Code Architecture

This document lists all classes, describes their responsibilities, and details how they fit together.

Core Classes

These classes are involved when the user calls Fill. The list is ordered "bottom-up" in terms of functionality.

RRegularAxis, RVariableBinAxis, RCategoricalAxis

These classes implement a concrete axis type. Their main task is to compute the linearized index for a single Fill argument:

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RLinearizedIndex ComputeLinearizedIndex(double x);

The bool is used to indicate if the return value is valid. For example, the argument may be outside the axis with the underflow and overflow bins disabled. RLinearizedIndex is a simple struct with a std::size_t index and bool valid. It is chosen over std::optional because it unifies the return value construction: If outside the axis, the validity is just determined by the member property fEnableFlowBins.

Internal::RAxes

This class is responsible for managing a histogram's axis configuration. It stores the axis objects as a std::vector of std::variants. Objects of this class are used internally and not exposed to the user. Relevant functionality is available through user-facing classes such as RHistEngine.

The main function is

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template <typename... A>
RLinearizedIndex ComputeGlobalIndex(const std::tuple<A...> &args) const;

that dispatches arguments to the individual ComputeLinearizedIndex and combines the results. If any of the linearized indices is invalid, so will be the combination.

RHistEngine

This class combines an RAxes object and storage of bin contents in a std::vector. During Fill, it calls RAxes::ComputeLinearizedIndex and then updates the bin content. In contrast to RHist, this class supports direct concurrent filling via FillAtomic.

RHistStats

Manages the (global) histogram statistics, such as the number of entries. It also keeps statistics of the unbinned values for each dimension.

RHist

This class combines RHistEngine, with its axes and bin contents, and RHistStats. During Fill, it delegates to RHistEngine::Fill but also updates the histogram statistics.

Classes for Weighted Filling

RDoubleBinWithError

A special bin content type that also accumulates the sum of weights squared. It can be used as a template argument to RHistEngine and RHist.

RWeight

A wrapper struct for a single double value, used for weighted filling to distinguish its type. Objects of this type are passed by value.

Auxiliary Classes

RBinIndex

A single bin index, which is just an integer for normal bins. Underflow() and Overflow() are special values and not ordered with respect to others. Objects of this type are passed by value; most notably to GetBinContent and SetBinContent.

RBinIndexRange

A range of RBinIndex from begin (inclusive) to end (exclusive). The class exposes an iterator interface that can be used in range-based loops.