Start working on rendering part

cheuph/render/tree_list.py

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+from .element import Id, RenderedElement
+
+__all__ = ["TreeList"]
+
+class TreeList:
+    def __init__(self,
+            tree: List[RenderedElement],
+            anchor_id: Id,
+            ) -> None:
+        self._deque = collections.deque()
+
+        self._anchor_id = anchor_id
+
+        # The offsets can be thought of as the index of a line relative to the
+        # anchor's first line.
+        #
+        # The upper offset is the index of the uppermost message's first line.
+        # upper_offset <= 0.
+        #
+        # The lower offset is the index of the lowermost message's LAST line.
+        # lower_offset >= 0.
+        self._upper_offset: Int
+        self._lower_offset: Int
+
+        # The upper and lower tree ids are the ids of the uppermost or
+        # lowermost tree added to the TreeList. They can be used to request the
+        # previous or next tree from an ElementSupply.
+        self._upper_tree_id: Id
+        self._lower_tree_id: Id
+
+        self._add_first_tree(tree)
+
+    @property
+    def upper_offset(self) -> Int:
+        return self._upper_offset
+
+    @property
+    def lower_offset(self) -> Int:
+        return self._lower_offset
+
+    @property
+    def upper_tree_id(self) -> Id:
+        return self._upper_tree_id
+
+    @property
+    def lower_tree_id(self) -> Id:
+        return self._lower_tree_id
+
+    def _add_first_tree(self, tree: List[RenderedElement]) -> None:
+        if len(tree) == 0:
+            raise ValueError("The tree must contain at least one element")
+
+        tree_id = tree[0].element.id
+        self._upper_tree_id = tree_id
+        self._lower_tree_id = tree_id
+
+        offset = 0
+        found_anchor = False
+
+        for rendered in elements:
+            if rendered.element.id == anchor_id:
+                found_anchor = True
+                self._upper_offset = -offset
+
+            offset += rendered.height
+
+        if not found_anchor:
+            raise ValueError("The initial tree must contain the anchor")
+
+        # Subtracting 1 because the lower offset is the index of the lowermost
+        # message's last line, not the first line of a hypothetical message
+        # below that.
+        self._lower_offset = offset - 1
+
+    def add_above(self, tree: List[RenderedElement]) -> None:
+        if len(tree) == 0:
+            raise ValueError("The tree must contain at least one element")
+
+        self._upper_tree_id = tree[0].element.id
+
+        for rendered in reversed(tree):
+            self._deque.appendleft(rendered)
+            self._upper_offset -= rendered.height
+
+        # Alternative to the above for loop
+        #delta = sum(map(lambda r: r.height, tree))
+        #self._upper_offset -= delta
+        #self._deque.extendLeft(reversed(tree))
+
+    def add_below(self, tree: List[RenderedElement]) -> None:
+        if len(tree) == 0:
+            raise ValueError("The tree must contain at least one element")
+
+        self._lower_tree_id = tree[0].element.id
+
+        for rendered in tree:
+            self._deque.append(rendered)
+            self._lower_offset += rendered.height
+
+        # Alternative to the above for loop
+        #delta = sum(map(lambda r: r.height, tree))
+        #self._lower_offset += delta
+        #self._deque.extend(tree)