5 files changed, 53 insertions(+), 8 deletions(-)

M lib/Data/Text/ParagraphLayout/Rich.hs
M src/Data/Text/ParagraphLayout/Internal/Layout.hs
M src/Data/Text/ParagraphLayout/Internal/ParagraphAlignment.hs
M src/Data/Text/ParagraphLayout/Internal/ParagraphOptions.hs
M src/Data/Text/ParagraphLayout/Internal/Rich.hs

@@ 5,7 5,13 @@ module Data.Text.ParagraphLayout.Rich
     -- * Input paragraph
     ( Paragraph (Paragraph)
     , constructParagraph
-    , ParagraphAlignment (AlignLeft, AlignRight, AlignCentreH)
+    , ParagraphAlignment
+        ( AlignStart
+        , AlignEnd
+        , AlignLeft
+        , AlignRight
+        , AlignCentreH
+        )
     , BoxSpacing (BoxSpacingLeftRight)
     , LineHeight (Absolute, Normal)
     , ParagraphOptions

@@ 121,7 121,7 @@ positionLineH dir align maxWidth originY (num, pl) = (nextY, frags)
         wpfs = PL.applyBoxes pl
         originX = paragraphOriginX + if lineWidth > maxWidth
             then overflowingLineOffset dir (lineWidth - maxWidth)
-            else fittingLineOffset align (maxWidth - lineWidth)
+            else fittingLineOffset align dir (maxWidth - lineWidth)
         lineWidth = PL.width pl
 
 -- | Inline offset of the first fragment on a line that overflows.


@@ 133,10 133,30 @@ overflowingLineOffset DirRTL excess = -excess
 overflowingLineOffset DirBTT excess = -excess
 
 -- | Inline offset of the first fragment on a line with extra blank space.
-fittingLineOffset :: ParagraphAlignment -> Int32 -> Int32
-fittingLineOffset AlignLeft _ = 0
-fittingLineOffset AlignRight slack = slack
-fittingLineOffset AlignCentreH slack = slack `div` 2
+fittingLineOffset :: ParagraphAlignment -> Direction -> Int32 -> Int32
+fittingLineOffset AlignLeft _ = leftAlignOffset
+fittingLineOffset AlignRight _ = rightAlignOffset
+fittingLineOffset AlignCentreH _ = centreAlignOffset
+fittingLineOffset AlignStart DirLTR = leftAlignOffset
+fittingLineOffset AlignEnd DirLTR = rightAlignOffset
+fittingLineOffset AlignStart DirRTL = rightAlignOffset
+fittingLineOffset AlignEnd DirRTL = leftAlignOffset
+-- For completeness, treat vertical directions as horizontal directions
+-- rotated 90° clockwise, thus left becomes top and right becomes bottom.
+-- TODO: Verify this when vertical text is implemented.
+fittingLineOffset AlignStart DirTTB = leftAlignOffset
+fittingLineOffset AlignEnd DirTTB = rightAlignOffset
+fittingLineOffset AlignStart DirBTT = rightAlignOffset
+fittingLineOffset AlignEnd DirBTT = leftAlignOffset
+
+leftAlignOffset :: Int32 -> Int32
+leftAlignOffset _ = 0
+
+rightAlignOffset :: Int32 -> Int32
+rightAlignOffset slack = slack
+
+centreAlignOffset :: Int32 -> Int32
+centreAlignOffset slack = slack `div` 2
 
 -- | Position the given horizontal fragment on a line,
 -- using @originY@ as its top edge and @originX@ as its left edge,

@@ 10,7 10,15 @@ where
 -- This has no effect on lines with overflowing content.
 data ParagraphAlignment
 
-    = AlignLeft
+    = AlignStart
+    -- ^ Behave like `AlignLeft` when the base direction of the paragraph
+    -- is LTR, and like `AlignRight` when the base direction is RTL.
+
+    | AlignEnd
+    -- ^ Behave like `AlignLeft` when the base direction of the paragraph
+    -- is RTL, and like `AlignRight` when the base direction is LTR.
+
+    | AlignLeft
     -- ^ Stack fragments to the left edge of each line,
     -- leaving free space on the right side.
     --

@@ 15,6 15,10 @@ import Data.Text.ParagraphLayout.Internal.ParagraphAlignment
 -- This record type is likely to be extended in the future.
 -- Use `defaultParagraphOptions` and update it with specific record selectors
 -- instead of constructing `ParagraphOptions` directly.
+--
+-- In order to get CSS defaults, use:
+--
+-- > defaultParagraphOptions { paragraphAlignment = AlignStart }
 data ParagraphOptions = ParagraphOptions
 
     { paragraphFont :: Font


@@ 38,7 42,10 @@ data ParagraphOptions = ParagraphOptions
     }
     deriving (Eq)
 
--- | `ParagraphOptions` with default values.
+-- | `ParagraphOptions` with backwards-compatible values.
+--
+-- Note that this sets `paragraphAlignment` to `AlignLeft`,
+-- whereas the CSS default should be `AlignStart`.
 defaultParagraphOptions :: ParagraphOptions
 defaultParagraphOptions = ParagraphOptions
     { paragraphFont = emptyFont

@@ 33,6 33,10 @@ layoutRich p = paragraphLayout $ map unwrap frags
             Just xs -> layoutAndAlignLines dir align maxWidth xs
             Nothing -> []
         wrappedRuns = spansToRunsWrapped spans
+        -- TODO: To support @unicode-bidi: plaintext@ as in CSS, allow ignoring
+        --       the text direction of the root box, and instead use the BiDi
+        --       rules P2 and P3 to determine the base directionality, which
+        --       may differ between lines.
         dir = textDirection rootTextOpts
         align = paragraphAlignment opts
         maxWidth = paragraphMaxWidth opts