Add optional SpinGlassPEPS backend

[bernalde]

Summary

• Adds internal structured PEPS backend scaffolding: SquareGrid, KingGrid, PEPSBackend, solve_ising, and PEPSSolution.
• Adds an optional TenSolverSpinGlassPEPSExt package extension that translates structured Ising/QUBO inputs into SpinGlassNetworks/SpinGlassEngine calls and decodes PEPS states back to TenSolver Boolean states.
• Keeps the PEPS backend/topology/result types non-public while the registered SpinGlass component stack cannot be exercised in default CI.
• Keeps DMRG as the default backend and preserves clear errors when the optional SpinGlass component stack is not available.
• Merges the updated PR Introduce solver backend interface #43 base, including current QUBODrivers/QUBOTools compat and backend error handling.
• Adds an internal IsingModel adapter so the PEPS and solve_ising boundary works with the current QUBOTools-form conversion API.
• Records per-transformation PEPS failures while continuing to try later transformations, and aggregates duplicate decoded-state probability mass.
• Removes the unrelated Solution export and keeps Solution documented as TenSolver.Solution.

Tests run

• git diff --check - passed.
• julia +1.12 --startup-file=no -e 'Meta.parse(read("ext/TenSolverSpinGlassPEPSExt.jl", String)); println("extension syntax parsed")' - passed.
• julia +1.12 --project=. -e 'using Pkg; Pkg.instantiate()' - passed.
• julia +1.12 --project=. -e 'using Test, Random, LinearAlgebra, TenSolver; include("test/utils.jl"); include("test/backend.jl"); include("test/peps_backend.jl")' - passed; backend interface 32 pass, PEPS backend core 37 pass, optional SpinGlassPEPS extension 1 expected broken.
• julia +1.12 --project=. -e 'using Test, Random, LinearAlgebra, TenSolver; include("test/ising_conversion.jl")' - passed, 3611 tests.
• julia +1.12 --project=docs/ -e 'using Pkg; Pkg.instantiate()' - passed.
• julia +1.12 --project=docs/ docs/make.jl - passed; deployment skipped outside CI.
• julia +1.12 --project=. -e 'using Pkg; Pkg.test(; coverage=false)' - passed.

Notes

• The first instantiate/precompile attempt after the base merge exposed the missing IsingModel adapter on the current QUBOTools-form base; that is fixed in edd04e479ecb1a3f6116ea6b32972be02f376963.
• The real SpinGlassPEPS CPU solve path still cannot be executed in the default environment because the optional SpinGlass component stack is unresolved/unsupported there. The extension remains experimental scaffolding.

Closes #38

[bernalde]

GitHub rejected REQUEST_CHANGES because the authenticated account owns this pull request. The findings below are blocking from a maintainer review perspective. I would not merge this until the blocking issues above are addressed.

[bernalde]

Blocking: This reintroduces the hard-coded Symbol backend normalization that the current base just fixed. Because every symbol reaches this method, a package extension cannot add TenSolver._normalize_backend(::Val{:peps}) or another symbol-specific mapping; backend = :peps will always throw before extension-specific _minimize or _solve_ising methods can run. It also drops the fake-symbol regression test from test/backend.jl relative to the current base.

Please keep the current-base Val dispatch structure and layer PEPSBackend on top of it: _normalize_backend(backend::Symbol) = _normalize_backend(Val(backend)), _normalize_backend(::Val{:dmrg}) = default_backend, and a Val fallback that throws _backend_error(backend). Then keep/add a test proving a symbol-specific backend can be registered by an extension.

[bernalde]

Addressed by the restacked base and preserved here. The branch now inherits 009bcd1, which restores _normalize_backend(backend::Symbol) = _normalize_backend(Val(backend)), the Val{:dmrg} method, the Val fallback, and the fake-symbol regression test in test/backend.jl. No additional change was needed in e122868556e03840b5124333c6410b974e35a3db.

