Agent guidelines

These rules apply throughout this repository. Keep purpose, status, usage, and development instructions in README.md; keep agent workflow rules here. Neither file is a session log.

Scope and repository map

This is a WIP .NET double-double arithmetic library, not arbitrary-precision math. Do not claim API completeness or accuracy beyond tested contracts.

0-source/Just.PreciseMath/: library implementation and package metadata.
1-tests/Just.PreciseMath.Tests/: xUnit v3 tests, Shouldly assertions, and Microsoft.Testing.Platform (MTP) with coverlet.MTP coverage.
2-benchmarks/Just.PreciseMath.Benchmarks/: source-free BenchmarkDotNet library scaffold, not yet a runnable benchmark suite.
Root Directory.Build.props holds shared settings. Each numbered directory's props explicitly imports it; preserve this import chain.
review-legacy/: optional local reference material, excluded by .gitignore. Start with review-legacy/Review-Revisited-DoubleDouble.md when present. The code and proposed fixes contain known defects: reproduce findings against current code rather than treating them as correctness oracles. Do not compile, copy wholesale, or force-add these files. If absent, proceed without them; do not invent their contents or make validation depend on them.
Use the ignored, repository-local .hermes/ directory for plans, checklists, investigation notes, and handoff context. Create or update notes as useful; keep them concise and revalidate them against current files. Optional .hermes/project-context.md holds setup and review background. Do not store secrets, force-add this directory, or make builds or tests depend on it.

Working rules

Check both staged and unstaged changes before starting. Preserve the user's work and index; do not stage, commit, push, publish packages, or rewrite history without an explicit request. Do not read or expose credentials.
Read definitions, callers, tests, and relevant configuration before editing. Limit changes to the request; do not implement the roadmap or repair unrelated WIP code.
Reproduce numerical bugs with a failing regression test before fixing them. For new behavior, define the contract and test cases before implementation. Exercise sibling overloads and operand orders that can share the defect.
Do not weaken analyzers, nullable checks, warnings-as-errors, or assertions to get a green build. Explain necessary policy changes before making them.
Keep dependencies minimal. Check existing references, compatibility, and current stable releases before adding or upgrading packages.

C# conventions

Follow .editorconfig, not incidental style in unfinished code.

Use file-scoped namespaces, explicit types rather than var, and block-bodied methods. Preserve the configured expression-bodied property/accessor preferences.
Use _camelCase for non-public instance fields, including internal fields; s_camelCase for non-public mutable static fields; PascalCase for constants and static readonly fields. Do not rename internal fields to remove underscores.
Preserve parentheses that make mathematical grouping readable. IDE0047 is intentionally disabled; do not re-enable it or remove grouping as style cleanup.
Document public APIs and non-obvious numerical preconditions. Explain algorithms, error behavior, and range constraints rather than narrating syntax.
Tests and benchmarks already have internal access through InternalsVisibleTo; do not widen the public API merely to make a helper testable.

Numerical correctness

Treat floating-point evaluation order as part of the algorithm. Do not reassociate expressions, discard residuals, replace fused multiply-add with multiply-plus-add, or simplify error-free transforms without justification and regression tests. Mathematically equivalent formulas can round differently.
State and verify algorithm preconditions, especially magnitude ordering for quick-sum transforms, normalization assumptions, and overflow/underflow limits.
The internal two-component DoubleDouble constructor currently does not normalize. Do not assume arbitrary pairs are canonical. Establish the intended normalization, NaN, infinity, and signed-zero contracts before changing construction, equality, hashing, ordering, or classification; keep those operations consistent.
For affected operations, cover cancellation, widely separated magnitudes, zero and signed zero, subnormals, extreme finite values, infinities, and NaNs. Check intermediate overflow/underflow even when the final result is representable.
Derive expected values from independent high-precision references or exact binary/rational cases, never from the implementation under test. Record the reference source or reproducible derivation and justify tolerances.
Do not collapse both components to double or decimal to validate double-double accuracy: that can discard precisely the bits being tested. Use component-aware or higher-precision comparisons, and inspect sign bits when testing signed zero.
Smoke tests, coverage percentages, and benchmark output are not proof of numerical correctness.

Build and verification

Run commands from the repository root. Read SDK and runner selection from global.json; read framework, language, and dependency versions from project and props files rather than duplicating version pins here.

For documentation-only changes, validate referenced paths and any new or changed commands, then run git diff --check.

Report commands actually run, their outcomes, and anything not verified. Distinguish pre-existing failures from failures introduced by the change.

Code or build changes

Use targeted tests while iterating, then run the full sequence:

dotnet restore Just.PreciseMath.slnx --locked-mode
dotnet build Just.PreciseMath.slnx -c Release --no-restore
dotnet test --solution Just.PreciseMath.slnx -c Release --no-build --minimum-expected-tests 1
dotnet format Just.PreciseMath.slnx --verify-no-changes --no-restore
git diff --check

Use verify-only formatting first. Report unrelated existing violations separately and validate the changed files; do not reformat the repository to hide failures.

Test or coverage pipeline changes

After the code/build checks, also run:

dotnet test --solution Just.PreciseMath.slnx -c Release --no-build --minimum-expected-tests 1 --report-xunit-trx --coverlet --coverlet-output-format cobertura --coverlet-include "[Just.PreciseMath]*"

Keep MTP-native options and the empty-suite failure. Do not substitute VSTest collector commands or remove compile assets from coverlet.MTP; generated MTP registration requires them. Inspect actual TRX and Cobertura output, including failed-test behavior when changing report collection.

Packaging changes

After the code/build checks, run:

dotnet pack 0-source/Just.PreciseMath/Just.PreciseMath.csproj -c Release --no-build

Inspect package metadata and the included README.md and LICENSE. Do not publish.

Dependency changes

For intentional dependency changes, run dotnet restore --force-evaluate and review all affected packages.lock.json files before the locked restore above. Include lock-file updates in the change; never regenerate them to bypass unexpected restore failures.

CI and benchmarks

.gitea/workflows/test-dotnet.yaml is the CI source of truth. Keep reports artifact-only: no badges, publication branches, or repository-write publishing jobs.
Preserve upload-on-failure and both report globs: 1-tests/**/TestResults/**/*.trx and 1-tests/**/TestResults/**/*cobertura*.xml. Cobertura filenames can be timestamped; *.cobertura.xml alone will miss them.
Cache NuGet packages, not bin/ or obj/; restore remains necessary on a cache hit. A local restore does not prove CI cache reuse or remote artifact upload succeeded.
Before upgrading actions, check each action's required Node runtime against both the Gitea runner and job image. A recent runner can still launch an old Node image; ubuntu-latest is a configured label, not a guarantee of GitHub's environment.
Do not add an empty benchmark job. When benchmarks are introduced, add an executable entry point and cases, then smoke-run them in Release with BenchmarkDotNet's Dry job. Require discovered/executed cases and verify that benchmark failures fail CI.
Keep smoke validation separate from performance measurement. Do not gate timing regressions on a shared runner or compare coverage-instrumented measurements. Use controlled, repeatable baseline/candidate runs before proposing performance gates.

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