[ntuple] Grow RVec capacity geometrically in ResizeRVec

tree/ntuple/src/RFieldSequenceContainer.cxx

@@ -291,13 +291,21 @@ unsigned char *ROOT::RRVecField::ResizeRVec(void *rvec, std::size_t nItems, std:
          }
       }
 
-      // TODO Increment capacity by a factor rather than just enough to fit the elements.
+      // Increment capacity geometrically rather than just enough to fit the elements.
+      std::size_t newCapacity = nItems;
+      if (*capacityPtr > 0) {
+         constexpr std::size_t maxSize = static_cast<std::size_t>(std::numeric_limits<std::int32_t>::max());
+         const std::size_t oldCap = static_cast<std::size_t>(*capacityPtr);
+         const std::size_t grown = oldCap <= maxSize / 2 ? oldCap * 2 : maxSize;
+         newCapacity = std::min(maxSize, std::max(nItems, grown));
+      }
+
       Internal::DestroyRVecWithChecks(itemField->GetAlignment(), beginPtr, capacityPtr);
       // We trust that malloc returns a buffer with large enough alignment.
       // This might not be the case if T in RVec<T> is over-aligned.
-      *beginPtr = static_cast<unsigned char *>(malloc(nItems * itemSize));
+      *beginPtr = static_cast<unsigned char *>(malloc(newCapacity * itemSize));
       R__ASSERT(*beginPtr != nullptr);
-      *capacityPtr = nItems;
+      *capacityPtr = static_cast<std::int32_t>(newCapacity);
 
       // Placement new for elements that were already there before the resize
       if (needsConstruct) {

tree/ntuple/test/rfield_vector.cxx

@@ -154,7 +154,6 @@ TEST(RNTuple, InsideCollection)
    // TODO: test reading of "klassVec.v1"
 }
 
-
 TEST(RNTuple, RVec)
 {
    FileRaii fileGuard("test_ntuple_rvec.root");
@@ -304,6 +303,46 @@ TEST(RNTuple, ReadVectorAsRVec)
    EXPECT_TRUE(rvec->empty());
 }
 
+TEST(RNTuple, RVecGeometricGrowth)
+{
+   FileRaii fileGuard("test_ntuple_rvec_geometric_growth.root");
+
+   {
+      auto m = RNTupleModel::Create();
+      auto rvec = m->MakeField<ROOT::RVec<int>>("v");
+      auto w = RNTupleWriter::Recreate(std::move(m), "ntpl", fileGuard.GetPath());
+
+      *rvec = {1, 2};
+      w->Fill();
+      *rvec = {1, 2, 3};
+      w->Fill();
+      *rvec = {1, 2, 3, 4};
+      w->Fill();
+      *rvec = {10};
+      w->Fill();
+   }
+
+   auto m = RNTupleModel::Create();
+   auto rvec = m->MakeField<ROOT::RVec<int>>("v");
+   auto reader = RNTupleReader::Open(std::move(m), "ntpl", fileGuard.GetPath());
+
+   reader->LoadEntry(0);
+   EXPECT_EQ(rvec->size(), 2u);
+   EXPECT_EQ(rvec->capacity(), 2u);
+
+   reader->LoadEntry(1);
+   EXPECT_EQ(rvec->size(), 3u);
+   EXPECT_GE(rvec->capacity(), 4u);
+
+   reader->LoadEntry(2);
+   EXPECT_EQ(rvec->size(), 4u);
+   EXPECT_GE(rvec->capacity(), 4u);
+
+   reader->LoadEntry(3);
+   EXPECT_EQ(rvec->size(), 1u);
+   EXPECT_GE(rvec->capacity(), 4u);
+}
+
 TEST(RNTuple, BoolVector)
 {
    FileRaii fileGuard("test_ntuple_boolvec.root");