[bernalde]

Blocking: This documents that the registered SpinGlass component compat bounds do not resolve with TenSolver, and the tests skip the only extension exercise when those packages are absent. That means the PR exports and documents PEPSBackend as current public behavior, but users cannot activate it from registered packages and CI never runs the implementation in ext/TenSolverSpinGlassPEPSExt.jl. I confirmed the resolution failure in a fresh environment by developing this PR and adding SpinGlassNetworks, SpinGlassEngine, and SpinGlassTensors; Pkg reports no compatible Parsers version because SpinGlassNetworks 1.4 constrains CSV/Parsers while the TenSolver environment constrains InlineStrings/Parsers.

Please either keep this as non-public scaffolding until the optional stack resolves, or update the dependency/compat strategy so the optional packages install together with TenSolver and add a CI/test path that exercises the extension.

[bernalde]

Addressed in e122868. PEPS backend/topology/result types are no longer exported or listed in public API docs, and docs/src/spinglasspeps_integration.md now describes the bridge as non-public scaffolding until the SpinGlass dependencies resolve and CI can cover the extension.

[bernalde]

Nonblocking: sample(::PEPSSolution) returns the most probable retained state deterministically rather than sampling according to probabilities, so sample(psi, n) repeats the same state. That is surprising next to sample(::Solution), which samples from the represented distribution, and next to the public probabilities field. Consider sampling from the cumulative probability distribution, or exposing deterministic best-state selection through a separate helper if that behavior is intentional.

[bernalde]

Addressed in e122868. sample(::PEPSSolution) now draws from cumulative probability weights and validates length, nonnegative weights, and positive total weight. test/peps_backend.jl covers the weighted path and malformed probability vectors.

[bernalde]

Pushed commit e122868556e03840b5124333c6410b974e35a3db.

Main changes:

• Kept the PEPS bridge as non-public scaffolding while the registered SpinGlass component stack does not resolve with TenSolver and CI cannot exercise the extension.
• Removed PEPS backend/topology/result exports and public API docs, while keeping internal qualified access for extension development.
• Updated the integration note to describe the PEPS path as gated internal scaffolding rather than current public behavior.
• Changed sample(::PEPSSolution) to sample by cumulative probability weights and added regression coverage for weighted sampling plus malformed probability vectors.
• The outdated Symbol-dispatch thread is already addressed by the current Introduce solver backend interface #43 base: PR Add optional SpinGlassPEPS backend #44 now has the Val dispatch hook and fake-symbol backend test.

Tests run:

• julia +1.12 --project=. -e 'using Test, Random, LinearAlgebra, TenSolver; include("test/utils.jl"); include("test/backend.jl"); include("test/peps_backend.jl")' - passed; backend interface 19 pass, PEPS backend core 34 pass, optional SpinGlassPEPS extension 1 expected broken.
• julia +1.12 --project=. -e 'using Pkg; Pkg.test()' - passed.
• julia +1.11.5 --project=docs/ docs/make.jl local - passed.
• git diff --check - passed.

Notes on tests not cleanly runnable:

• julia +1.12 --project=docs/ docs/make.jl local reaches doctests and document checks but fails during HTML rendering with could not find source path for package MbedTLS_jll from docs/Manifest.toml. The docs manifest was resolved with Julia 1.11.5, and the same docs command passes under +1.11.5.

Comments intentionally not addressed:

• The outdated Symbol-dispatch thread did not need new code in this commit because the restacked Introduce solver backend interface #43 base already contains the fix.

Remaining risks/follow-up:

• Downstream stacked PRs Add JuMP PEPS backend attributes #45 and Add PEPS docs, examples, and benchmarks #46 will need to be rebased after this branch update.
• The SpinGlass component stack still needs a resolvable compat/CI path before the PEPS backend should become public.

[bernalde]

Fresh review of the current net diff: I did not find blocking issues. The PEPS scaffold is now kept out of exports/public API docs, and the restored Val backend dispatch preserves the extension hook. I left two nonblocking comments on test coverage and documentation wording.

[bernalde]

Nonblocking: solve_ising is now exported and listed in the API reference, but the PR only tests a one-spin field-only call through the IsingModel method. That does not exercise pair couplings, offset preservation, or the public solve_ising(J, h, offset) overload.

Please add a small exact 2- or 3-spin regression test that brute-forces the Ising objective and asserts both solve_ising(model; ...) and solve_ising(J, h, offset; ...) return the optimum and a Boolean sample whose converted spin state has matching Ising energy. That would make the new public API safer to maintain while the PEPS path remains internal.

