    {-
    Copyright (C) 2012-2015 John MacFarlane <jgm@berkeley.edu>

    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 2 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program; if not, write to the Free Software
    Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
    -}

    {- |
       Module      : Text.Pandoc.Slides
       Copyright   : Copyright (C) 2012-2015 John MacFarlane
       License     : GNU GPL, version 2 or above

       Maintainer  : John MacFarlane <jgm@berkeley.edu>
       Stability   : alpha
       Portability : portable

    Utility functions for splitting documents into slides for slide
    show formats (dzslides, revealjs, s5, slidy, slideous, beamer).
    -}
    module Text.Pandoc.Slides ( getSlideLevel, prepSlides ) where
    import Text.Pandoc.Definition

    -- | Find level of header that starts slides (defined as the least header
    -- level that occurs before a non-header/non-hrule in the blocks).
    getSlideLevel :: [Block] -> Int
    getSlideLevel = go 6
      where go least (Header n _ _ : x : xs)
                     | n < least && nonHOrHR x = go n xs
                     | otherwise               = go least (x:xs)
            go least (_ : xs) = go least xs
            go least [] = least
            nonHOrHR (Header{}) = False
            nonHOrHR (HorizontalRule) = False
            nonHOrHR _ = True

    -- | Prepare a block list to be passed to hierarchicalize.
    prepSlides :: Int -> [Block] -> [Block]
    prepSlides slideLevel = ensureStartWithH . splitHrule . extractRefsHeader
      where splitHrule (HorizontalRule : Header n attr xs : ys)
                           | n == slideLevel = Header slideLevel attr xs : splitHrule ys
            splitHrule (HorizontalRule : xs) = Header slideLevel nullAttr [Str "\0"] :
                                               splitHrule xs
            splitHrule (x : xs)              = x : splitHrule xs
            splitHrule []                    = []
            extractRefsHeader bs             =
              case reverse bs of
                   (Div ("",["references"],[]) (Header n attrs xs : ys) : zs)
                     -> reverse zs ++ (Header n attrs xs : [Div ("",["references"],[]) ys])
                   _ -> bs
            ensureStartWithH bs@(Header n _ _:_)
                           | n <= slideLevel = bs
            ensureStartWithH bs              = Header slideLevel nullAttr [Str "\0"] : bs