]\n")) <* ("]" <* optional ":")
      c <- takeLine
      (cs, items) <- more indent p
      let contents = parseString . dropNLs . unlines $ c : cs
      return $ case items of
        Left x -> [(label, contents `docAppend` x)]
        Right i -> (label, contents) : i

-- | Drops all trailing newlines.
dropNLs :: String -> String
dropNLs = reverse . dropWhile (== '\n') . reverse

-- | Main worker for 'innerList' and 'definitionList'.
-- We need the 'Either' here to be able to tell in the respective functions
-- whether we're dealing with the next list or a nested paragraph.
more :: Monoid a => BS.ByteString -> Parser a
     -> Parser ([String], Either (DocH mod Identifier) a)
more indent item = innerParagraphs indent
               <|> moreListItems indent item
               <|> moreContent indent item
               <|> pure ([], Right mempty)

-- | Used by 'innerList' and 'definitionList' to parse any nested paragraphs.
innerParagraphs :: BS.ByteString
                -> Parser ([String], Either (DocH mod Identifier) a)
innerParagraphs indent = (,) [] . Left <$> ("\n" *> indentedParagraphs indent)

-- | Attempts to fetch the next list if possibly. Used by 'innerList' and
-- 'definitionList' to recursively grab lists that aren't separated by a whole
-- paragraph.
moreListItems :: BS.ByteString -> Parser a
              -> Parser ([String], Either (DocH mod Identifier) a)
moreListItems indent item = (,) [] . Right <$> indentedItem
  where
    indentedItem = string indent *> skipSpace *> item

-- | Helper for 'innerList' and 'definitionList' which simply takes
-- a line of text and attempts to parse more list content with 'more'.
moreContent :: Monoid a => BS.ByteString -> Parser a
            -> Parser ([String], Either (DocH mod Identifier) a)
moreContent indent item = first . (:) <$> nonEmptyLine <*> more indent item

-- | Parses an indented paragraph.
-- The indentation is 4 spaces.
indentedParagraphs :: BS.ByteString -> Parser (DocH mod Identifier)
indentedParagraphs indent =
    (concat <$> dropFrontOfPara indent') >>= parseParagraphs
  where
    indent' = string $ BS.append indent "    "

-- | Grab as many fully indented paragraphs as we can.
dropFrontOfPara :: Parser BS.ByteString -> Parser [String]
dropFrontOfPara sp = do
  currentParagraph <- some (sp *> takeNonEmptyLine)
  followingParagraphs <-
    skipHorizontalSpace *> nextPar -- we have more paragraphs to take
    <|> skipHorizontalSpace *> nlList -- end of the ride, remember the newline
    <|> endOfInput *> return [] -- nothing more to take at all
  return (currentParagraph ++ followingParagraphs)
  where
    nextPar = (++) <$> nlList <*> dropFrontOfPara sp
    nlList = "\n" *> return ["\n"]

nonSpace :: BS.ByteString -> Parser BS.ByteString
nonSpace xs
  | not $ any (not . isSpace) $ decodeUtf8 xs = fail "empty line"
  | otherwise = return xs

-- | Takes a non-empty, not fully whitespace line.
--
--  Doesn't discard the trailing newline.
takeNonEmptyLine :: Parser String
takeNonEmptyLine = do
    (++ "\n") . decodeUtf8 <$> (takeWhile1 (/= '\n') >>= nonSpace) <* "\n"

-- | Takes indentation of first non-empty line.
--
-- More precisely: skips all whitespace-only lines and returns indentation
-- (horizontal space, might be empty) of that non-empty line.
takeIndent :: Parser BS.ByteString
takeIndent = do
  indent <- takeHorizontalSpace
  "\n" *> takeIndent <|> return indent

-- | Blocks of text of the form:
--
-- >> foo
-- >> bar
-- >> baz
--
birdtracks :: Parser (DocH mod a)
birdtracks = DocCodeBlock . DocString . intercalate "\n" . stripSpace <$> many1 line
  where
    line = skipHorizontalSpace *> ">" *> takeLine

stripSpace :: [String] -> [String]
stripSpace = fromMaybe <*> mapM strip'
  where
    strip' (' ':xs') = Just xs'
    strip' "" = Just ""
    strip' _  = Nothing

-- | Parses examples. Examples are a paragraph level entitity (separated by an empty line).
-- Consecutive examples are accepted.
examples :: Parser (DocH mod a)
examples = DocExamples <$> (many (skipHorizontalSpace *> "\n") *> go)
  where
    go :: Parser [Example]
    go = do
      prefix <- decodeUtf8 <$> takeHorizontalSpace <* ">>>"
      expr <- takeLine
      (rs, es) <- resultAndMoreExamples
      return (makeExample prefix expr rs : es)
      where
        resultAndMoreExamples :: Parser ([String], [Example])
        resultAndMoreExamples = moreExamples <|> result <|> pure ([], [])
          where
            moreExamples :: Parser ([String], [Example])
            moreExamples = (,) [] <$> go

            result :: Parser ([String], [Example])
            result = first . (:) <$> nonEmptyLine <*> resultAndMoreExamples