[bernalde]

Addressed in f3c4db1. test/backend.jl now brute-forces a two-spin Ising objective with pair coupling and offset, then checks both solve_ising(model; ...) and solve_ising(J, h, offset; ...) return the optimum and a sample whose spin energy matches it.

[bernalde]

Nonblocking: This wording groups solve_ising into the non-public scaffolding, but solve_ising is exported and included in the public API reference. To keep the API boundary clear, please reword this to say the PEPS backend/topology/result types are internal scaffolding, while solve_ising is the public Ising boundary that optional structured backends may implement.

[bernalde]

Addressed in f3c4db1. The integration note now says backend/topology/result boundaries are internal scaffolding, while exported solve_ising is the public Ising boundary optional structured backends may implement.

[bernalde]

Pushed commit f3c4db10db4af978bb04f1ef9c804604f8b92c1a.

Main changes:

• Added an exact two-spin solve_ising regression covering pair couplings, offset preservation, the IsingModel method, and the public solve_ising(J, h, offset) overload.
• Clarified the SpinGlassPEPS integration note so PEPS backend/topology/result boundaries are internal scaffolding while exported solve_ising remains the public Ising boundary optional structured backends may implement.

Tests run:

• julia +1.12 --project=. -e 'using Test, Random, LinearAlgebra, TenSolver; include("test/utils.jl"); include("test/backend.jl")' - passed; backend interface 25 pass.
• julia +1.12 --project=. -e 'using Pkg; Pkg.test()' - passed.
• julia +1.11.5 --project=docs/ docs/make.jl local - passed.
• git diff --check - passed.

Comments intentionally not addressed:

• None in the fresh review. Previously replied outdated threads remain addressed by 009bcd1584589592e135d6e7524d34cd3c03e712 and e122868556e03840b5124333c6410b974e35a3db.

Remaining risks/follow-up:

• Downstream stacked PRs Add JuMP PEPS backend attributes #45 and Add PEPS docs, examples, and benchmarks #46 need to be rebased after this branch update.
• The SpinGlass component stack still needs a resolvable compat/CI path before the PEPS backend should become public.

[codecov]

Codecov Report

❌ Patch coverage is 33.14917% with 121 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (feature/issue-37-backend-interface@009bcd1). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
ext/TenSolverSpinGlassPEPSExt.jl	0.00%	112 Missing ⚠️
src/solver.jl	82.50%	7 Missing ⚠️
src/solution.jl	93.10%	2 Missing ⚠️

Additional details and impacted files

@@                          Coverage Diff                          @@
##             feature/issue-37-backend-interface      #44   +/-   ##
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  Branches                                      ?        0           
=====================================================================
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  Misses                                        ?      129           
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[bernalde]

Reviewed the current net diff against feature/issue-37-backend-interface. No blocking or nonblocking inline issues found. The PEPS scaffolding remains internal, solve_ising is covered as public API, and the updated tests cover the newly public behavior and internal boundary sufficiently for this PR.

GitHub would not let me submit APPROVE because this account is the PR author: "Review Can not approve your own pull request".

Tests run locally:

• git diff --check origin/feature/issue-37-backend-interface...HEAD
• julia --project=. -e "using Pkg; Pkg.test()"
• julia --project=. -e "using Test, Random, LinearAlgebra, TenSolver; include("test/utils.jl"); include("test/backend.jl"); include("test/peps_backend.jl")"
• julia +1.12 --project=. -e "using Test, Random, LinearAlgebra, TenSolver; include("test/utils.jl"); include("test/backend.jl"); include("test/peps_backend.jl")"
• julia --project=. targeted solve_ising exact-energy probe
• julia --project=docs/ -e "using Pkg; Pkg.develop(PackageSpec(path=pwd())); Pkg.instantiate()"
• julia --project=docs/ docs/make.jl local

The optional SpinGlassPEPS extension path still cannot be exercised locally because SpinGlassNetworks/SpinGlassEngine/SpinGlassTensors cannot currently resolve with this package dependency stack; the PR documents that limitation and keeps the PEPS backend internal.

