`THnSparse::CreateSparse` creates histograms that are not sparse

[Zehvogel]

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Description

According to the documentation "THnSparse only allocates memory for the bins that have non-zero bin content". Therefore, I expected to obtain a THnSparse that is sparse when calling THnSparse::CreateSparse on a sparsely filled THn, i.e. that the new "sparse" histogram does not contain any entries for the empty bins in the original histogram. However, when creating a "sparse" histogram this way, also the empty bins are filled, losing every benefit of the sparseness.

Reproducer

from array import array
# not the most python friendly constructor
hn = ROOT.THnD("", "", 1, array("i", [2]), [[0., 1., 2.]])
# fill a bin
hn.Fill(0.5)
print(hn.GetBinContent(1))
# 1
print(hn.GetBinContent(2))
# 0
# convert to sparse
hn_sparse = ROOT.THnSparseD.CreateSparse("", "", hn)
print(hn_sparse.GetNbins())
# 4 (2 normal + 2 flow)
print(hn_sparse.GetSparseFractionBins())
# 1.0 -> the "sparse" histogram is 100% dense :(

ROOT version

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Installation method

source /cvmfs/sw-nightlies.hsf.org/key4hep/setup.sh

Operating system

Alma9

Additional context

I suspect the issue to be in the kTRUE here in the Add called eventually by THnBase::CreateHnAny, but just removing it will probably break something else ;)

root/hist/hist/src/THnBase.cxx

Line 851 in c035b1d

mybinidx = GetBin(coord, kTRUE /*allocate*/);