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.

4 files changed, 29 insertions, 41 deletions

diff --git a/MANUAL.txt b/MANUAL.txt
index 2edef40d9..865fa8cb1 100644
--- a/MANUAL.txt
+++ b/MANUAL.txt
@@ -1107,14 +1107,17 @@ header when requesting a document from a URL:
 
 `--number-offset=`*NUMBER*[`,`*NUMBER*`,`*...*]
 
-:   Offset for section headings in HTML output (ignored in other
-    output formats).  The first number is added to the section number for
-    top-level headings, the second for second-level headings, and so on.
-    So, for example, if you want the first top-level heading in your
-    document to be numbered "6", specify `--number-offset=5`.
-    If your document starts with a level-2 heading which you want to
-    be numbered "1.5", specify `--number-offset=1,4`.
-    Offsets are 0 by default.  Implies `--number-sections`.
+:   Offsets for section heading numbers. The first number is added
+    to the section number for level-1 headings, the second for
+    level-2 headings, and so on. So, for example, if you
+    want the first level-1 heading in your document to be
+    numbered "6" instead of "1", specify `--number-offset=5`.
+    If your document starts with a level-2 heading which you want
+    to be numbered "1.5", specify `--number-offset=1,4`.
+    `--number-offset` only directly affects the number of the
+    first section heading in a document; subsequent numbers
+    increment in the normal way. Implies `--number-sections`.
+    Currently this feature only affects HTML output.
 
 `--listings[=true|false]`
 
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
diff --git a/test/command/5071.md b/test/command/5071.md
index 6bbeb85fc..0347d3c46 100644
--- a/test/command/5071.md
+++ b/test/command/5071.md
@@ -49,8 +49,8 @@ giving numbers like 0.1, when `--number-offset` is used:
 ^D
 <h2 data-number="2.3" id="first-section"><span
 class="header-section-number">2.3</span> First section</h2>
-<h3 data-number="2.3.3" id="subhead"><span
-class="header-section-number">2.3.3</span> Subhead</h3>
+<h3 data-number="2.3.1" id="subhead"><span
+class="header-section-number">2.3.1</span> Subhead</h3>
 
 ```
 
@@ -62,7 +62,7 @@ class="header-section-number">2.3.3</span> Subhead</h3>
 ^D
 <h2 data-number="0.3" id="first-section"><span
 class="header-section-number">0.3</span> First section</h2>
-<h3 data-number="0.3.3" id="subhead"><span
-class="header-section-number">0.3.3</span> Subhead</h3>
+<h3 data-number="0.3.1" id="subhead"><span
+class="header-section-number">0.3.1</span> Subhead</h3>
 
 ```