Merge recommendation: merge-ready from this review pass.

[bernalde]

Senior maintainer review. Note: I am the PR author, so GitHub only permits a COMMENT event from me; a formal approval or change-request must come from another maintainer. No blocking issues; one substantive Question for the reviewer below.

Diff verification: confirmed against the stacked base feature/issue-37-backend-interface (not main). merge-base 009bcd1; git diff <merge-base>..HEAD --stat = 10 files, +646 / -5, matching the PR's reported totals.

Issue intent (#38): add an optional SpinGlassPEPS-backed path without making SpinGlassPEPS a hard dependency; reuse the #42 conversions; decode back to Boolean and report the objective in TenSolver's convention; keep DMRG default; guard tests so absence of the heavy dep / Julia 1.10 does not break core CI. The PR satisfies all of these.

Optionality — PASS: the three SpinGlass packages are in [weakdeps] + [extensions] (TenSolverSpinGlassPEPSExt), not in [deps]. julia = "1.10" is left unchanged (no silent breaking bump); the extension mechanism is valid on 1.10.

Julia-compat reality (Question, not a blocker): I confirmed empirically on Julia 1.10.11 that SpinGlassNetworks@1.4/SpinGlassTensors@1.3 are unresolvable (resolver: "restricted by julia compatibility ... no versions left"). So on the only CI-able Julia (1.10) the weakdeps can never install, the extension never loads, and ordinary users are unaffected — but the entire SpinGlassPEPS bridge is therefore dead/untested-on-CI until TenSolver raises compat to 1.11. The PR is honest about this and keeps the PEPS types internal/unexported. For the reviewer to decide: merge now as the "experimental scaffolding" #38 explicitly permits (option 3), or defer activation until compat is raised to 1.11. Not blocking under #38's stated acceptance.

Bridge correctness (static; heavy dep not runnable here) — looks correct: _minimize(::PEPSBackend, ...) reuses qubo_to_ising; the reported objective is recomputed independently in TenSolver's convention via _decoded_records (decode Potts -> spins -> spin_to_bool -> ising_energy), not trusting SpinGlassPEPS's raw_energy (kept as metadata) — this sidesteps any sign/offset mismatch between the libraries. _ising_instance maps diagonal=biases, off-diag=couplings, consistent with the upper-triangular canonical J. The DMRG solve_ising round trip is verified by the new passing test.

Default unchanged — PASS. Tests guarded — PASS: test/peps_backend.jl gates the extension test on Base.find_package for all three components and uses @test_skip (observed as "Broken 1", i.e. skipped, on 1.10); the always-run "PEPS backend core" tests only the available types/validation.

Nonblocking

1. src/solution.jl narrows Base.in(bs, psi) to Base.in(bs::AbstractVector, psi). All in-repo callers pass vectors and tests pass, but this is a subtle public-API signature narrowing — confirm intended.
2. src/TenSolver.jl adds export Solution (previously unexported). Purely additive/non-breaking; PEPSSolution/PEPSBackend/SquareGrid/KingGrid are intentionally not exported.
3. ext/...Ext.jl _metadata reads SpinGlassEngine Solution fields (energies/probabilities/largest_discarded_probability); plausible but unverifiable without the dep — confirm against SpinGlassEngine 1.6 when the path is first exercised.

Tests run (Julia 1.10.11):

• julia --project=. -e 'using Pkg; Pkg.instantiate(); Pkg.test()' — passed (Testing TenSolver tests passed): Backend interface 25/25, PEPS backend core 34/34, Optional SpinGlassPEPS extension = 1 Broken (skipped as expected), QUBODrivers 128/128, Aqua 11/11, VRP/HDF5/Doctests pass.
• Adversarial install: Pkg.add(SpinGlassNetworks@1.4) on fresh 1.10 -> resolve failed with explicit julia-compat exclusion (confirms uninstallable on 1.10; expected, not a PR failure).

Merge readiness: No blocking issues. The PR honors #38: optionality via package extension (not [deps]), DMRG default preserved, conversions reused, objective decoded into TenSolver's Boolean convention, tests properly guarded, core suite green on 1.10. The substantive caveat is the Question above: the bridge is uninstallable/untested on CI Julia and remains so until compat is raised to 1.11; merge as documented scaffolding or defer, and track a follow-up to exercise it under 1.11. Stacked on #43 and must merge after it. The formal verdict must come from another maintainer.

