Check min number of children invariant

CC #9

ConflictSet.cpp

@@ -718,10 +718,23 @@ void maybeDownsize(Node *self, Node *&root, NodeAllocators *allocators,
   switch (self->type) {
   case Type::Node0:
     __builtin_unreachable(); // GCOVR_EXCL_LINE
-  case Type::Node4:
+  case Type::Node4: {
-    if (self->numChildren < 2) {
+    auto *self4 = (Node4 *)self;
+    if (self->numChildren == 0) {
+      auto *newSelf = allocators->node0.allocate(self->partialKeyLen);
+      memcpy((char *)newSelf + kNodeCopyBegin, (char *)self + kNodeCopyBegin,
+             kNodeCopySize);
+      memcpy(newSelf->partialKey(), self4->partialKey(), self->partialKeyLen);
+
+      if (self->parent == nullptr) {
+        root = newSelf;
+      } else {
+        getChildExists(self->parent, self->parentsIndex) = newSelf;
+      }
+
+      allocators->node4.release(self4);
+    } else if (self->numChildren == 1) {
       if (!self->entryPresent) {
-        auto *self4 = (Node4 *)self;
         auto *child = self4->children[0].child;
         int minCapacity = self4->partialKeyLen + 1 + child->partialKeyLen;
 
@@ -763,7 +776,7 @@ void maybeDownsize(Node *self, Node *&root, NodeAllocators *allocators,
         allocators->node4.release(self4);
       }
     }
-    break;
+  } break;
   case Type::Node16:
     if (self->numChildren < kMinChildrenNode16) {
       auto *self16 = (Node16 *)self;
@@ -2365,6 +2378,39 @@ Iterator firstGeq(Node *n, std::string_view key) {
   return total;
 }
 
+[[maybe_unused]] void checkMinChildCount(Node *node, bool &success) {
+  int minNumChildren;
+  switch (node->type) {
+  case Type::Node0:
+    minNumChildren = 0;
+    break;
+  case Type::Node4:
+    minNumChildren = kMinChildrenNode4;
+    break;
+  case Type::Node16:
+    minNumChildren = kMinChildrenNode16;
+    break;
+  case Type::Node48:
+    minNumChildren = kMinChildrenNode48;
+    break;
+  case Type::Node256:
+    minNumChildren = kMinChildrenNode256;
+    break;
+  }
+  if (node->numChildren < minNumChildren) {
+    Arena arena;
+    fprintf(stderr,
+            "%s has %d children, which is less than the minimum required %d\n",
+            getSearchPathPrintable(node).c_str(), node->numChildren,
+            minNumChildren);
+    success = false;
+  }
+  for (int i = getChildGeq(node, 0); i >= 0; i = getChildGeq(node, i + 1)) {
+    auto *child = getChildExists(node, i);
+    checkMinChildCount(child, success);
+  }
+}
+
 bool checkCorrectness(Node *node, int64_t oldestVersion,
                       ConflictSet::Impl *impl) {
   bool success = true;
@@ -2372,6 +2418,7 @@ bool checkCorrectness(Node *node, int64_t oldestVersion,
   checkParentPointers(node, success);
   checkMaxVersion(node, node, oldestVersion, success, impl);
   checkEntriesExist(node, success);
+  checkMinChildCount(node, success);
 
   return success;
 }