6 files changed, 56 insertions(+), 5 deletions(-)

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

@@ 114,6 114,7 @@ library balkon-internal
         Data.Text.ParagraphLayout.Internal.LineHeight,
         Data.Text.ParagraphLayout.Internal.LinePagination,
         Data.Text.ParagraphLayout.Internal.Paginable,
+        Data.Text.ParagraphLayout.Internal.ParagraphAlignment,
         Data.Text.ParagraphLayout.Internal.ParagraphConstruction,
         Data.Text.ParagraphLayout.Internal.ParagraphOptions,
         Data.Text.ParagraphLayout.Internal.Plain,

@@ 5,6 5,7 @@ module Data.Text.ParagraphLayout.Rich
     -- * Input paragraph
     ( Paragraph (Paragraph)
     , constructParagraph
+    , ParagraphAlignment (AlignLeft, AlignRight, AlignCentreH)
     , BoxSpacing (BoxSpacingLeftRight)
     , LineHeight (Absolute, Normal)
     , ParagraphOptions


@@ 12,6 13,7 @@ module Data.Text.ParagraphLayout.Rich
     -- ** Paragraph options
     -- | These are record selectors that can be used for reading
     -- as well as updating specific option fields.
+    , paragraphAlignment
     , paragraphMaxWidth
     -- NOTE: `paragraphFont` is only used by the legacy plain text interface,
     --       and is therefore not exported here.


@@ 67,6 69,7 @@ import Data.Text.ParagraphLayout.Internal.AncestorBox
 import Data.Text.ParagraphLayout.Internal.BoxOptions
 import Data.Text.ParagraphLayout.Internal.Fragment
 import Data.Text.ParagraphLayout.Internal.LineHeight
+import Data.Text.ParagraphLayout.Internal.ParagraphAlignment
 import Data.Text.ParagraphLayout.Internal.ParagraphOptions
 import Data.Text.ParagraphLayout.Internal.Rich
 import Data.Text.ParagraphLayout.Internal.Rich.Paragraph

@@ 32,6 32,7 @@ import Data.Text.ParagraphLayout.Internal.BiDiReorder
 import Data.Text.ParagraphLayout.Internal.Break
 import Data.Text.ParagraphLayout.Internal.Fragment
 import Data.Text.ParagraphLayout.Internal.LineHeight
+import Data.Text.ParagraphLayout.Internal.ParagraphAlignment
 import Data.Text.ParagraphLayout.Internal.ParagraphExtents
 import qualified Data.Text.ParagraphLayout.Internal.ProtoFragment as PF
 import qualified Data.Text.ParagraphLayout.Internal.ProtoLine as PL


@@ 57,14 58,15 @@ type ProtoFragmentWithBoxes d = WithBoxes d (ProtoFragmentWithSpan d)
 -- The output is a flat list of fragments positioned in both dimensions.
 layoutAndAlignLines
     :: Direction
+    -> ParagraphAlignment
     -> Int32
     -> NonEmpty (WithSpan d Run)
     -> [FragmentWithSpan d]
-layoutAndAlignLines dir maxWidth runs = frags
+layoutAndAlignLines dir align maxWidth runs = frags
     where
         frags = concatMap NonEmpty.toList fragsInLines
         (_, fragsInLines) = mapAccumL positionLine originY numberedLines
-        positionLine = positionLineH dir maxWidth
+        positionLine = positionLineH dir align maxWidth
         numberedLines = zip [1 ..] canonicalLines
         canonicalLines = fmap reorderProtoFragments logicalLines
         logicalLines = nonEmptyItems $ layoutLines maxWidth [] runs


@@ 106,11 108,12 @@ layoutLines maxWidth openBoxes runs = case nonEmpty rest of
 -- TODO: For rich text, allow other types of vertical alignment.
 positionLineH
     :: Direction
+    -> ParagraphAlignment
     -> Int32
     -> Int32
     -> (Int, PL.ProtoLine NonEmpty d)
     -> (Int32, NonEmpty (FragmentWithSpan d))
-positionLineH dir maxWidth originY (num, pl) = (nextY, frags)
+positionLineH dir align maxWidth originY (num, pl) = (nextY, frags)
     where
         nextY = minimum $ fmap y_min rects
         rects = fmap (\ (WithSpan _ r) -> fragmentRect r) frags