[bernalde]

Question for the reviewer: these weakdeps require Julia 1.11, but TenSolver declares julia = "1.10". I confirmed SpinGlassNetworks@1.4 / SpinGlassTensors@1.3 are unresolvable on 1.10, so the extension can never load on the only CI-able Julia — the whole bridge is dead/untested on CI until compat is raised to 1.11. Optionality is correct (weakdeps, not deps), so not a blocker; decide whether to merge as experimental scaffolding (allowed by #38) or defer activation, and track a follow-up to exercise it under 1.11.

[bernalde]

Not changed in this pass. I am keeping this as experimental scaffolding rather than raising TenSolver's Julia compat or adding an activation path here. Follow-up remains to exercise the extension under Julia 1.11 or once the SpinGlass compat stack resolves.

[bernalde]

Senior maintainer review of PR #44 ("Add optional SpinGlassPEPS backend"), diffed against the merge-base of origin/feature/issue-37-backend-interface and the PR head (009bcd1). Reproduced totals: 10 files, +646/-5. Only issues introduced by #44 itself are flagged; the #41-#43 base chain is treated as inherited.

Process note: I am the PR author, so GitHub only permits an event = COMMENT from this account. A formal approve/request-changes must come from another maintainer.

Test execution: the review sandbox denied julia and git (including with sandbox disabled), so I could not run the suite locally. Findings are from static review of the head sources plus CI. CI is green at the head SHA across all 12 check-runs (Julia 1/lts/pre x ubuntu/macOS/windows, build, docs, codecov). Caveat: CI ran with the stack cut from old main, and the optional SpinGlassPEPS path is @test_skip-ped because the component packages are not installed in CI, so the real PEPS solve and decode path is NOT exercised by any automated test.

Optionality mechanism (verified correct): the SpinGlass components are declared under [weakdeps] + [extensions] (TenSolverSpinGlassPEPSExt), not hard [deps]. Compat bounds are set (SpinGlassEngine 1.6, SpinGlassNetworks 1.4, SpinGlassTensors 1.3; QUBOTools widened to "0.10, 0.11"). When the extension is not loaded, PEPSBackend solves throw a clear ArgumentError naming the three packages and pointing to backend = :dmrg (covered by test/peps_backend.jl). This satisfies issue #38's "do not add SpinGlassPEPS as mandatory deps" requirement and the packaging-option-1 (extension) preference. Ordinary installs keep the existing dependency footprint.

Mapping correctness (static): _ising_instance builds the SpinGlassNetworks dict from model.h (diagonal) and the upper-triangle of model.J, which matches IsingModel's objective offset + sum(h_i s_i) + sum_{i<j} J_ij s_i s_j. The constant offset is intentionally dropped from the instance and reintroduced by recomputing the final energy via ising_energy(model, spins), so returned energies are in TenSolver's original convention. Decode uses decode_potts_hamiltonian_state then spin_to_bool. Square/king super_square_lattice clustering and the SquareSingleNode/KingSingleNode{GaugesEnergy} networks follow the issue's implementation sketch. Multi-spin-per-site is handled in _check_layout_edges by skipping intra-cell couplings (ci == cj). I did not find a correctness bug in the mapping by inspection, but see the no-coverage caveat below.

1. Blocking

None introduced by this PR. The mapping/decoding code is plausible but completely unexercised by CI (the only test that would run it is skipped). I am not raising this to Blocking because issue #38 explicitly anticipates this as packaging-option-3 ("clearly experimental branch-only prototype if dependency/version constraints block an extension") and the PR keeps the PEPS types unexported and out of the public API docs. But it is the single largest risk: 244 lines of extension logic ship with zero executed-path coverage. See Questions.

2. Nonblocking

Nonblocking: sample/prob/in on PEPSSolution operate on the deduplicated retained states, and _deduplicated_records keeps only the first record per Boolean state. Duplicate states' probability mass is therefore dropped rather than summed, and sample's cumulative-weight draw uses the un-renormalized retained probabilities. This is acceptable for "retained states" semantics but is a latent fidelity gap if two PEPS records decode to the same Boolean vector; a one-line comment or summing duplicates in prob would make the intent explicit.

