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 // Author: Enrico Guiraud, Danilo Piparo CERN  02/2018
 
 /*************************************************************************
  * Copyright (C) 1995-2018, 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_RLAZYDSIMPL
 #define ROOT_RLAZYDSIMPL
 
 #include "ROOT/RDataSource.hxx"
 #include "ROOT/RResultPtr.hxx"
 #include "ROOT/TSeq.hxx"
 
 #include <algorithm>
 #include <map>
 #include <memory>
 #include <tuple>
 #include <string>
 #include <typeinfo>
 #include <utility> // std::index_sequence
 #include <vector>
 
 namespace ROOT {
 
 namespace RDF {
 ////////////////////////////////////////////////////////////////////////////////////////////////
 /// \brief A RDataSource implementation which is built on top of result proxies
 ///
 /// This component allows to create a data source on a set of columns coming from
 /// one or multiple data frames. The processing of the parent data frames starts
 /// only when the event loop is triggered in the data frame initialized with a
 /// RLazyDS.
 ///
 /// The implementation takes care of matching compile time information with runtime
 /// information, e.g. expanding in a smart way the template parameters packs.
 template <typename... ColumnTypes>
 class RLazyDS final : public ROOT::RDF::RDataSource {
    using PointerHolderPtrs_t = std::vector<ROOT::Internal::TDS::TPointerHolder *>;
 
    std::tuple<RResultPtr<std::vector<ColumnTypes>>...> fColumns;
    const std::vector<std::string> fColNames;
    const std::map<std::string, std::string> fColTypesMap;
    // The role of the fPointerHoldersModels is to be initialized with the pack
    // of arguments in the constrcutor signature at construction time
    // Once the number of slots is known, the fPointerHolders are initialized
    // according to the models.
    const PointerHolderPtrs_t fPointerHoldersModels;
    std::vector<PointerHolderPtrs_t> fPointerHolders;
    std::vector<std::pair<ULong64_t, ULong64_t>> fEntryRanges{};
    unsigned int fNSlots{0};
 
    Record_t GetColumnReadersImpl(std::string_view colName, const std::type_info &id) final
    {
       auto colNameStr = std::string(colName);
       // This could be optimised and done statically
       const auto idName = ROOT::Internal::RDF::TypeID2TypeName(id);
       auto it = fColTypesMap.find(colNameStr);
       if (fColTypesMap.end() == it) {
          std::string err = "The specified column name, \"" + colNameStr + "\" is not known to the data source.";
          throw std::runtime_error(err);
       }
 
       const auto colIdName = it->second;
       if (colIdName != idName) {
          std::string err = "Column " + colNameStr + " has type " + colIdName +
                            " while the id specified is associated to type " + idName;
          throw std::runtime_error(err);
       }
 
       const auto colBegin = fColNames.begin();
       const auto colEnd = fColNames.end();
       const auto namesIt = std::find(colBegin, colEnd, colName);
       const auto index = std::distance(colBegin, namesIt);
 
       Record_t ret(fNSlots);
       for (auto slot : ROOT::TSeqU(fNSlots)) {
          ret[slot] = fPointerHolders[index][slot]->GetPointerAddr();
       }
       return ret;
    }
 
    size_t GetEntriesNumber() { return std::get<0>(fColumns)->size(); }
    template <std::size_t... S>
    void SetEntryHelper(unsigned int slot, ULong64_t entry, std::index_sequence<S...>)
    {
       std::initializer_list<int> expander{
          (*static_cast<ColumnTypes *>(fPointerHolders[S][slot]->GetPointer()) = (*std::get<S>(fColumns))[entry], 0)...};
       (void)expander; // avoid unused variable warnings
    }
 
    template <std::size_t... S>
    void ColLengthChecker(std::index_sequence<S...>)
    {
       if (sizeof...(S) < 2)
          return;
 
       const std::vector<size_t> colLengths{std::get<S>(fColumns)->size()...};
       const auto expectedLen = colLengths[0];
       std::string err;
       for (auto i : TSeqI(1, colLengths.size())) {
          if (expectedLen != colLengths[i]) {
             err += "Column \"" + fColNames[i] + "\" and column \"" + fColNames[0] +
                    "\" have different lengths: " + std::to_string(expectedLen) + " and " +
                    std::to_string(colLengths[i]);
          }
       }
       if (!err.empty()) {
          throw std::runtime_error(err);
       }
    }
 
 protected:
    std::string AsString() final { return "lazy data source"; };
 
 public:
    RLazyDS(std::pair<std::string, RResultPtr<std::vector<ColumnTypes>>>... colsNameVals)
       : fColumns(std::tuple<RResultPtr<std::vector<ColumnTypes>>...>(colsNameVals.second...)),
         fColNames({colsNameVals.first...}),
         fColTypesMap({{colsNameVals.first, ROOT::Internal::RDF::TypeID2TypeName(typeid(ColumnTypes))}...}),
         fPointerHoldersModels({new ROOT::Internal::TDS::TTypedPointerHolder<ColumnTypes>(new ColumnTypes())...})
    {
    }
 
    ~RLazyDS()
    {
       for (auto &&ptrHolderv : fPointerHolders) {
          for (auto &&ptrHolder : ptrHolderv) {
             delete ptrHolder;
          }
       }
    }
 
    const std::vector<std::string> &GetColumnNames() const final { return fColNames; }
 
    std::vector<std::pair<ULong64_t, ULong64_t>> GetEntryRanges() final
    {
       auto entryRanges(std::move(fEntryRanges)); // empty fEntryRanges
       return entryRanges;
    }
 
    std::string GetTypeName(std::string_view colName) const final
    {
       const auto key = std::string(colName);
       return fColTypesMap.at(key);
    }
 
    bool HasColumn(std::string_view colName) const final
    {
       const auto key = std::string(colName);
       const auto endIt = fColTypesMap.end();
       return endIt != fColTypesMap.find(key);
    }
 
    bool SetEntry(unsigned int slot, ULong64_t entry) final
    {
       SetEntryHelper(slot, entry, std::index_sequence_for<ColumnTypes...>());
       return true;
    }
 
    void SetNSlots(unsigned int nSlots) final
    {
       fNSlots = nSlots;
       const auto nCols = fColNames.size();
       fPointerHolders.resize(nCols); // now we need to fill it with the slots, all of the same type
       auto colIndex = 0U;
       for (auto &&ptrHolderv : fPointerHolders) {
          for (auto slot : ROOT::TSeqI(fNSlots)) {
             auto ptrHolder = fPointerHoldersModels[colIndex]->GetDeepCopy();
             ptrHolderv.emplace_back(ptrHolder);
             (void)slot;
          }
          colIndex++;
       }
       for (auto &&ptrHolder : fPointerHoldersModels)
          delete ptrHolder;
    }
 
    void Initialize() final
    {
       ColLengthChecker(std::index_sequence_for<ColumnTypes...>());
       const auto nEntries = GetEntriesNumber();
       const auto nEntriesInRange = nEntries / fNSlots; // between integers. Should make smaller?
       auto reminder = 1U == fNSlots ? 0 : nEntries % fNSlots;
       fEntryRanges.resize(fNSlots);
       auto init = 0ULL;
       auto end = 0ULL;
       for (auto &&range : fEntryRanges) {
          end = init + nEntriesInRange;
          if (0 != reminder) { // Distribute the reminder among the first chunks
             reminder--;
             end += 1;
          }
          range.first = init;
          range.second = end;
          init = end;
       }
    }
 
    std::string GetLabel() final { return "LazyDS"; }
 };
 
 } // ns RDF
 
 } // ns ROOT
 
 #endif

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