Implement multicolumn support for slide formats.

The structure expected is:

    <div class="columns">
      <div class="column" width="40%">
        contents...
      </div>
      <div class="column" width="60%">
        contents...
      </div>
    </div>

Support has been added for beamer and all HTML slide formats.

Closes #1710.

Note:  later we could add a more elegant way to create
this structure in Markdown than to use raw HTML div elements.
This would come for free with a "native div syntax" (#168).
Or we could devise something specific to slides

2 files changed, 28 insertions, 3 deletions

diff --git a/src/Text/Pandoc/Writers/HTML.hs b/src/Text/Pandoc/Writers/HTML.hs
index b899ce96a..c73af4604 100644
--- a/src/Text/Pandoc/Writers/HTML.hs
+++ b/src/Text/Pandoc/Writers/HTML.hs
@@ -640,8 +640,14 @@ blockToHtml opts (LineBlock lns) =
     let lf = preEscapedString "\n"
     htmlLines <- mconcat . intersperse lf <$> mapM (inlineListToHtml opts) lns
     return $ H.div ! A.class_ "line-block" $ htmlLines
-blockToHtml opts (Div attr@(ident, classes, kvs) bs) = do
+blockToHtml opts (Div attr@(ident, classes, kvs') bs) = do
   html5 <- gets stHtml5
+  let kvs = kvs' ++
+        if "column" `elem` classes
+           then let w = fromMaybe "48%" (lookup "width" kvs')
+                in  [("style", "width:" ++ w ++ ";min-width:" ++ w ++
+                      ";vertical-align:top;")]
+           else []
   let speakerNotes = "notes" `elem` classes
   -- we don't want incremental output inside speaker notes, see #1394
   let opts' = if speakerNotes then opts{ writerIncremental = False } else opts
diff --git a/src/Text/Pandoc/Writers/LaTeX.hs b/src/Text/Pandoc/Writers/LaTeX.hs
index fcc5ad1c6..4a81cd245 100644
--- a/src/Text/Pandoc/Writers/LaTeX.hs
+++ b/src/Text/Pandoc/Writers/LaTeX.hs
@@ -452,7 +452,25 @@ blockToLaTeX (Div (identifier,classes,kvs) bs) = do
             _ -> linkAnchor'
   let align dir txt = inCmd "begin" dir $$ txt $$ inCmd "end" dir
   lang <- toLang $ lookup "lang" kvs
-  let wrapDir = case lookup "dir" kvs of
+  let wrapColumns = if "columns" `elem` classes
+                       then \contents ->
+                          inCmd "begin" "columns" <> brackets "T"
+                              $$ contents
+                              $$ inCmd "end" "columns"
+                       else id
+      wrapColumn  = if "column" `elem` classes
+                       then \contents ->
+                          let fromPct xs =
+                                case reverse xs of
+                                     '%':ds -> '0':'.': reverse ds
+                                     _      -> xs
+                              w = maybe "0.48" fromPct (lookup "width" kvs)
+                          in  inCmd "begin" "column" <>
+                              braces (text w <> "\\textwidth")
+                              $$ contents
+                              $$ inCmd "end" "column"
+                       else id
+      wrapDir = case lookup "dir" kvs of
                   Just "rtl" -> align "RTL"
                   Just "ltr" -> align "LTR"
                   _          -> id
@@ -468,7 +486,8 @@ blockToLaTeX (Div (identifier,classes,kvs) bs) = do
       wrapNotes txt = if beamer && "notes" `elem` classes
                           then "\\note" <> braces txt -- speaker notes
                           else linkAnchor $$ txt
-  fmap (wrapDir . wrapLang . wrapNotes) $ blockListToLaTeX bs
+  fmap (wrapColumns . wrapColumn . wrapDir . wrapLang . wrapNotes)
+    $ blockListToLaTeX bs
 blockToLaTeX (Plain lst) =
   inlineListToLaTeX $ dropWhile isLineBreakOrSpace lst
 -- title beginning with fig: indicates that the image is a figure