CommonMark reader: support `gfm_auto_identifiers`.

Added `Ext_gfm_auto_identifiers`: new constructor for `Extension`
in `Text.Pandoc.Extensions` [API change].

Use this in githubExtensions.

Closes #2821.

2 files changed, 34 insertions, 1 deletions

diff --git a/src/Text/Pandoc/Extensions.hs b/src/Text/Pandoc/Extensions.hs
index 9e49c5907..e6a3ca044 100644
--- a/src/Text/Pandoc/Extensions.hs
+++ b/src/Text/Pandoc/Extensions.hs
@@ -133,6 +133,8 @@ data Extension =
     | Ext_abbreviations       -- ^ PHP markdown extra abbreviation definitions
     | Ext_emoji               -- ^ Support emoji like :smile:
     | Ext_auto_identifiers    -- ^ Automatic identifiers for headers
+    | Ext_gfm_auto_identifiers  -- ^ Automatic identifiers for headers,
+                                -- using GitHub's method for generating identifiers
     | Ext_ascii_identifiers   -- ^ ascii-only identifiers for headers
     | Ext_header_attributes   -- ^ Explicit header attributes {#id .class k=v}
     | Ext_mmd_header_identifiers -- ^ Multimarkdown style header identifiers [myid]
@@ -237,7 +239,7 @@ githubMarkdownExtensions = extensionsFromList
   , Ext_pipe_tables
   , Ext_raw_html
   , Ext_fenced_code_blocks
-  , Ext_auto_identifiers
+  , Ext_gfm_auto_identifiers
   , Ext_ascii_identifiers
   , Ext_backtick_code_blocks
   , Ext_autolink_bare_uris
diff --git a/src/Text/Pandoc/Readers/CommonMark.hs b/src/Text/Pandoc/Readers/CommonMark.hs
index 4b24ea374..ea9d696cb 100644
--- a/src/Text/Pandoc/Readers/CommonMark.hs
+++ b/src/Text/Pandoc/Readers/CommonMark.hs
@@ -33,15 +33,23 @@ module Text.Pandoc.Readers.CommonMark (readCommonMark)
 where
 
 import CMarkGFM
+import Control.Monad.State
+import Data.Char (isLetter, isAlphaNum, isSpace, toLower)
 import Data.List (groupBy)
 import Data.Text (Text, unpack)
+import qualified Data.Map as Map
 import Text.Pandoc.Class (PandocMonad)
 import Text.Pandoc.Definition
 import Text.Pandoc.Options
+import Text.Pandoc.Shared (stringify)
+import Text.Pandoc.Walk (walkM)
 
 -- | Parse a CommonMark formatted string into a 'Pandoc' structure.
 readCommonMark :: PandocMonad m => ReaderOptions -> Text -> m Pandoc
 readCommonMark opts s = return $
+  (if enabled Ext_gfm_auto_identifiers
+      then addHeaderIdentifiers
+      else id) $
   nodeToPandoc $ commonmarkToNode opts' exts s
   where opts' = [ optSmart | enabled Ext_smart ]
         exts = [ extStrikethrough | enabled Ext_strikeout ] ++
@@ -49,6 +57,29 @@ readCommonMark opts s = return $
                [ extAutolink | enabled Ext_autolink_bare_uris ]
         enabled x = extensionEnabled x (readerExtensions opts)
 
+addHeaderIdentifiers :: Pandoc -> Pandoc
+addHeaderIdentifiers doc = evalState (walkM addHeaderId doc) mempty
+
+addHeaderId :: Block -> State (Map.Map String Int) Block
+addHeaderId (Header lev (_,classes,kvs) ils) = do
+  idmap <- get
+  let ident = toIdent ils
+  ident' <- case Map.lookup ident idmap of
+                 Nothing -> do
+                   put (Map.insert ident 1 idmap)
+                   return ident
+                 Just i -> do
+                   put (Map.adjust (+ 1) ident idmap)
+                   return (ident ++ "-" ++ show i)
+  return $ Header lev (ident',classes,kvs) ils
+addHeaderId x = return x
+
+toIdent :: [Inline] -> String
+toIdent =   map (\c -> if isSpace c then '-' else c)
+          . filter (\c -> isLetter c || isAlphaNum c || isSpace c ||
+                           c == '_' || c == '-')
+          . map toLower . stringify
+
 nodeToPandoc :: Node -> Pandoc
 nodeToPandoc (Node _ DOCUMENT nodes) =
   Pandoc nullMeta $ foldr addBlock [] nodes