    makeExample :: String -> String -> [String] -> Example
    makeExample prefix expression res =
      Example (strip expression) result
      where
        result = map (substituteBlankLine . tryStripPrefix) res

        tryStripPrefix xs = fromMaybe xs (stripPrefix prefix xs)

        substituteBlankLine "<BLANKLINE>" = ""
        substituteBlankLine xs = xs

nonEmptyLine :: Parser String
nonEmptyLine = mfilter (any (not . isSpace)) takeLine

takeLine :: Parser String
takeLine = decodeUtf8 <$> takeWhile (/= '\n') <* endOfLine

endOfLine :: Parser ()
endOfLine = void "\n" <|> endOfInput

-- | Property parser.
--
-- >>> snd <$> parseOnly property "prop> hello world"
-- Right (DocProperty "hello world")
property :: Parser (DocH mod a)
property = DocProperty . strip . decodeUtf8 <$> ("prop>" *> takeWhile1 (/= '\n'))

-- |
-- Paragraph level codeblock. Anything between the two delimiting \@ is parsed
-- for markup.
codeblock :: Parser (DocH mod Identifier)
codeblock =
  DocCodeBlock . parseStringBS . dropSpaces
  <$> ("@" *> skipHorizontalSpace *> "\n" *> block' <* "@")
  where
    dropSpaces xs =
      let rs = decodeUtf8 xs
      in case splitByNl rs of
        [] -> xs
        ys -> case last ys of
          ' ':_ -> case mapM dropSpace ys of
            Nothing -> xs
            Just zs -> encodeUtf8 $ intercalate "\n" zs
          _ -> xs

    -- This is necessary because ‘lines’ swallows up a trailing newline
    -- and we lose information about whether the last line belongs to @ or to
    -- text which we need to decide whether we actually want to be dropping
    -- anything at all.
    splitByNl = unfoldr (\x -> case x of
                                 '\n':s -> Just (span (/= '\n') s)
                                 _      -> Nothing)
                . ('\n' :)

    dropSpace "" = Just ""
    dropSpace (' ':xs) = Just xs
    dropSpace _ = Nothing

    block' = scan False p
      where
        p isNewline c
          | isNewline && c == '@' = Nothing
          | isNewline && isSpace c = Just isNewline
          | otherwise = Just $ c == '\n'

hyperlink :: Parser (DocH mod a)
hyperlink = angleBracketLink <|> markdownLink <|> autoUrl

angleBracketLink :: Parser (DocH mod a)
angleBracketLink =
    DocHyperlink . makeLabeled Hyperlink . decodeUtf8
    <$> disallowNewline ("<" *> takeUntil ">")

markdownLink :: Parser (DocH mod a)
markdownLink = DocHyperlink <$> linkParser

linkParser :: Parser Hyperlink
linkParser = flip Hyperlink <$> label <*> (whitespace *> url)
  where
    label :: Parser (Maybe String)
    label = Just . strip . decode <$> ("[" *> takeUntil "]")

    whitespace :: Parser ()
    whitespace = skipHorizontalSpace <* optional ("\n" *> skipHorizontalSpace)

    url :: Parser String
    url = rejectWhitespace (decode <$> ("(" *> takeUntil ")"))

    rejectWhitespace :: MonadPlus m => m String -> m String
    rejectWhitespace = mfilter (all (not . isSpace))

    decode :: BS.ByteString -> String
    decode = removeEscapes . decodeUtf8

-- | Looks for URL-like things to automatically hyperlink even if they
-- weren't marked as links.
autoUrl :: Parser (DocH mod a)
autoUrl = mkLink <$> url
  where
    url = mappend <$> ("http://" <|> "https://" <|> "ftp://") <*> takeWhile1 (not . isSpace)
    mkLink :: BS.ByteString -> DocH mod a
    mkLink s = case unsnoc s of
      Just (xs, x) | inClass ",.!?" x -> DocHyperlink (Hyperlink (decodeUtf8 xs) Nothing) `docAppend` DocString [x]
      _ -> DocHyperlink (Hyperlink (decodeUtf8 s) Nothing)

-- | Parses strings between identifier delimiters. Consumes all input that it
-- deems to be valid in an identifier. Note that it simply blindly consumes
-- characters and does no actual validation itself.
parseValid :: Parser String
parseValid = p some
  where
    idChar = satisfyUnicode (\c -> isAlphaNum c || isSymbolChar c || c == '_')

    p p' = do
      vs <- p' idChar
      c <- peekChar'
      case c of
        '`' -> return vs
        '\'' -> (\x -> vs ++ "'" ++ x) <$> ("'" *> p many') <|> return vs
        _ -> fail "outofvalid"

-- | Parses identifiers with help of 'parseValid'. Asks GHC for
-- 'String' from the string it deems valid.
identifier :: Parser (DocH mod Identifier)
identifier = do
  o <- idDelim
  vid <- parseValid
  e <- idDelim
  return $ DocIdentifier (o, vid, e)
  where
    idDelim = satisfy (\c -> c == '\'' || c == '`')