[ntuple] Add GetNEntries() to RPageSink

This is needed by the RNTupleMerger to properly know the initial
number of entries in the destination sink in case of incremental
merging. Since the Descriptor's NEntries is not updated until the
first cluster group is committed - and since we don't commit a cluster
group in InitFromDescriptor() - it cannot be used for that purpose.

Author: silverweed

Diff for: tree/ntuple/v7/inc/ROOT/RNTupleDescriptor.hxx

@@ -1405,6 +1405,8 @@ public:
    RResult<void> AddClusterGroup(RClusterGroupDescriptor &&clusterGroup);
    RResult<void> AddCluster(RClusterDescriptor &&clusterDesc);
 
+   RResult<void> AddNEntries(std::uint64_t nEntries);
+
    RResult<void> AddExtraTypeInfo(RExtraTypeInfoDescriptor &&extraTypeInfoDesc);
 
    /// Clears so-far stored clusters, fields, and columns and return to a pristine ntuple descriptor

Diff for: tree/ntuple/v7/inc/ROOT/RPageNullSink.hxx

@@ -48,6 +48,8 @@ public:
       return descriptor;
    }
 
+   NTupleSize_t GetNEntries() const final { return 0; }
+
    void ConnectFields(const std::vector<RFieldBase *> &fields, NTupleSize_t firstEntry)
    {
       auto connectField = [&](RFieldBase &f) { CallConnectPageSinkOnField(f, *this, firstEntry); };

Diff for: tree/ntuple/v7/inc/ROOT/RPageSinkBuf.hxx

@@ -133,6 +133,8 @@ public:
 
    const RNTupleDescriptor &GetDescriptor() const final;
 
+   NTupleSize_t GetNEntries() const final { return fInnerSink->GetNEntries(); }
+
    void InitImpl(RNTupleModel &model) final;
    void UpdateSchema(const RNTupleModelChangeset &changeset, NTupleSize_t firstEntry) final;
    void UpdateExtraTypeInfo(const RExtraTypeInfoDescriptor &extraTypeInfo) final;

Diff for: tree/ntuple/v7/inc/ROOT/RPageStorage.hxx

@@ -317,6 +317,8 @@ public:
    /// Return the RNTupleDescriptor being constructed.
    virtual const RNTupleDescriptor &GetDescriptor() const = 0;
 
+   virtual NTupleSize_t GetNEntries() const = 0;
+
    /// Physically creates the storage container to hold the ntuple (e.g., a keys a TFile or an S3 bucket)
    /// Init() associates column handles to the columns referenced by the model
    void Init(RNTupleModel &model)
@@ -519,6 +521,8 @@ public:
 
    const RNTupleDescriptor &GetDescriptor() const final { return fDescriptorBuilder.GetDescriptor(); }
 
+   NTupleSize_t GetNEntries() const final { return fPrevClusterNEntries; }
+
    /// Updates the descriptor and calls InitImpl() that handles the backend-specific details (file, DAOS, etc.)
    void InitImpl(RNTupleModel &model) final;
    void UpdateSchema(const RNTupleModelChangeset &changeset, NTupleSize_t firstEntry) final;

Diff for: tree/ntuple/v7/src/RNTupleDescriptor.cxx

@@ -595,11 +595,9 @@ ROOT::Experimental::RNTupleDescriptor::AddClusterGroupDetails(DescriptorId_t clu
          return R__FAIL("invalid attempt to re-populate existing cluster");
       }
    }
-   std::sort(clusterIds.begin(), clusterIds.end(),
-             [this](DescriptorId_t a, DescriptorId_t b)
-             {
-                return fClusterDescriptors[a].GetFirstEntryIndex() < fClusterDescriptors[b].GetFirstEntryIndex();
-             });
+   std::sort(clusterIds.begin(), clusterIds.end(), [this](DescriptorId_t a, DescriptorId_t b) {
+      return fClusterDescriptors[a].GetFirstEntryIndex() < fClusterDescriptors[b].GetFirstEntryIndex();
+   });
    auto cgBuilder = Internal::RClusterGroupDescriptorBuilder::FromSummary(iter->second);
    cgBuilder.AddSortedClusters(clusterIds);
    iter->second = cgBuilder.MoveDescriptor().Unwrap();
@@ -949,10 +947,9 @@ ROOT::Experimental::RNTupleDescriptor ROOT::Experimental::Internal::RNTupleDescr
    for (const auto &[id, _] : fDescriptor.fClusterGroupDescriptors)
       fDescriptor.fSortedClusterGroupIds.emplace_back(id);
    std::sort(fDescriptor.fSortedClusterGroupIds.begin(), fDescriptor.fSortedClusterGroupIds.end(),
-             [this](DescriptorId_t a, DescriptorId_t b)
-             {
-               return fDescriptor.fClusterGroupDescriptors[a].GetMinEntry() <
-                      fDescriptor.fClusterGroupDescriptors[b].GetMinEntry();
+             [this](DescriptorId_t a, DescriptorId_t b) {
+                return fDescriptor.fClusterGroupDescriptors[a].GetMinEntry() <
+                       fDescriptor.fClusterGroupDescriptors[b].GetMinEntry();
              });
    RNTupleDescriptor result;
    std::swap(result, fDescriptor);
@@ -1173,6 +1170,12 @@ ROOT::Experimental::Internal::RNTupleDescriptorBuilder::AddClusterGroup(RCluster
    return RResult<void>::Success();
 }
 
