1 files changed, 25 insertions(+), 0 deletions(-)

M src/Data/Text/ParagraphLayout/Internal/ResolvedBox.hs

@@ 4,6 4,8 @@ module Data.Text.ParagraphLayout.Internal.ResolvedBox
     , boxLeftSpacing
     , boxRightSpacing
     , boxStartSpacing
+    , diff
+    , union
     )
 where
 


@@ 24,6 26,29 @@ data ResolvedBox d = ResolvedBox
 instance Eq (ResolvedBox d) where
     a == b = boxIndex a == boxIndex b
 
+instance Ord (ResolvedBox d) where
+    a `compare` b = boxIndex a `compare` boxIndex b
+
+-- | Calculate the union of two lists of boxes that are ordered
+-- from highest index to lowest and do not contain duplicates.
+union :: Ord a => [a] -> [a] -> [a]
+union xs [] = xs
+union [] ys = ys
+union (x : xs) (y : ys) = case x `compare` y of
+    EQ -> x : union xs ys
+    GT -> x : union xs (y : ys)
+    LT -> y : union (x : xs) ys
+
+-- | Calculate the difference of two lists of boxes that are ordered
+-- from highest index to lowest and do not contain duplicates.
+diff :: Ord a => [a] -> [a] -> [a]
+diff xs [] = xs
+diff [] _ = []
+diff (x : xs) (y : ys) = case x `compare` y of
+    EQ -> diff xs ys
+    GT -> x : diff xs (y : ys)
+    LT -> diff (x : xs) ys
+
 boxLeftSpacing :: ResolvedBox d -> Int32
 boxLeftSpacing rb = case boxSpacing $ boxOptions rb of
     BoxSpacingLeftRight s _ -> s