Text.Pandoc.Shared: add `makeSectionsWithOffsets` [API change].

This is like `makeSections` but has an additional parameter
specifying number offsets, for use with the `--number-offset` option.

Use `makeSectionsWithOffsets` in HTML writer instead of ad hoc
and inefficient number-adjusting code.

Clarify MANUAL.txt: the `--number-offset` option should only
directly affect numbering of the first section heading in
a document; subsequent headings will increment normally.
Fix test output for #5071 to reflect this.

2 files changed, 14 insertions, 29 deletions

diff --git a/src/Text/Pandoc/Shared.hs b/src/Text/Pandoc/Shared.hs
index a2a39ac43..6aa8601f2 100644
--- a/src/Text/Pandoc/Shared.hs
+++ b/src/Text/Pandoc/Shared.hs
@@ -50,6 +50,7 @@ module Text.Pandoc.Shared (
                      linesToPara,
                      figureDiv,
                      makeSections,
+                     makeSectionsWithOffsets,
                      combineAttr,
                      uniqueIdent,
                      inlineListToIdentifier,
@@ -512,12 +513,20 @@ textToIdentifier exts =
 -- adjusted so that the lowest header level is n.
 -- (There may still be gaps in header level if the author leaves them.)
 makeSections :: Bool -> Maybe Int -> [Block] -> [Block]
-makeSections numbering mbBaseLevel bs =
-  S.evalState (go bs) []
+makeSections = makeSectionsWithOffsets []
+
+-- | Like 'makeSections', but with a parameter for number offsets
+-- (a list of 'Int's, the first of which is added to the level 1
+-- section number, the second to the level 2, and so on).
+makeSectionsWithOffsets :: [Int] -> Bool -> Maybe Int -> [Block] -> [Block]
+makeSectionsWithOffsets numoffsets numbering mbBaseLevel bs =
+  S.evalState (go bs) numoffsets
  where
   getLevel (Header level _ _) = Min level
   getLevel _ = Min 99
-  minLevel = getMin $ query getLevel bs
+  minLevel = if null numoffsets || all (== 0) numoffsets
+                then getMin $ query getLevel bs
+                else 1 -- see #5071, for backwards compatibility
   go :: [Block] -> S.State [Int] [Block]
   go (Header level (ident,classes,kvs) title':xs) = do
     lastnum <- S.get
diff --git a/src/Text/Pandoc/Writers/HTML.hs b/src/Text/Pandoc/Writers/HTML.hs
index 076f11d7a..916aee675 100644
--- a/src/Text/Pandoc/Writers/HTML.hs
+++ b/src/Text/Pandoc/Writers/HTML.hs
@@ -289,8 +289,8 @@ pandocToHtml opts (Pandoc meta blocks) = do
                           lookupMetaString "description" meta
   slideVariant <- gets stSlideVariant
   abstractTitle <- translateTerm Abstract
-  let sects = adjustNumbers opts $
-              makeSections (writerNumberSections opts) Nothing $
+  let sects = makeSectionsWithOffsets
+                (writerNumberOffset opts) (writerNumberSections opts) Nothing $
               if slideVariant == NoSlides
                  then blocks
                  else prepSlides slideLevel blocks
@@ -729,30 +729,6 @@ dimensionsToAttrList attr = go Width ++ go Height
                (Just x)         -> [("style", tshow dir <> ":" <> tshow x)]
                Nothing          -> []
 
-adjustNumbers :: WriterOptions -> [Block] -> [Block]
-adjustNumbers opts doc =
-  if all (==0) (writerNumberOffset opts)
-     then doc
-     else walk go doc
-  where
-   go (Div (ident,"section":classes,kvs) lst@(Header level _ _ : _)) =
-     Div (ident,"section":classes,map (fixnum level) kvs) lst
-   go (Header level (ident,classes,kvs) lst) =
-     Header level (ident,classes,map (fixnum level) kvs) lst
-   go x = x
-   fixnum level ("number",num) = ("number",
-                               showSecNum $ zipWith (+)
-                               (writerNumberOffset opts ++ repeat 0)
-                               (padTo level $
-                                map (fromMaybe 0 . safeRead) $
-                                T.split (=='.') num))
-   fixnum _ x = x
-   padTo n xs =
-     case n - length xs of
-       x | x > 0 -> replicate x 0 ++ xs
-         | otherwise -> xs
-   showSecNum = T.intercalate "." . map tshow
-
 blockToHtmlInner :: PandocMonad m => WriterOptions -> Block -> StateT WriterState m Html
 blockToHtmlInner opts (Plain lst) = inlineListToHtml opts lst
 blockToHtmlInner opts (Para lst) = do