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LibWeb: Avoid full tree traversal in Node::compare_document_position()

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Introduce optimization that determines if one node is preceding,
following, ancestor or descdendant of another node by comparing
ancestors chains of both nodes which is much cheaper than using
Node::is_before() that can walk whole subtree in the worst case.
This commit is contained in:
Aliaksandr Kalenik 2023-05-24 00:18:35 +03:00 committed by Andreas Kling
parent 884d8b14ac
commit e5618b17c4
Notes: sideshowbarker 2024-07-17 08:38:37 +09:00 
Author: https://github.com/kalenikaliaksandr
Commit: https://github.com/SerenityOS/serenity/commit/e5618b17c4
Pull-request: https://github.com/SerenityOS/serenity/pull/19003
Reviewed-by: https://github.com/awesomekling
1 changed files with 39 additions and 5 deletions
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44
Userland/Libraries/LibWeb/DOM/Node.cpp
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@ -968,8 +968,8 @@ u16 Node::compare_document_position(JS::GCPtr<Node> other)
Node* node2 = this;
// 3. Let attr1 and attr2 be null.
Attr* attr1;
Attr* attr2;
Attr* attr1 = nullptr;
Attr* attr2 = nullptr;
// 4. If node1 is an attribute, then set attr1 to node1 and node1 to attr1’s element.
if (is<Attr>(node1)) {
@ -996,16 +996,50 @@ u16 Node::compare_document_position(JS::GCPtr<Node> other)
if ((node1 == nullptr || node2 == nullptr) || (&node1->root() != &node2->root()))
return DOCUMENT_POSITION_DISCONNECTED | DOCUMENT_POSITION_IMPLEMENTATION_SPECIFIC | (node1 > node2 ? DOCUMENT_POSITION_PRECEDING : DOCUMENT_POSITION_FOLLOWING);
Vector<Node*> node1_ancestors;
for (auto* node = node1; node; node = node->parent())
node1_ancestors.append(node);
Vector<Node*> node2_ancestors;
for (auto* node = node2; node; node = node->parent())
node2_ancestors.append(node);
auto it_node1_ancestors = node1_ancestors.rbegin();
auto it_node2_ancestors = node2_ancestors.rbegin();
// Walk ancestor chains of both nodes starting from root
while (it_node1_ancestors != node1_ancestors.rend() && it_node2_ancestors != node2_ancestors.rend()) {
auto* ancestor1 = *it_node1_ancestors;
auto* ancestor2 = *it_node2_ancestors;
// If ancestors of nodes at the same level in the tree are different then preceding node is the one with lower sibling position
if (ancestor1 != ancestor2) {
auto* node = ancestor1;
while (node) {
if (node == ancestor2)
return DOCUMENT_POSITION_PRECEDING;
node = node->next_sibling();
}
return DOCUMENT_POSITION_FOLLOWING;
}
it_node1_ancestors++;
it_node2_ancestors++;
}
// NOTE: If nodes in ancestors chains are the same but one chain is longer, then one node is ancestor of another.
// The node with shorter ancestors chain is the ancestor.
// The node with longer ancestors chain is the descendant.
// 7. If node1 is an ancestor of node2 and attr1 is null, or node1 is node2 and attr2 is non-null, then return the result of adding DOCUMENT_POSITION_CONTAINS to DOCUMENT_POSITION_PRECEDING.
if ((node1->is_ancestor_of(*node2) && !attr1) || (node1 == node2 && attr2))
if ((node1_ancestors.size() < node2_ancestors.size() && !attr1) || (node1 == node2 && attr2))
return DOCUMENT_POSITION_CONTAINS | DOCUMENT_POSITION_PRECEDING;
// 8. If node1 is a descendant of node2 and attr2 is null, or node1 is node2 and attr1 is non-null, then return the result of adding DOCUMENT_POSITION_CONTAINED_BY to DOCUMENT_POSITION_FOLLOWING.
if ((node2->is_ancestor_of(*node1) && !attr2) || (node1 == node2 && attr1))
if ((node1_ancestors.size() > node2_ancestors.size() && !attr2) || (node1 == node2 && attr1))
return DOCUMENT_POSITION_CONTAINED_BY | DOCUMENT_POSITION_FOLLOWING;
// 9. If node1 is preceding node2, then return DOCUMENT_POSITION_PRECEDING.
if (node1->is_before(*node2))
if (node1_ancestors.size() < node2_ancestors.size())
return DOCUMENT_POSITION_PRECEDING;
// 10. Return DOCUMENT_POSITION_FOLLOWING.