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[hist] Implement initial RHistConcurrentFiller
In order to support efficient concurrent filling of RHist with global histogram statistics, we keep one local RHistStats object per RHistFillContext.
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‎hist/histv7/doc/CodeArchitecture.md‎

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A wrapper `struct` for a single `double` value, used for weighted filling to distinguish its type.
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Objects of this type are passed by value.
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## Classes for Concurrent Filling
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### `RHistConcurrentFiller`
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A class to orchestrate concurrent filling of `RHist` by creating (multiple) fill contexts.
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### `RHistFillContext`
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Parallel user code uses contexts to fill `RHist`s concurrently.
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To uses a local `RHistStats` object to avoid contention on the global histogram statistics.
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## Auxiliary Classes
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### `RBinIndex`

‎hist/histv7/doc/ConcurrentFilling.md‎

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On the other hand, updates of the (global) histogram statistics (`RHistStats`) can easily lead to contention.
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For this reason, `RHist` does **not** offer a `FillAtomic` method because it cannot be implemented efficiently.
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Instead, the user has to create a `RHistConcurrentFiller` and (potentially many) `RHistFillContext`s.
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These will work together to accumulate the (global) histogram statistics during concurrent filling.
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These work together to accumulate the (global) histogram statistics during concurrent filling.

‎hist/histv7/headers.cmake‎

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ROOT/RCategoricalAxis.hxx
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ROOT/RHist.hxx
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ROOT/RHistAutoAxisFiller.hxx
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ROOT/RHistConcurrentFiller.hxx
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ROOT/RHistEngine.hxx
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ROOT/RHistFillContext.hxx
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ROOT/RHistStats.hxx
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ROOT/RHistUtils.hxx
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ROOT/RLinearizedIndex.hxx

‎hist/histv7/inc/ROOT/RHist.hxx‎

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namespace ROOT {
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namespace Experimental {
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// forward declaration for friend declaration
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template <typename BinContentType>
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class RHistFillContext;
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/**
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A histogram for aggregation of data along multiple dimensions.
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@@ -53,6 +57,8 @@ Feedback is welcome!
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*/
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template <typename BinContentType>
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class RHist final {
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friend class RHistFillContext<BinContentType>;
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/// The histogram engine including the bin contents.
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RHistEngine<BinContentType> fEngine;
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/// The global histogram statistics.

‎hist/histv7/inc/ROOT/RHistConcurrentFiller.hxx‎

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/// \file
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/// \warning This is part of the %ROOT 7 prototype! It will change without notice. It might trigger earthquakes.
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/// Feedback is welcome!
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#ifndef ROOT_RHistConcurrentFiller
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#define ROOT_RHistConcurrentFiller
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#include "RHist.hxx"
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#include "RHistEngine.hxx"
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#include "RHistFillContext.hxx"
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#include "RWeight.hxx"
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#include <exception>
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#include <memory>
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#include <mutex>
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#include <stdexcept>
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#include <vector>
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namespace ROOT {
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namespace Experimental {
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/**
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A histogram filler to concurrently fill an RHist.
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\code
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auto hist = std::make_shared<ROOT::Experimental::RHist<int>>(10, std::make_pair(5, 15));
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{
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ROOT::Experimental::RHistConcurrentFiller filler(hist);
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auto context = filler.CreateFillContext();
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context.Fill(8.5);
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}
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// hist->GetBinContent(ROOT::Experimental::RBinIndex(3)) will return 1
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\endcode
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\warning This is part of the %ROOT 7 prototype! It will change without notice. It might trigger earthquakes.
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Feedback is welcome!
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*/
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template <typename BinContentType>
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class RHistConcurrentFiller final {
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/// A pointer to the filled histogram
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std::shared_ptr<RHist<BinContentType>> fHist;
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/// Mutex to protect access to the list of fill contexts (not for filling itself!)
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std::mutex fMutex;
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/// The list of fill contexts, for checks during destruction
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std::vector<std::weak_ptr<RHistFillContext<BinContentType>>> fFillContexts;
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public:
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/// Create a filler object.
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///
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/// \param[in] hist a pointer to the histogram
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explicit RHistConcurrentFiller(std::shared_ptr<RHist<BinContentType>> hist) : fHist(hist)
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{
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if (!hist) {
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throw std::invalid_argument("hist must not be nullptr");
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}
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}
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RHistConcurrentFiller(const RHistConcurrentFiller<BinContentType> &) = delete;
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RHistConcurrentFiller(RHistConcurrentFiller<BinContentType> &&) = delete;
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RHistConcurrentFiller<BinContentType> &operator=(const RHistConcurrentFiller<BinContentType> &) = delete;
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RHistConcurrentFiller<BinContentType> &operator=(RHistConcurrentFiller<BinContentType> &&) = delete;
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~RHistConcurrentFiller()
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{
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for (const auto &context : fFillContexts) {
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if (!context.expired()) {
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// According to C++ Core Guideline C.36 "A destructor must not fail" and (C.37) "If a destructor tries to
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// exit with an exception, it’s a bad design error and the program had better terminate".
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std::terminate(); // GCOVR_EXCL_LINE
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}
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}
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}
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const std::shared_ptr<RHist<BinContentType>> &GetHist() const { return fHist; }
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/// Create a new context for concurrent filling.
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std::shared_ptr<RHistFillContext<BinContentType>> CreateFillContext()
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{
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std::lock_guard g(fMutex);
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// Cannot use std::make_shared because the constructor of RHistFillContext is private. Also it would mean that the
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// (direct) memory of all contexts stays around until the vector of weak_ptr's is cleared.
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std::shared_ptr<RHistFillContext<BinContentType>> context(new RHistFillContext<BinContentType>(*fHist));
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fFillContexts.push_back(context);
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return context;
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}
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};
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} // namespace Experimental
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} // namespace ROOT
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#endif

