Add unproject to invert project given the codomain/domain split#211
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Adds `unproject(a, ndims_codomain::Val)`, the inverse of `project`: it recovers the dense array `project` maps to `a`, taking the codomain/domain split as a `Val`. The default is `convert(Array, a)`. A backend that changes basis during `project` (for example a graded array that bends its domain legs into the codomain to carry a fermion sign) overloads it to undo that change. `project` and `tryproject` now verify their result through a split-aware `is_projected(dest, src, ndims_codomain::Val)` that compares against `unproject` rather than `convert(Array, dest)`, so such a backend passes its own round-trip check. Behavior is unchanged for backends that store the projection in the plain dense layout such as a dense array or a `TensorMap`, for which `unproject` is `convert(Array, ·)`. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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The two-argument `is_projected` now forwards to the split-aware form using the destination's own codomain rank, given by a new `ndims_codomain` accessor (`ndims` by default, `numout` for a `TensorMap`). This removes the duplicated method body and makes the two-argument form the split-aware check at the destination's own split. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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## Summary `project(A, codomain, domain)` for a fermionic operator now bends its domain (bra) legs, multiplying each stored block by the `±1` fermion sign of moving them past the codomain, so the operator contracts like the corresponding `TensorMap`. Previously the three-argument operator projection collapsed to the naive dual form and dropped that sign, so an operator with a both-odd domain block (for example a hopping gate's doubly-occupied diagonal) contracted with the wrong sign. `unproject(a, Val(K))` inverts the bend given the codomain/domain split, and `project` runs its self-check through it so the round trip holds. This builds on the `unproject` verb from ITensor/TensorAlgebra.jl#211. This also fixes the fermion permutation and bend signs in `matricizeopperm`, `unmatricizeperm!`, and `bipermutedimsopadd!`, so fermionic contraction no longer depends on contraction order. Breaking (`0.14.0`) because it changes the results of fermionic operator contraction. --------- Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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Summary
Adds
unproject(a, ndims_codomain::Val), the inverse ofproject. It recovers the dense array thatprojectmapsafrom, given the codomain/domain split as aVal. The default isconvert(Array, a). A backend that changes basis duringprojectoverloadsunprojectto undo that change, so thatunproject(project(raw, codomain_axes, domain_axes), Val(length(codomain_axes)))recoversraw.projectandtryprojectnow run theiris_projectedself-check throughunprojectwith the split they were handed, instead of comparing againstconvert(Array, dest). A backend that stores the projection in the plain dense layout is unaffected. A backend that changes basis inprojectis now checked in the frame the input was given in, so its self-check passes. The two-argumentis_projectedforwards to this split-aware form using the destination's own codomain rank, a newndims_codomainaccessor that aTensorMapoverloads withnumout.The TensorKit extension defines
unprojectforAbstractTensorMapasconvert(Array, ·), which already undoes the domain braiding, guarded by a check that the passed split matches the map's codomain rank.