+ROOT::RResult<void> ROOT::Experimental::Internal::RNTupleDescriptorBuilder::AddNEntries(std::uint64_t nEntries)
+{
+   fDescriptor.fNEntries += nEntries;
+   return RResult<void>::Success();
+}
+
 void ROOT::Experimental::Internal::RNTupleDescriptorBuilder::Reset()
 {
    fDescriptor.fName = "";

Diff for: tree/ntuple/v7/src/RNTupleParallelWriter.cxx

@@ -72,6 +72,8 @@ class RPageSynchronizingSink : public RPageSink {
 
    const RNTupleDescriptor &GetDescriptor() const final { return fInnerSink->GetDescriptor(); }
 
+   NTupleSize_t GetNEntries() const final { return fInnerSink->GetNEntries(); }
+
    ColumnHandle_t AddColumn(DescriptorId_t, RColumn &) final { return {}; }
    void InitImpl(RNTupleModel &) final {}
    void UpdateSchema(const RNTupleModelChangeset &, NTupleSize_t) final

Diff for: tree/ntuple/v7/src/RPageStorage.cxx

@@ -257,6 +257,7 @@ void ROOT::Experimental::Internal::RPageSource::UnzipClusterImpl(RCluster *clust
          continue;
       const auto &columnInfos = fActivePhysicalColumns.GetColumnInfos(columnId);
 
+      allElements.reserve(allElements.size() + columnInfos.size());
       for (const auto &info : columnInfos) {
          allElements.emplace_back(GenerateColumnElement(info.fElementId));
 
@@ -857,6 +858,8 @@ void ROOT::Experimental::Internal::RPagePersistentSink::UpdateSchema(const RNTup
    }
 
    const auto nColumns = descriptor.GetNPhysicalColumns();
+   fOpenColumnRanges.reserve(fOpenColumnRanges.size() + (nColumns - nColumnsBeforeUpdate));
+   fOpenPageRanges.reserve(fOpenPageRanges.size() + (nColumns - nColumnsBeforeUpdate));
    for (DescriptorId_t i = nColumnsBeforeUpdate; i < nColumns; ++i) {
       RClusterDescriptor::RColumnRange columnRange;
       columnRange.fPhysicalColumnId = i;
@@ -899,8 +902,10 @@ void ROOT::Experimental::Internal::RPagePersistentSink::InitImpl(RNTupleModel &m
    projectedFields.GetFieldZero().SetOnDiskId(0);
 
    RNTupleModelChangeset initialChangeset{model};
+   initialChangeset.fAddedFields.reserve(fieldZero.GetSubFields().size());
    for (auto f : fieldZero.GetSubFields())
       initialChangeset.fAddedFields.emplace_back(f);
+   initialChangeset.fAddedProjectedFields.reserve(projectedFields.GetFieldZero().GetSubFields().size());
    for (auto f : projectedFields.GetFieldZero().GetSubFields())
       initialChangeset.fAddedProjectedFields.emplace_back(f);
    UpdateSchema(initialChangeset, 0U);
@@ -1133,6 +1138,7 @@ void ROOT::Experimental::Internal::RPagePersistentSink::CommitClusterGroup()
 
    const auto nClusters = descriptor.GetNActiveClusters();
    std::vector<DescriptorId_t> physClusterIDs;
+   physClusterIDs.reserve(nClusters);
    for (auto i = fNextClusterInGroup; i < nClusters; ++i) {
       physClusterIDs.emplace_back(fSerializationContext.MapClusterId(i));
    }
@@ -1156,6 +1162,7 @@ void ROOT::Experimental::Internal::RPagePersistentSink::CommitClusterGroup()
          .NClusters(nClusters - fNextClusterInGroup);
    }
    std::vector<DescriptorId_t> clusterIds;
+   clusterIds.reserve(nClusters);
    for (auto i = fNextClusterInGroup; i < nClusters; ++i) {
       clusterIds.emplace_back(i);
    }

Diff for: tree/ntuple/v7/test/ntuple_endian.cxx

@@ -49,6 +49,8 @@ class RPageSinkMock : public RPageSink {
       return descriptor;
    }
 
+   NTupleSize_t GetNEntries() const final { return 0; }
+
    void InitImpl(RNTupleModel &) final {}
    void UpdateSchema(const ROOT::Experimental::Internal::RNTupleModelChangeset &, NTupleSize_t) final {}
    void UpdateExtraTypeInfo(const ROOT::Experimental::RExtraTypeInfoDescriptor &) final {}

Diff for: tree/ntuple/v7/test/ntuple_storage.cxx

@@ -41,6 +41,8 @@ class RPageSinkMock : public RPageSink {
       return descriptor;
    }
 
+   NTupleSize_t GetNEntries() const final { return 0; }
+
    void InitImpl(RNTupleModel &) final {}
    void UpdateSchema(const ROOT::Experimental::Internal::RNTupleModelChangeset &, NTupleSize_t) final {}
    void UpdateExtraTypeInfo(const ROOT::Experimental::RExtraTypeInfoDescriptor &) final {}