From 39ec2358151d0d99dbeb4528efde98158031920a Mon Sep 17 00:00:00 2001
From: Jaro <jaro@argonaut-constellation.org>
Date: Wed, 26 Apr 2023 23:34:10 +0200
Subject: [PATCH] Define tree for structuring paragraph content.

---
 balkon.cabal                                  |  1 +
 .../Text/ParagraphLayout/Internal/Tree.hs     | 88 +++++++++++++++++++
 2 files changed, 89 insertions(+)
 create mode 100644 src/Data/Text/ParagraphLayout/Internal/Tree.hs

diff --git a/balkon.cabal b/balkon.cabal
index 31af3d7..880889b 100644
--- a/balkon.cabal
+++ b/balkon.cabal
@@ -119,6 +119,7 @@ library balkon-internal
         Data.Text.ParagraphLayout.Internal.Span,
         Data.Text.ParagraphLayout.Internal.TextContainer,
         Data.Text.ParagraphLayout.Internal.TextOptions,
+        Data.Text.ParagraphLayout.Internal.Tree,
         Data.Text.ParagraphLayout.Internal.Zipper
 
     -- Modules used purely internally and not in any tests.
diff --git a/src/Data/Text/ParagraphLayout/Internal/Tree.hs b/src/Data/Text/ParagraphLayout/Internal/Tree.hs
new file mode 100644
index 0000000..2ea1d28
--- /dev/null
+++ b/src/Data/Text/ParagraphLayout/Internal/Tree.hs
@@ -0,0 +1,88 @@
+-- | Represents the contents of a paragraph as a tree.
+--
+-- The tree is a hierarchy of boxes, with one box always present as the root.
+-- Each box may contain any combination of text sequences and other boxes.
+module Data.Text.ParagraphLayout.Internal.Tree
+    ( RootNode (..)
+    , InnerNode (..)
+    , Box (..)
+    , Leaf (..)
+    , flatten
+    )
+where
+
+import Data.Text.ParagraphLayout.Internal.BoxOptions
+import Data.Text.ParagraphLayout.Internal.TextOptions
+
+-- | Root of the paragraph tree.
+data RootNode d
+
+    = RootBox
+    -- ^ The root inline box. Always present in a paragraph.
+    --
+    -- Cannot be styled directly, but can still set options for formatting
+    -- text sequences directly descending from this box.
+        (Box d)
+        -- ^ Contents of the box.
+
+-- | Non-root node of the paragraph tree.
+data InnerNode d
+
+    = InlineBox
+    -- ^ An inline box, nested in another box.
+        d
+        -- ^ User-defined data associated with the box.
+        (Box d)
+        -- ^ Contents of the box.
+        BoxOptions
+        -- ^ Style options to apply to the inline box.
+
+    | TextSequence
+    -- ^ A leaf node containing text.
+        d
+        -- ^ User-defined data associated with the text node.
+        Int
+        -- ^ Byte offset to the next text node or the end of the paragraph text.
+
+-- | A box with content and a defined format. Corresponds to a DOM element.
+data Box d = Box
+
+    [InnerNode d]
+    -- ^ Text nodes and other boxes contained by this box.
+
+    TextOptions
+    -- ^ Style options to apply to text sequences directly contained
+    -- by this box.
+
+-- TODO: we need a way to test box equality, maybe add indexes?
+type BoxPath d = [(d, BoxOptions)]
+
+-- | Representation of a leaf node of the tree after flattening.
+data Leaf d = TextLeaf
+
+    d
+    -- ^ User-defined data associated with the text node.
+
+    Int
+    -- ^ Byte offset to the next text node or the end of the paragraph text.
+
+    TextOptions
+    -- ^ Style options to apply to this text sequence.
+
+    (BoxPath d)
+    -- ^ Inline boxes found on the path from this text sequence to the root
+    -- of the original tree.
+
+-- | Convert the tree to a flat list of its leaf nodes (text sequences).
+flatten :: RootNode d -> [Leaf d]
+flatten (RootBox (Box ns textOpts)) = flattenNodes [] textOpts ns
+
+flattenNodes :: BoxPath d -> TextOptions -> [InnerNode d] -> [Leaf d]
+flattenNodes _ _ [] = []
+flattenNodes path textOpts (n : ns) =
+    flattenNode path textOpts n ++ flattenNodes path textOpts ns
+
+flattenNode :: BoxPath d -> TextOptions -> InnerNode d -> [Leaf d]
+flattenNode path textOpts (TextSequence d len) = [TextLeaf d len textOpts path]
+flattenNode path _ (InlineBox d (Box ns textOpts) boxOpts) =
+    flattenNodes ((d, boxOpts) : path) textOpts ns
-- 
2.30.2