Operand-driven diagonal algebra for the dense path - #224
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Preserve the Diagonal type through the operations whose result is genuinely diagonal: factorizations return the spectrum bare, unmatricize keeps the (1,1) endomorphism split, and single-contracted-leg Diagonal times Diagonal stays Diagonal. Densify only where the result is not representable as a Diagonal.
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The dense `unmatricize` reshaped without validating the split, so a mismatched codomain/domain boundary with the right total element count would reshape silently. Add a `check_input` size guard, matching the `Diagonal` and graded paths.
Replaces a runtime length branch in the two-`Diagonal` `contract` output allocation with dispatch on the perm tuples, matching the operand-driven convention. Also drops the `Diagonal`-specific `unmatricize` `check_input`, whose general-looking signature only admitted the `{1,1}` split, in favor of the generic size check.
A dense `unmatricize` is a `reshape`, so it only needs the total element count to match, not the per-side codomain/domain split. The earlier per-side check rejected valid regroupings such as `unmatricize(m, (), axes)`.
`promote_type` returns the plain element type, but a product accumulation can widen (for example `Bool` inputs accumulate to `Int`), so model it with `Base.promote_op(matprod, ...)` as matmul does.
The helper is only ever called with `Diagonal` operands and reads `a1.diag`, so annotate `a1::Diagonal` to make that explicit.
The output type is a function of both operands, so take `a1::Diagonal, a2::Diagonal` even though the allocation reads only the first.
Re-opens 0.20.0 as a prerelease accumulator (its standalone registration was removed), so these changes ship in a single 0.20.0 release rather than a separate patch.
The matricized input's rows must be the fused codomain and its columns the fused domain, matching what `matricize` produces, so a matrix with the right element count but the wrong split is rejected instead of reshaped silently.
Mechanical rename matching the `axes_codomain`/`axes_domain` naming used across the ecosystem.
The standalone 0.20.0 registration was pulled, so this ships as the 0.20.0 release (registered manually, since the version matches main).
mtfishman
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August 21, 2026 01:09
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Summary
Make diagonal algebra operand-driven:
Diagonalis kept when the result is genuinely diagonal and densified otherwise. Also renames[co]domain_axestoaxes_[co]domain. Companion to ITensor/GradedArrays.jl#264.