@@ 118,7 121,7 @@ positionLineH dir maxWidth originY (num, pl) = (nextY, frags)
         wpfs = PL.applyBoxes pl
         originX = paragraphOriginX + if lineWidth > maxWidth
             then overflowingLineOffset dir (lineWidth - maxWidth)
-            else 0
+            else fittingLineOffset align (maxWidth - lineWidth)
         lineWidth = PL.width pl
 
 -- | Inline offset of the first fragment on a line that overflows.


@@ 129,6 132,12 @@ overflowingLineOffset DirRTL excess = -excess
 -- TODO: Check if the sign needs to be flipped for vertical text.
 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
+
 -- | Position the given horizontal fragment on a line,
 -- using @originY@ as its top edge and @originX@ as its left edge,
 -- returning the X coordinate of its right edge for continuation.

@@ 0,0 1,32 @@
+module Data.Text.ParagraphLayout.Internal.ParagraphAlignment
+    ( ParagraphAlignment (..)
+    )
+where
+
+-- | Determines how the contents of the paragraph should be aligned
+-- horizontally, that is, how should fragments be stacked inside each line
+-- when there is space available.
+--
+-- This has no effect on lines with overflowing content.
+data ParagraphAlignment
+
+    = AlignLeft
+    -- ^ Stack fragments to the left edge of each line,
+    -- leaving free space on the right side.
+    --
+    -- The leftmost fragment on each non-overflowing line will have its
+    -- `Data.Text.ParagraphLayout.Rect.x_min` equal to @0@.
+
+    | AlignRight
+    -- ^ Stack fragments to the right edge of each line,
+    -- leaving free space on the left side.
+    --
+    -- The rightmost fragment on each non-overflowing line will have its
+    -- `Data.Text.ParagraphLayout.Rect.x_max` equal to
+    -- `Data.Text.ParagraphLayout.Rich.paragraphMaxWidth`.
+
+    | AlignCentreH
+    -- ^ Stack fragments to the horizontal centre of each line,
+    -- leaving free space split evenly between each side.
+
+    deriving (Eq, Read, Show)

@@ 8,6 8,7 @@ import Data.Int (Int32)
 import Data.Text.Glyphize (Font, emptyFont)
 
 import Data.Text.ParagraphLayout.Internal.LineHeight
+import Data.Text.ParagraphLayout.Internal.ParagraphAlignment
 
 -- | Defines options relevant to the entire paragraph.
 --


@@ 24,6 25,9 @@ data ParagraphOptions = ParagraphOptions
     , paragraphLineHeight :: LineHeight
     -- ^ Preferred line height of the resulting fragments.
 
+    , paragraphAlignment :: ParagraphAlignment
+    -- ^ Alignment of non-overflowing lines within the paragraph.
+
     , paragraphMaxWidth :: Int32
     -- ^ Line width at which line breaking should occur.
     -- Lines will be broken at language-appropriate boundaries.


@@ 39,5 43,6 @@ defaultParagraphOptions :: ParagraphOptions
 defaultParagraphOptions = ParagraphOptions
     { paragraphFont = emptyFont
     , paragraphLineHeight = Normal
+    , paragraphAlignment = AlignLeft
     , paragraphMaxWidth = maxBound
     }

@@ 30,10 30,11 @@ layoutRich p = paragraphLayout $ map unwrap frags
         unwrap (WithSpan rs frag) =
             frag { fragmentUserData = RS.spanUserData rs }
         frags = case nonEmpty wrappedRuns of
-            Just xs -> layoutAndAlignLines dir maxWidth xs
+            Just xs -> layoutAndAlignLines dir align maxWidth xs
             Nothing -> []
         wrappedRuns = spansToRunsWrapped spans
         dir = textDirection rootTextOpts
+        align = paragraphAlignment opts
         maxWidth = paragraphMaxWidth opts
         spans = resolveSpans p