Nonblocking: in TenSolver._solve_ising the per-transformation loop uses try ... finally clear_memoize_cache() with no catch. If any single transformation in transformations = :all throws inside SpinGlassEngine, the whole solve aborts even when other transformations would have succeeded. Issue #38 frames transformations as a set "to try"; consider catching and recording per-transform failures so one bad transform does not fail the solve. Not blocking since the default-path correctness is unaffected.

Nonblocking: the cutoff keyword (always injected by minimize(...; cutoff = 1e-8)) is silently absorbed by the extension's _solve_ising(...; cutoff = nothing, ...) and ignored. That is the right behavior for a DMRG-specific knob, but the extension's strict "Unsupported PEPS backend keyword(s)" guard relies on cutoff (and verbosity) being explicitly captured. A short comment noting that cutoff is intentionally accepted-and-ignored would prevent a future refactor from accidentally routing it into the kwargs guard and breaking minimize(Q; backend = PEPSBackend(...)).

3. Questions

Question: Closes #38. Issue #38's acceptance criteria require "A PEPS-backed direct API works for at least one small structured grid case on CPU" and "Exact small tests prove objective consistency." Because the SpinGlass stack does not resolve in the TenSolver environment (documented in the PR body and in docs/src/spinglasspeps_integration.md), that acceptance test is skipped, not passing. The core scaffolding, error handling, and DMRG-path solve_ising tests do pass. Given the real PEPS solve was never executed end-to-end, should this be Refs #38 (keeping #38 open until the component stack resolves and the gated test actually runs) rather than Closes #38? As written, merging would auto-close #38 while its primary acceptance criterion remains unverified.

Question: export Solution is added to src/TenSolver.jl. Solution was previously unexported (documented via TenSolver.Solution in api.md). Exporting it is a public-API surface change that is unrelated to the SpinGlassPEPS backend goal of this PR. Is this intentional and in scope for #44, or stack drift that belongs in a separate change?

4. Tests run and outcomes

Local execution blocked: julia and git were both denied by the review sandbox (confirmed even with sandbox disabled; not retried). Verified statically from head sources via gh pr diff/gh api contents. CI: all 12 check-runs success at head f3c4db1 (build, docs deploy, codecov patch+project, and the 9 Julia matrix jobs). The optional SpinGlassPEPS test is skipped in CI (Base.find_package returns nothing for the components), so the actual PEPS solve/decode path has no executed coverage anywhere.

5. Merge-readiness

Not mergeable yet, primarily due to the stacked-base dependency, not code defects. PR #44's base is feature/issue-37-backend-interface (PR #43, still OPEN), which itself chains through #42 -> #41 -> main. #44 cannot merge until #41, #42, and #43 merge first. Additionally, the whole #41..#46 stack was cut from old main (ca27880); current origin/main is 9a3dd5c (v0.2.0, with #49/#53/#54 merged), so the stack is stale and carries rebase/conflict risk against current main (Project.toml compat and exports are likely touch points). CI green here was computed on the stale base. No code changes are required from a correctness standpoint; the nonblocking items and the Closes #38 vs Refs #38 question above are the substantive follow-ups.

I would not merge this until the stacked base chain (#41-#43) merges and the stack is rebased onto current main; the Closes #38 linkage should also be reconsidered given the PEPS solve path is not exercised by CI.

[bernalde]

Optionality verified: SpinGlass components are weakdeps with an extension entry, not hard deps, and compat bounds are set. This matches issue #38's requirement not to add SpinGlassPEPS as mandatory deps. Note for the rebase onto current main (v0.2.0 / 9a3dd5c): the QUBOTools compat widening to "0.10, 0.11" and the SpinGlass compat additions are likely conflict points against the post-#49/#53/#54 Project.toml.

[bernalde]

Addressed by c31f1e6. The restack keeps the SpinGlass weakdeps/compat and resolves the conflict onto the current QUBODrivers 0.6.1 / QUBOTools 0.13 base. Pkg.instantiate() passed.

