Allow em for image height/width in HTML, LaTeX.

- Export `inEm` from ImageSize [API change].
- Change `showFl` and `show` instance for `Dimension` so
  extra decimal places are omitted.
- Added `Em` as a constructor of `Dimension` [API change].
- Allow `em`, `cm`, `in` to pass through without conversion
  in HTML, LaTeX.

Closes #3450.

2 files changed, 21 insertions, 6 deletions

diff --git a/src/Text/Pandoc/ImageSize.hs b/src/Text/Pandoc/ImageSize.hs
index 4d914a10c..eec8658c5 100644
--- a/src/Text/Pandoc/ImageSize.hs
+++ b/src/Text/Pandoc/ImageSize.hs
@@ -43,6 +43,7 @@ module Text.Pandoc.ImageSize ( ImageType(..)
                              , inInch
                              , inPixel
                              , inPoints
+                             , inEm
                              , numUnit
                              , showInInch
                              , showInPixel
@@ -80,12 +81,14 @@ data Dimension = Pixel Integer
                | Centimeter Double
                | Inch Double
                | Percent Double
+               | Em Double
 
 instance Show Dimension where
   show (Pixel a)      = show   a ++ "px"
   show (Centimeter a) = showFl a ++ "cm"
   show (Inch a)       = showFl a ++ "in"
   show (Percent a)    = show   a ++ "%"
+  show (Em a)         = showFl a ++ "em"
 
 data ImageSize = ImageSize{
                      pxX   :: Integer
@@ -97,7 +100,13 @@ instance Default ImageSize where
   def = ImageSize 300 200 72 72
 
 showFl :: (RealFloat a) => a -> String
-showFl a = showFFloat (Just 5) a ""
+showFl a = removeExtra0s $ showFFloat (Just 5) a ""
+
+removeExtra0s :: String -> String
+removeExtra0s s =
+  case dropWhile (=='0') $ reverse s of
+       '.':xs -> reverse xs
+       xs     -> reverse xs
 
 imageType :: ByteString -> Maybe ImageType
 imageType img = case B.take 4 img of
@@ -167,6 +176,9 @@ desiredSizeInPoints opts attr s =
 inPoints :: WriterOptions -> Dimension -> Double
 inPoints opts dim = 72 * inInch opts dim
 
+inEm :: WriterOptions -> Dimension -> Double
+inEm opts dim = (64/11) * inInch opts dim
+
 inInch :: WriterOptions -> Dimension -> Double
 inInch opts dim =
   case dim of
@@ -174,6 +186,7 @@ inInch opts dim =
     (Centimeter a) -> a * 0.3937007874
     (Inch a)       -> a
     (Percent _)    -> 0
+    (Em a)         -> a * (11/64)
 
 inPixel :: WriterOptions -> Dimension -> Integer
 inPixel opts dim =
@@ -181,7 +194,8 @@ inPixel opts dim =
     (Pixel a)      -> a
     (Centimeter a) -> floor $ dpi * a * 0.3937007874 :: Integer
     (Inch a)       -> floor $ dpi * a :: Integer
-    _              -> 0
+    (Percent _)    -> 0
+    (Em a)         -> floor $ dpi * a * (11/64) :: Integer
   where
     dpi = fromIntegral $ writerDpi opts
 
@@ -213,6 +227,7 @@ scaleDimension factor dim =
         Centimeter x -> Centimeter (factor * x)
         Inch x       -> Inch (factor * x)
         Percent x    -> Percent (factor * x)
+        Em x         -> Em (factor * x)
 
 -- | Read a Dimension from an Attr attribute.
 -- `dimension Width attr` might return `Just (Pixel 3)` or for example `Just (Centimeter 2.0)`, etc.
@@ -236,6 +251,7 @@ lengthToDim s = numUnit s >>= uncurry toDim
     toDim a ""     = Just $ Pixel (floor a::Integer)
     toDim a "pt"   = Just $ Inch (a / 72)
     toDim a "pc"   = Just $ Inch (a / 6)
+    toDim a "em"   = Just $ Em a
     toDim _ _      = Nothing
 
 epsSize :: ByteString -> Maybe ImageSize
diff --git a/src/Text/Pandoc/Writers/HTML.hs b/src/Text/Pandoc/Writers/HTML.hs
index 63e839684..2a72f6f3d 100644
--- a/src/Text/Pandoc/Writers/HTML.hs
+++ b/src/Text/Pandoc/Writers/HTML.hs
@@ -533,10 +533,9 @@ dimensionsToAttrList :: WriterOptions -> Attr -> [(String, String)]
 dimensionsToAttrList opts attr = (go Width) ++ (go Height)
   where
     go dir = case (dimension dir attr) of
-               (Just (Percent a)) -> [("style", show dir ++ ":" ++ show (Percent a))]
-               (Just dim)         -> [(show dir, showInPixel opts dim)]
-               _ -> []
-
+               (Just (Pixel a))  -> [(show dir, show a)]
+               (Just x)          -> [("style", show dir ++ ":" ++ show x)]
+               Nothing           -> []
 
 imageExts :: [String]
 imageExts = [ "art", "bmp", "cdr", "cdt", "cpt", "cr2", "crw", "djvu", "erf",