Address duplicate nodeIDs in Tree.Split (#724)

We need to define ID of split element node for concurrent editing.
Text has fixed content and its split nodes could have limited offset
range. But element node could have arbitrary children and its split
nodes could have arbitrary offset range. So, id could be duplicated
and its order could be broken when concurrent editing happens.

This commit temporarily avoids duplication by issuing a new ID.

pkg/document/crdt/tree.go

@@ -219,7 +219,7 @@ func (n *TreeNode) Child(offset int) (*TreeNode, error) {
 }
 
 // Split splits the node at the given offset.
-func (n *TreeNode) Split(tree *Tree, offset int) error {
+func (n *TreeNode) Split(tree *Tree, offset int, issueTimeTicket func() *time.Ticket) error {
 	var split *TreeNode
 	var err error
 	if n.IsText() {
@@ -228,7 +228,7 @@ func (n *TreeNode) Split(tree *Tree, offset int) error {
 			return err
 		}
 	} else {
-		split, err = n.SplitElement(offset, n.ID.Offset)
+		split, err = n.SplitElement(offset, issueTimeTicket)
 		if err != nil {
 			return err
 		}
@@ -282,14 +282,14 @@ func (n *TreeNode) SplitText(offset, absOffset int) (*TreeNode, error) {
 }
 
 // SplitElement splits the given element at the given offset.
-func (n *TreeNode) SplitElement(offset, absOffset int) (*TreeNode, error) {
+func (n *TreeNode) SplitElement(offset int, issueTimeTicket func() *time.Ticket) (*TreeNode, error) {
 	// TODO(hackerwins): Define ID of split node for concurrent editing.
 	// Text has fixed content and its split nodes could have limited offset
 	// range. But element node could have arbitrary children and its split
 	// nodes could have arbitrary offset range. So, id could be duplicated
 	// and its order could be broken when concurrent editing happens.
 	// Currently, we use the similar ID of split element with the split text.
-	split := NewTreeNode(&TreeNodeID{CreatedAt: n.ID.CreatedAt, Offset: offset + absOffset}, n.Type(), nil)
+	split := NewTreeNode(&TreeNodeID{CreatedAt: issueTimeTicket(), Offset: 0}, n.Type(), nil)
 	split.RemovedAt = n.RemovedAt
 	if err := n.Index.Parent.InsertAfterInternal(split.Index, n.Index); err != nil {
 		return nil, err
@@ -563,6 +563,7 @@ func (t *Tree) EditT(
 	contents []*TreeNode,
 	splitLevel int,
 	editedAt *time.Ticket,
+	issueTimeTicket func() *time.Ticket,
 ) (map[string]*time.Ticket, error) {
 	fromPos, err := t.FindPos(start)
 	if err != nil {
@@ -573,7 +574,7 @@ func (t *Tree) EditT(
 		return nil, err
 	}
 
-	return t.Edit(fromPos, toPos, contents, splitLevel, editedAt, nil)
+	return t.Edit(fromPos, toPos, contents, splitLevel, editedAt, issueTimeTicket, nil)
 }
 
 // FindPos finds the position of the given index in the tree.
@@ -618,6 +619,7 @@ func (t *Tree) Edit(
 	contents []*TreeNode,
 	splitLevel int,
 	editedAt *time.Ticket,
+	issueTimeTicket func() *time.Ticket,
 	latestCreatedAtMapByActor map[string]*time.Ticket,
 ) (map[string]*time.Ticket, error) {
 	// 01. find nodes from the given range and split nodes.
@@ -653,7 +655,7 @@ func (t *Tree) Edit(
 	}
 
 	// 04. Split: split the element nodes for the given splitLevel.
-	if err := t.split(fromParent, fromLeft, splitLevel); err != nil {
+	if err := t.split(fromParent, fromLeft, splitLevel, issueTimeTicket); err != nil {
 		return nil, err
 	}
 
@@ -773,7 +775,12 @@ func (t *Tree) collectBetween(
 	return toBeRemoveds, toBeMovedToFromParents, createdAtMapByActor, nil
 }
 
-func (t *Tree) split(fromParent *TreeNode, fromLeft *TreeNode, splitLevel int) error {
+func (t *Tree) split(
+	fromParent *TreeNode,
+	fromLeft *TreeNode,
+	splitLevel int,
+	issueTimeTicket func() *time.Ticket,
+) error {
 	if splitLevel == 0 {
 		return nil
 	}
@@ -792,7 +799,7 @@ func (t *Tree) split(fromParent *TreeNode, fromLeft *TreeNode, splitLevel int) e
 
 			offset++
 		}
-		if err := parent.Split(t, offset); err != nil {
+		if err := parent.Split(t, offset, issueTimeTicket); err != nil {
 			return err
 		}
 		left = parent
@@ -881,7 +888,7 @@ func (t *Tree) FindTreeNodesWithSplitText(pos *TreePos, editedAt *time.Ticket) (
 
 	// 02. Split text node if the left node is text node.
 	if leftNode.IsText() {
-		err := leftNode.Split(t, pos.LeftSiblingID.Offset-leftNode.ID.Offset)
+		err := leftNode.Split(t, pos.LeftSiblingID.Offset-leftNode.ID.Offset, nil)
 		if err != nil {
 			return nil, nil, err
 		}