EPUB: Correctly handle internal links.

Previously they were ignored.  Now all links are preserved,
but purely internal links are modified so that they point
to the proper place in the EPUB.

This is nontrivial, since the heading you refer to in your
markdown source with 'my-section-1' might end up as
'ch16.xhtml#my-section' in the EPUB.

Closes #76.

src/Text/Pandoc/Writers/EPUB.hs

@@ -40,13 +40,13 @@ import System.Time
 import Text.Pandoc.Shared hiding ( Element )
 import Text.Pandoc.Definition
 import Text.Pandoc.Generic
-import Control.Monad (liftM)
+import Control.Monad.State
 import Text.XML.Light hiding (ppTopElement)
 import Text.Pandoc.UUID
 import Text.Pandoc.Writers.HTML
 import Text.Pandoc.Writers.Markdown ( writePlain )
 import Data.Char ( toLower )
-import Network.URI ( unEscapeString, isRelativeReference )
+import Network.URI ( unEscapeString )
 
 -- | Produce an EPUB file from a Pandoc document.
 writeEPUB :: Maybe String   -- ^ EPUB stylesheet specified at command line
@@ -104,12 +104,16 @@ writeEPUB mbStylesheet opts doc@(Pandoc meta _) = do
   -- body pages
   let isH1 (Header 1 _) = True
       isH1 _            = False
-  let h1Indices = dropWhile (== 0) $ findIndices isH1 blocks
-  let chunks = splitByIndices h1Indices blocks
+  -- internal reference IDs change when we chunk the file,
+  -- so the next two lines fix that:
+  let reftable = correlateRefs blocks
+  let blocks' = replaceRefs reftable blocks
+  let h1Indices = dropWhile (== 0) $ findIndices isH1 blocks'
+  let chunks = splitByIndices h1Indices blocks'
   let titleize (Header 1 xs : ys) = Pandoc meta{docTitle = xs} ys
       titleize xs                 = Pandoc meta xs
-  let chapToHtml = writeHtmlString opts'{ writerTemplate = pageTemplate }
   let chapters = map titleize chunks
+  let chapToHtml = writeHtmlString opts'{ writerTemplate = pageTemplate }
   let chapterToEntry :: Int -> Pandoc -> Entry
       chapterToEntry num chap = mkEntry ("ch" ++ show num ++ ".xhtml") $
                                    fromString $ chapToHtml chap
@@ -271,7 +275,6 @@ transformInlines (MathML _) _ _ (x@(Math _ _) : xs) = do
       result = if "<math" `isPrefixOf` mathml then inOps else mathml
   return $ RawInline "html" result : xs
 transformInlines _ _ _ (RawInline _ _ : xs) = return $ Str "" : xs
-transformInlines _ _ _ (Link lab (src,_) : xs) | isRelativeReference src = return $ lab ++ xs
 transformInlines _ _ _ xs = return xs
 
 transformBlock :: Block -> Block
@@ -305,3 +308,49 @@ imageTypeOf x = case drop 1 (map toLower (takeExtension x)) of
                      "svg"       -> Just "image/svg+xml"
                      _           -> Nothing
 
+
+data IdentState = IdentState{
+       chapterNumber :: Int,
+       runningIdents :: [String],
+       chapterIdents :: [String],
+       identTable    :: [(String,String)]
+       } deriving (Read, Show)
+
+-- Go through a block list and construct a table
+-- correlating the automatically constructed references
+-- that would be used in a normal pandoc document with
+-- new URLs to be used in the EPUB.  For example, what
+-- was "header-1" might turn into "ch6.xhtml#header".
+correlateRefs :: [Block] -> [(String,String)]
+correlateRefs bs = identTable $ execState (mapM_ go bs)
+                                IdentState{ chapterNumber = 0
+                                          , runningIdents = []
+                                          , chapterIdents = []
+                                          , identTable = [] }
+ where go :: Block -> State IdentState ()
+       go (Header n ils) = do
+          when (n == 1) $
+              modify $ \s -> s{ chapterNumber = chapterNumber s + 1
+                              , chapterIdents = [] }
+          st <- get
+          let runningid = uniqueIdent ils (runningIdents st)
+          let chapid    = if n == 1
+                             then Nothing
+                             else Just $ uniqueIdent ils (chapterIdents st)
+          modify $ \s -> s{ runningIdents = runningid : runningIdents st
+                          , chapterIdents = maybe (chapterIdents st)
+                                              (: chapterIdents st) chapid
+                          , identTable = (runningid, "ch" ++ show (chapterNumber st) ++
+                              ".xhtml" ++ maybe "" ('#':) chapid) : identTable st
+                           }
+       go _ = return ()
+
+-- Replace internal link references using the table produced
+-- by correlateRefs.
+replaceRefs :: [(String,String)] -> [Block] -> [Block]
+replaceRefs refTable = bottomUp replaceOneRef
+  where replaceOneRef x@(Link lab ('#':xs,tit)) =
+          case lookup xs refTable of
+                Just url -> Link lab (url,tit)
+                Nothing  -> x
+        replaceOneRef x = x