‎hist/histv7/inc/ROOT/RHistFillContext.hxx‎

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/// \file
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/// \warning This is part of the %ROOT 7 prototype! It will change without notice. It might trigger earthquakes.
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/// Feedback is welcome!
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#ifndef ROOT_RHistFillContext
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#define ROOT_RHistFillContext
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#include "RHist.hxx"
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#include "RHistEngine.hxx"
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#include "RHistStats.hxx"
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namespace ROOT {
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namespace Experimental {
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// forward declaration for friend declaration
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template <typename BinContentType>
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class RHistConcurrentFiller;
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/**
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A context to concurrently fill an RHist.
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\sa RHistConcurrentFiller
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\warning This is part of the %ROOT 7 prototype! It will change without notice. It might trigger earthquakes.
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Feedback is welcome!
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*/
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template <typename BinContentType>
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class RHistFillContext final {
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friend class RHistConcurrentFiller<BinContentType>;
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private:
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/// A pointer to the filled histogram
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RHist<BinContentType> *fHist;
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/// Local histogram statistics
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RHistStats fStats;
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/// \sa RHistConcurrentFiller::CreateFillContent()
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explicit RHistFillContext(RHist<BinContentType> &hist) : fHist(&hist), fStats(hist.GetStats())
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{
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// The histogram may already have some statistics. Reset them after copying the structure.
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fStats.Clear();
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}
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RHistFillContext(const RHistFillContext<BinContentType> &) = delete;
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RHistFillContext(RHistFillContext<BinContentType> &&) = default;
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RHistFillContext<BinContentType> &operator=(const RHistFillContext<BinContentType> &) = delete;
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RHistFillContext<BinContentType> &operator=(RHistFillContext<BinContentType> &&) = default;
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public:
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~RHistFillContext() { Flush(); }
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/// Fill an entry into the histogram.
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///
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/// If one of the arguments is outside the corresponding axis and flow bins are disabled, the entry will be silently
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/// discarded.
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///
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/// Throws an exception if the number of arguments does not match the axis configuration, or if an argument cannot be
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/// converted for the axis type at run-time.
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///
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/// \param[in] args the arguments for each axis
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/// \sa RHist::Fill(const std::tuple<A...> &args)
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template <typename... A>
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void Fill(const std::tuple<A...> &args)
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{
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fHist->fEngine.FillAtomic(args);
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fStats.Fill(args);
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}
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/// Fill an entry into the histogram with a weight.
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///
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/// This overload is not available for integral bin content types (see \ref RHistEngine::SupportsWeightedFilling).
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///
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/// If one of the arguments is outside the corresponding axis and flow bins are disabled, the entry will be silently
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/// discarded.
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///
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/// Throws an exception if the number of arguments does not match the axis configuration, or if an argument cannot be
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/// converted for the axis type at run-time.
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///
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/// \param[in] args the arguments for each axis
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/// \param[in] weight the weight for this entry
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/// \sa RHist::Fill(const std::tuple<A...> &args, RWeight weight)
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template <typename... A>
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void Fill(const std::tuple<A...> &args, RWeight weight)
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{
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fHist->fEngine.FillAtomic(args, weight);
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fStats.Fill(args, weight);
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}
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/// Fill an entry into the histogram.
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///
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/// For weighted filling, pass an RWeight as the last argument. This is not available for integral bin content types
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/// (see \ref RHistEngine::SupportsWeightedFilling).
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///
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/// If one of the arguments is outside the corresponding axis and flow bins are disabled, the entry will be silently
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/// discarded.
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///
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/// Throws an exception if the number of arguments does not match the axis configuration, or if an argument cannot be
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/// converted for the axis type at run-time.
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///
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/// \param[in] args the arguments for each axis
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/// \sa RHist::Fill(const A &...args)
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template <typename... A>
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void Fill(const A &...args)
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{
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fHist->fEngine.FillAtomic(args...);
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fStats.Fill(args...);
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}
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/// Flush locally accumulated entries to the histogram.
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void Flush()
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{
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fHist->fStats.AddAtomic(fStats);
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fStats.Clear();
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}
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};
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} // namespace Experimental
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} // namespace ROOT
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#endif

‎hist/histv7/test/CMakeLists.txt‎

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HIST_ADD_GTEST(hist_auto hist_auto.cxx)
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HIST_ADD_GTEST(hist_axes hist_axes.cxx)
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HIST_ADD_GTEST(hist_categorical hist_categorical.cxx)
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HIST_ADD_GTEST(hist_concurrent hist_concurrent.cxx)
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HIST_ADD_GTEST(hist_engine hist_engine.cxx)
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HIST_ADD_GTEST(hist_engine_atomic hist_engine_atomic.cxx)
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HIST_ADD_GTEST(hist_hist hist_hist.cxx)

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