ENH: Declare front() and back() of Index, Offset, and Size constexpr

[N-Dekker]

Follow-up to pull request #3236 commit 47bce26
"ENH: Declare begin(), end() of FixedArray, Index, Offset, Size constexpr"

[N-Dekker]

@thewtex Would this PR still be feasible for v5.4.0? It just adds constexpr keywords to front() and back() member functions.

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FYI, Adding constexpr to itk::Index::back() would be a stepping stone towards evaluating the OffsetToIndexTable of BSplineInterpolationWeightFunction at compile-time, instead of at run-time:

ITK/Modules/Core/Common/include/itkBSplineInterpolationWeightFunction.h

Line 124 in 4d75072

const TableType m_OffsetToIndexTable{ [] {

Which is likely to speed up BSplineInterpolationWeightFunction::Evaluate:

ITK/Modules/Core/Common/include/itkBSplineInterpolationWeightFunction.hxx

Lines 45 to 48 in 4d75072

	BSplineInterpolationWeightFunction<TCoordRep, VSpaceDimension, VSplineOrder>::Evaluate(
	const ContinuousIndexType & index,
	WeightsType & weights,
	IndexType & startIndex) const

[blowekamp]

@thewtex Would this PR still be feasible for v5.4.0? It just adds constexpr keywords to front() and back() member functions.

Out of my curiosity, is this for a particular issue? Has it given any measurable performance benefit?

[N-Dekker]

Out of my curiosity, is this for a particular issue? Has it given any measurable performance benefit?

This PR would pave the way to adding constexpr to itk::Index iteration, by itk::IndexRange. Which would allow creating the OffsetToIndexTable of BSplineInterpolationWeightFunction at compile-time, instead of at run-time. I haven't yet been able to benchmark, but it may certainly benefit its performance.

Note that itk::Index<N> is similar to std::array<itk::IndexValueType, N>, whose front() and back() are also marked "constexpr": https://en.cppreference.com/w/cpp/container/array/back

[thewtex]

@N-Dekker this is awesome!

I think we should save it for ITK 6 to avoid any unforeseen issues.

[N-Dekker]

I think we should save it for ITK 6 to avoid any unforeseen issues.

@thewtex OK, thanks. Marked "draft" to avoid that this PR might accidentally be merged before the release of ITK 5.4!

[N-Dekker]

I think this pull request may also be merged now, as v5.4.0 has been tagged (#4603 (comment))