Change kMinChildrenNode4 to 2, fixing the induction

ConflictSet.cpp

@@ -259,6 +259,10 @@ struct Node256 : Node {
   uint8_t *partialKey() { return (uint8_t *)(this + 1); }
 };
 
+namespace {
+std::string getSearchPathPrintable(Node *n);
+}
+
 // Bound memory usage following the analysis in the ART paper
 
 constexpr int kBytesPerKey = 120;
@@ -839,6 +843,11 @@ void maybeDecreaseCapacity(Node *&self, NodeAllocators *allocators,
 // TODO fuse into erase child so we don't need to repeat branches on type
 void maybeDownsize(Node *self, NodeAllocators *allocators,
                    ConflictSet::Impl *impl, Node *&dontInvalidate) {
+
+#if DEBUG_VERBOSE && !defined(NDEBUG)
+  fprintf(stderr, "maybeDownsize: %s\n", getSearchPathPrintable(self).c_str());
+#endif
+
   switch (self->type) {
   case Type::Node0:
     __builtin_unreachable(); // GCOVR_EXCL_LINE
@@ -858,11 +867,11 @@ void maybeDownsize(Node *self, NodeAllocators *allocators,
         int minCapacity = self4->partialKeyLen + 1 + child->partialKeyLen;
 
         if (minCapacity > child->partialKeyCapacity) {
-          // TODO resize child? It seems to be quite challenging to implement,
-          // since callers would now have to account for erase invalidating
-          // not on the search path. We could lower kBytesPerKey by doing this
-          // though.
-          return;
+          const bool update = child == dontInvalidate;
+          makeCapacityAtLeast(child, minCapacity, allocators, impl);
+          if (update) {
+            dontInvalidate = child;
+          }
         }
 
         // Merge partial key with child
@@ -964,16 +973,27 @@ void maybeDownsize(Node *self, NodeAllocators *allocators,
 
 // Precondition: self is not the root. May invalidate nodes along the search
 // path to self. May invalidate children of self->parent. Returns a pointer to
-// the node after self.
-Node *erase(Node *self, NodeAllocators *allocators, ConflictSet::Impl *impl) {
+// the node after self. If erase invalidates the pointee of `dontInvalidate`, it
+// will update it to its new pointee as well.
+Node *erase(Node *self, NodeAllocators *allocators, ConflictSet::Impl *impl,
+            Node *&dontInvalidate) {
   assert(self->parent != nullptr);
 
+#if DEBUG_VERBOSE && !defined(NDEBUG)
+  fprintf(stderr, "Erase: %s\n", getSearchPathPrintable(self).c_str());
+#endif
+
   Node *parent = self->parent;
   uint8_t parentsIndex = self->parentsIndex;
 
   auto *result = nextLogical(self);
   self->entryPresent = false;
   if (self->numChildren != 0) {
+    const bool update = result == dontInvalidate;
+    maybeDownsize(self, allocators, impl, result);
+    if (update) {
+      dontInvalidate = result;
+    }
     return result;
   }
 
@@ -1027,9 +1047,13 @@ Node *erase(Node *self, NodeAllocators *allocators, ConflictSet::Impl *impl) {
   --parent->numChildren;
   if (parent->numChildren == 0 && !parent->entryPresent &&
       parent->parent != nullptr) {
-    erase(parent, allocators, impl);
+    return erase(parent, allocators, impl, dontInvalidate);
   } else {
+    const bool update = result == dontInvalidate;
     maybeDownsize(parent, allocators, impl, result);
+    if (update) {
+      dontInvalidate = result;
+    }
   }
   return result;
 }
@@ -1233,10 +1257,6 @@ struct SearchStepWise {
   }
 };
 
-namespace {
-std::string getSearchPathPrintable(Node *n);
-}
-
 // Logically this is the same as performing firstGeq and then checking against
 // point or range version according to cmp, but this version short circuits as
 // soon as it can prove that there's no conflict.
@@ -2024,14 +2044,8 @@ void addWriteRange(Node *&root, int64_t oldestVersion,
     assert(beginNode->entryPresent);
   }
 
-  for (beginNode = nextLogical(beginNode); beginNode != endNode;) {
-    auto *next = nextLogical(beginNode);
-    bool done = next == endNode;
-    erase(beginNode, allocators, impl);
-    if (done) {
-      break;
-    }
-    beginNode = next;
+  for (beginNode = nextLogical(beginNode); beginNode != endNode;
+       beginNode = erase(beginNode, allocators, impl, endNode)) {
   }
 }
 
@@ -2157,7 +2171,8 @@ struct __attribute__((visibility("hidden"))) ConflictSet::Impl {
         // There's no way to insert a range such that range version of the right
         // node is greater than the point version of the left node
         assert(n->entry.rangeVersion <= oldestVersion);
-        n = erase(n, &allocators, this);
+        Node *dummy = nullptr;
+        n = erase(n, &allocators, this, dummy);
       } else {
         maybeDecreaseCapacity(n, &allocators, this);
         n = nextLogical(n);
@@ -2511,9 +2526,10 @@ Iterator firstGeq(Node *n, std::string_view key) {
   }
   if (node->numChildren + int(node->entryPresent) < minNumChildren) {
     fprintf(stderr,
-            "%s has %d children, which is less than the minimum required %d\n",
+            "%s has %d children + %d entries, which is less than the minimum "
+            "required %d\n",
             getSearchPathPrintable(node).c_str(), node->numChildren,
-            minNumChildren);
+            int(node->entryPresent), minNumChildren);
     success = false;
   }
   // TODO check that the max capacity property eventually holds