[bernalde]

Nonblocking: this loop uses try ... finally with no catch. With transformations = :all, a throw from any single transformation aborts the entire solve even if other transformations would have produced valid states. Consider recording per-transform failures and continuing, since #38 frames transformations as a set to try.

[bernalde]

Addressed in edd04e4. The extension now records per-transform failures, continues to later transformations, clears cache in finally, and only throws after the loop if no records were produced.

[bernalde]

Nonblocking: prob sums over retained states, but _deduplicated_records in the extension keeps only the first record per Boolean state, so duplicate decodes' probability mass is dropped (and not renormalized for sample). Acceptable as "retained states" semantics, but a brief comment or summing duplicates would make the intent explicit and avoid surprising callers who treat prob as a true marginal.

[bernalde]

Addressed in edd04e4. _deduplicated_records now sums duplicate decoded-state probability mass, and test/peps_backend.jl covers duplicate retained-state probabilities.

[bernalde]

Question: export Solution is a public-API surface change unrelated to the SpinGlassPEPS backend that is the subject of this PR (Solution was previously unexported and documented as TenSolver.Solution). Intentional and in scope for #44, or stack drift?

[bernalde]

Addressed in edd04e4. export Solution was removed, and test/peps_backend.jl now asserts Solution remains unexported while docs keep using TenSolver.Solution.

[bernalde]

Pushed commits:

• c31f1e6 - merge updated PR Introduce solver backend interface #43 base into this stacked branch.
• edd04e4 - address PEPS backend review follow-ups.

Main changes:

• Resolved the Project.toml merge by keeping SpinGlass weakdeps/compat and the current QUBODrivers/QUBOTools compat from the updated base.
• Added an internal IsingModel adapter for the current QUBOTools-form conversion API so solve_ising and PEPS scaffolding still compile and test on the restacked branch.
• Removed the unrelated Solution export and added a regression check that Solution remains unexported.
• Changed the PEPS extension loop to record per-transformation failures and continue trying later transformations.
• Aggregated duplicate decoded-state probability mass in _deduplicated_records.
• Added/updated tests for the unexported surface, duplicate probability semantics, and restacked solve_ising path.

Tests run:

• git diff --check - passed.
• julia +1.12 --startup-file=no -e 'Meta.parse(read("ext/TenSolverSpinGlassPEPSExt.jl", String)); println("extension syntax parsed")' - passed.
• julia +1.12 --project=. -e 'using Pkg; Pkg.instantiate()' - passed.
• julia +1.12 --project=. -e 'using Test, Random, LinearAlgebra, TenSolver; include("test/utils.jl"); include("test/backend.jl"); include("test/peps_backend.jl")' - passed; backend interface 32 pass, PEPS backend core 37 pass, optional SpinGlassPEPS extension 1 expected broken.
• julia +1.12 --project=. -e 'using Test, Random, LinearAlgebra, TenSolver; include("test/ising_conversion.jl")' - passed, 3611 tests.
• julia +1.12 --project=docs/ -e 'using Pkg; Pkg.instantiate()' - passed.
• julia +1.12 --project=docs/ docs/make.jl - passed; deployment skipped outside CI.
• julia +1.12 --project=. -e 'using Pkg; Pkg.test(; coverage=false)' - passed.

Comments intentionally not addressed:

• 3376275944: no code change to raise TenSolver's Julia compat or activate the SpinGlass stack. This remains experimental scaffolding allowed by [SpinGlassPEPS 4/6] Add optional SpinGlassPEPS-backed structured solver path #38; follow-up is still needed to exercise the extension under Julia 1.11 or once the SpinGlass compat stack resolves.
• Codecov coverage comment: not addressed in this PR because the real extension path remains gated by optional dependencies that default CI cannot currently resolve.

Remaining risks or follow-up:

• GitHub CI is pending for the pushed branch.
• The real SpinGlassPEPS solve path remains unexercised in default CI.
• Downstream stacked PRs Add JuMP PEPS backend attributes #45 and Add PEPS docs, examples, and benchmarks #46 need to merge this updated Add optional SpinGlassPEPS backend #44 base before their review comments are revisited.