[TDF] Add TDataSource abstract class

Author: eguiraud

tree/treeplayer/inc/ROOT/TDataSource.hxx

@@ -0,0 +1,61 @@
+// Author: Enrico Guiraud, Danilo Piparo CERN  09/2017
+
+/*************************************************************************
+ * Copyright (C) 1995-2016, Rene Brun and Fons Rademakers.               *
+ * All rights reserved.                                                  *
+ *                                                                       *
+ * For the licensing terms see $ROOTSYS/LICENSE.                         *
+ * For the list of contributors see $ROOTSYS/README/CREDITS.             *
+ *************************************************************************/
+
+#ifndef ROOT_TDATASOURCE
+#define ROOT_TDATASOURCE
+
+#include "RStringView.h"
+#include "RtypesCore.h" // ULong64_t
+#include <algorithm> // std::transform
+#include <vector>
+#include <typeinfo>
+
+namespace ROOT {
+namespace Experimental {
+namespace TDF {
+
+class TDataSource {
+public:
+   virtual ~TDataSource(){};
+   virtual const std::vector<std::string> &GetColumnNames() const = 0;
+   virtual bool HasColumn(std::string_view) const = 0;
+   /// Type of a column as a string, e.g. `GetTypeName("x") == "double"`. Required for jitting e.g. `df.Filter("x>0")`.
+   virtual std::string GetTypeName(std::string_view) const = 0;
+   /// Called at most once per column by TDF. Return vector of pointers to pointers to column values - one per slot.
+   template <typename T>
+   std::vector<T **> GetColumnReaders(std::string_view name, unsigned int nSlots)
+   {
+      auto typeErasedVec = GetColumnReadersImpl(name, nSlots, typeid(T));
+      std::vector<T **> typedVec(typeErasedVec.size());
+      std::transform(typeErasedVec.begin(), typeErasedVec.end(), typedVec.begin(),
+                     [](void *p) { return static_cast<T **>(p); });
+      return typedVec;
+   }
+   /// Return chunks of entries to distribute to tasks. They are required to be continguous intervals with no entries
+   /// skipped, starting at 0 and ending at nEntries, e.g. [0-5],[5-10] for 10 entries.
+   virtual const std::vector<std::pair<ULong64_t, ULong64_t>> &GetEntryRanges() const = 0;
+   /// Different threads will loop over different ranges and will pass different "slot" values.
+   virtual void SetEntry(ULong64_t entry, unsigned int slot) = 0;
+   /// Convenience method called at the start of each task, before processing a range of entries.
+   /// DataSources can implement it if needed (does nothing by default).
+   /// firstEntry is the first entry of the range that the task will process.
+   virtual void InitSlot(unsigned int slot, ULong64_t firstEntry) {}
+
+protected:
+   /// type-erased vector of pointers to pointers to column values - one per slot
+   virtual std::vector<void *>
+   GetColumnReadersImpl(std::string_view name, unsigned int nSlots, const std::type_info &) = 0;
+};
+
+} // ns TDF
+} // ns Experimental
+} // ns ROOT
+
+#endif // ROOT_TDATASOURCE