    --------------------------------------------------------------------------------
    -- | Internal module to parse metadata
    module Hakyll.Core.Provider.Metadata
        ( loadMetadata
        , metadata
        , page

          -- This parser can be reused in some places
        , metadataKey
        ) where

    --------------------------------------------------------------------------------
    import           Control.Applicative
    import           Control.Arrow                 (second)
    import qualified Data.ByteString.Char8         as BC
    import           Data.List                     (intercalate)
    import qualified Data.Map                      as M
    import           System.IO                     as IO
    import           Text.Parsec                   ((<?>))
    import qualified Text.Parsec                   as P
    import           Text.Parsec.String            (Parser)

    --------------------------------------------------------------------------------
    import           Hakyll.Core.Identifier
    import           Hakyll.Core.Metadata
    import           Hakyll.Core.Provider.Internal
    import           Hakyll.Core.Util.Parser
    import           Hakyll.Core.Util.String

    --------------------------------------------------------------------------------
    loadMetadata :: Provider -> Identifier -> IO (Metadata, Maybe String)
    loadMetadata p identifier = do
        hasHeader  <- probablyHasMetadataHeader fp
        (md, body) <- if hasHeader
            then second Just <$> loadMetadataHeader fp
            else return (M.empty, Nothing)

        emd <- case mi of
            Nothing  -> return M.empty
            Just mi' -> loadMetadataFile $ resourceFilePath p mi'

        return (M.union md emd, body)
      where
        normal = setVersion Nothing identifier
        fp     = resourceFilePath p identifier
        mi     = M.lookup normal (providerFiles p) >>= resourceInfoMetadata

    --------------------------------------------------------------------------------
    loadMetadataHeader :: FilePath -> IO (Metadata, String)
    loadMetadataHeader fp = do
        contents <- readFile fp
        case P.parse page fp contents of
            Left err      -> error (show err)
            Right (md, b) -> return (M.fromList md, b)

    --------------------------------------------------------------------------------
    loadMetadataFile :: FilePath -> IO Metadata
    loadMetadataFile fp = do
        contents <- readFile fp
        case P.parse metadata fp contents of
            Left err  -> error (show err)
            Right md  -> return $ M.fromList md

    --------------------------------------------------------------------------------
    -- | Check if a file "probably" has a metadata header. The main goal of this is
    -- to exclude binary files (which are unlikely to start with "---").
    probablyHasMetadataHeader :: FilePath -> IO Bool
    probablyHasMetadataHeader fp = do
        handle <- IO.openFile fp IO.ReadMode
        bs     <- BC.hGet handle 1024
        IO.hClose handle
        return $ isMetadataHeader bs
      where
        isMetadataHeader bs =
            let pre = BC.takeWhile (\x -> x /= '\n' && x /= '\r') bs
            in  BC.length pre >= 3 && BC.all (== '-') pre

    --------------------------------------------------------------------------------
    -- | Space or tab, no newline
    inlineSpace :: Parser Char
    inlineSpace = P.oneOf ['\t', ' '] <?> "space"

    --------------------------------------------------------------------------------
    -- | Parse Windows newlines as well (i.e. "\n" or "\r\n")
    newline :: Parser String
    newline = P.string "\n" <|> P.string "\r\n"

    --------------------------------------------------------------------------------
    -- | Parse a single metadata field
    metadataField :: Parser (String, String)
    metadataField = do
        key <- metadataKey
        _   <- P.char ':'
        P.skipMany1 inlineSpace <?> "space followed by metadata for: " ++ key
        value     <- P.manyTill P.anyChar newline
        trailing' <- P.many trailing
        return (key, trim $ intercalate " " $ value : trailing')
      where
        trailing = P.many1 inlineSpace *> P.manyTill P.anyChar newline

    --------------------------------------------------------------------------------
    -- | Parse a metadata block
    metadata :: Parser [(String, String)]
    metadata = P.many metadataField

    --------------------------------------------------------------------------------
    -- | Parse a metadata block, including delimiters and trailing newlines
    metadataBlock :: Parser [(String, String)]
    metadataBlock = do
        open      <- P.many1 (P.char '-') <* P.many inlineSpace <* newline
        metadata' <- metadata
        _         <- P.choice $ map (P.string . replicate (length open)) ['-', '.']
        P.skipMany inlineSpace
        P.skipMany1 newline
        return metadata'

    --------------------------------------------------------------------------------
    -- | Parse a page consisting of a metadata header and a body
    page :: Parser ([(String, String)], String)
    page = do
        metadata' <- P.option [] metadataBlock
        body      <- P.many P.anyChar
        return (metadata', body)