Fabric: a new efficient implementation of ShadowNode::getAncestors()

Summary:
The algorithm is quite simply:
1. Get family objects of the two nodes (inner and outer).
2. Traverse the list of family objects starting from the inner one and form a reversed list of them.
3. tarting from the inner node, traverse the tree layer-by-layer choosing a next element of the path by comparing the family object of each node on the level and an object from the list.

Reviewed By: JoshuaGross

Differential Revision: D14416950

fbshipit-source-id: 23c659a9e01690f90174193650a2b0ef09eadb4d

ReactCommon/fabric/core/shadownode/ShadowNode.cpp

@@ -7,6 +7,7 @@
 
 #include "ShadowNode.h"
 
+#include <better/small_vector.h>
 #include <string>
 
 #include <react/core/ComponentDescriptor.h>
@@ -17,6 +18,8 @@
 namespace facebook {
 namespace react {
 
+using AncestorList = ShadowNode::AncestorList;
+
 SharedShadowNodeSharedList ShadowNode::emptySharedShadowNodeSharedList() {
   static const auto emptySharedShadowNodeSharedList =
       std::make_shared<SharedShadowNodeList>();
@@ -197,6 +200,48 @@ void ShadowNode::setMounted(bool mounted) const {
   }
 }
 
+AncestorList ShadowNode::getAncestors(
+    ShadowNode const &ancestorShadowNode) const {
+  auto ancestors = AncestorList{};
+  auto families = better::small_vector<ShadowNodeFamily const *, 64>{};
+  auto ancestorFamily = ancestorShadowNode.family_.get();
+  auto descendantFamily = family_.get();
+
+  auto family = descendantFamily;
+  while (family && family != ancestorFamily) {
+    families.push_back(family);
+    family = family->parent_.lock().get();
+  }
+
+  if (family != ancestorFamily) {
+    ancestors.clear();
+    return ancestors;
+  }
+
+  auto parentNode = &ancestorShadowNode;
+  for (auto it = families.rbegin(); it != families.rend(); it++) {
+    auto childFamily = *it;
+    auto found = bool{false};
+    auto childIndex = int{0};
+    for (const auto &childNode : *parentNode->children_) {
+      if (childNode->family_.get() == childFamily) {
+        ancestors.push_back({*parentNode, childIndex});
+        parentNode = childNode.get();
+        found = true;
+        break;
+      }
+      childIndex++;
+    }
+
+    if (!found) {
+      ancestors.clear();
+      return ancestors;
+    }
+  }
+
+  return ancestors;
+}
+
 bool ShadowNode::constructAncestorPath(
     const ShadowNode &ancestorShadowNode,
     std::vector<std::reference_wrapper<const ShadowNode>> &ancestors) const {

ReactCommon/fabric/core/shadownode/ShadowNode.h

@@ -45,6 +45,11 @@ class ShadowNode : public virtual Sealable,
  public:
   using Shared = std::shared_ptr<const ShadowNode>;
   using Weak = std::weak_ptr<const ShadowNode>;
+  using AncestorList = better::small_vector<
+      std::pair<
+          std::reference_wrapper<ShadowNode const> /* parentNode */,
+          int /* childIndex */>,
+      64>;
 
   static SharedShadowNodeSharedList emptySharedShadowNodeSharedList();
 
@@ -138,6 +143,7 @@ class ShadowNode : public virtual Sealable,
   void setMounted(bool mounted) const;
 
   /*
+   * Deprecated. Use `getAncestors` instead.
    * Forms a list of all ancestors of the node relative to the given ancestor.
    * The list starts from the parent node and ends with the given ancestor node.
    * Returns `true` if successful, `false` otherwise.
@@ -149,9 +155,19 @@ class ShadowNode : public virtual Sealable,
    * `childIndex` and `nodeId` tracking.
    */
   bool constructAncestorPath(
-      const ShadowNode &rootShadowNode,
+      const ShadowNode &ancestorShadowNode,
       std::vector<std::reference_wrapper<const ShadowNode>> &ancestors) const;
 
+  /*
+   * Returns a list of all ancestors of the node relative to the given ancestor.
+   * The list starts from the given ancestor node and ends with the parent node
+   * of `this` node. The elements of the list have a reference to some parent
+   * node and an index of the child of the parent node.
+   * Can be called from any thread.
+   * The theoretical complexity of the algorithm is `O(ln(n))`. Use it wisely.
+   */
+  AncestorList getAncestors(ShadowNode const &ancestorShadowNode) const;
+
 #pragma mark - DebugStringConvertible
 
 #if RN_DEBUG_STRING_CONVERTIBLE