Add a hardcoded reader for TMatrixTSym<T> (no file ever carries its StreamerInfo) #73

🤖 AI generated content

Line item of #9: Missing derived class streamer for TMatrixTSym<double> (uproot-issue-359.root; formerly "unknown type TMatrixTSym3cdouble3e"). Currently a special-cased NotImplementedError at src/rootfilespec/dynamic.py:95-100.

Spec: Matrices and vectors §2.2 — TMatrixTSym<Element>::Streamer passes ReadClassBuffer the TClass of TMatrixTBase<Element>, not its own, so "the file carries no streamer info for TMatrixTSym<T> at all … a reader … must dispatch on the name." The error message's premise (a missing derived streamer) is therefore wrong: it is never there. Confirmed: this file's entire streamer-info list is one entry, TMatrixTBase<double> v5.

§4 gives the read procedure: read the frame (the version word is TMatrixTBase's, 5), decode the base's members from the base's info, then — past the end implied by the byte count — read fNrows × (fNrows + 1) / 2 values of T in row order, row i holding fNcols − i of them. TMatrixTSym is one of the three extending classes of Buffer framing §2.4: "A byte count is a lower bound on the length of the object." The spec uses this very file as its independent witness: five records, 29×29 and 58×58, accounting to the byte. data/classes/matrix.root in the vendored spec is a second test vector.

Suggested fix: one hardcoded class in bootstrap/ dispatched on the name prefix TMatrixTSym<, which reads its base via the file's TMatrixTBase<T> info and then the packed triangle; element width from the template argument (double → 8, float → 4).

Severity / size: low, S–M, self-contained. Clears 1 xfail. The "object may extend past its byte count" handling overlaps #98.

Related: #9, #101.

Assisted-by: claude-code:claude-fable-5-1