[ntuple] add support for leaf count arrays in classes #20633
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jblomer
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Test Results 22 files 22 suites 3d 19h 25m 52s ⏱️ Results for commit 83ebc41. ♻️ This comment has been updated with latest results. |
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| struct LeafCountInClass : public LeafCountInClassBase { | ||
| unsigned char *fPayload1 = nullptr; //[fSize]; | ||
| unsigned char *fPayload2 = nullptr; //[fSize]; |
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I'm probably missing the context behind this PR, but looking at this class makes me wonder if we're subtly introducing support for raw pointers in RNTuple on-disk format.
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One could "emulate" a raw pointer by using fSize = 1, but since this only works for owned payloads it's not conceptually different from supporting unique_ptr (I believe the problematic part of raw pointers is the fact that they may not be owning and reference data outside the parent class)
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Also note that on-disk this is stored as an RVec<char>. This PR adds support for a peculiar memory layout of a vector, if you will.
| friend class RClassField; // only class fields are able to construct this | ||
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| private: | ||
| std::size_t fItemSize{0}; ///< The size of a child field's item |
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For better or worse, the leaf count is currently limited (hard coded) to 32bits.
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This member does not store the count (size) of the collection but the size of the collection item (as in sizeof(T) for std::vector<T>).
pcanal
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LGTM code wise. Inclusion shall be discussed at next ROOT I/O workshop.
jblomer
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New field for internal use: maps an on-disk collection to a leaf count array in a class. Writes RVec collection. Must only be used inside a class field and thus can only be constructed by RClassField.
Maps leaf count arrays in classes to RVecs using Internal::RLeafCountArrayField
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Transparently translates a leaf count array in a custom class in an RVec (read & write). Classes with leaf count arrays are used, e.g., in the ALICE RAW EDM but most likely there are other relevant cases, too.
EDIT: also needed for LHCb DST files.
Since this approach diverges the in-memory representation from the on-disk representation, we should discuss if we actually want this.
In any case, we probably want the first commit that detects leaf count arrays in classes and fails gracefully in such a case.