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Testing Strategy

culvert's test suite is built around deterministic register-access tests. All tests run against in-memory transport implementations; no real hardware is required at any point.

Test structure

Tests are split between inline unit tests in each src/client/ module and integration tests in tests/scdc.rs.

Unit tests (src/client/)

Each client module contains tests immediately below the code it covers. They use TestTransport, a 256-byte register array backed transport defined in src/client/test_transport.rs.

TestTransport has two constructors:

  • TestTransport::new() — succeeds on all operations; used for happy-path tests.
  • TestTransport::failing_after(n) — succeeds for the first n operations then returns Err(()); used to exercise every ? error branch.

The single generic instantiation (Scdc<TestTransport>) is intentional: using one concrete type avoids LLVM counting per-monomorphisation ? branches as uncovered, which would inflate the coverage denominator without corresponding tests.

Each register group has tests covering:

  • Encoding correctness — every field of a write config maps to the correct bit position in the output register(s). One assertion per field, not per struct.
  • Decoding correctness — every field of a read result is extracted from the correct bit position. Register values are crafted to isolate individual bits where necessary.
  • Protocol errors — invalid enum values returned by the sink produce the correct ProtocolError variant with the raw register value preserved (e.g. all eleven undefined LtpReq nibbles 5–15 are each tested individually).
  • Transport error propagation — every read and write call site has a TestTransport::failing_after(n) test that triggers failure at that exact operation and asserts the error bubbles through as ScdcError::Transport.

The register module also contains unit tests for the CedCount newtype (validity bit masking, 15-bit value preservation) and UpdateFlags::new field ordering.

Integration tests (tests/scdc.rs)

The integration tests use SimulatedScdc, a separate infallible transport (Error = Infallible) that exercises the public API through the crate boundary. These tests confirm that the full round-trip — pre-load registers, call an Scdc method, assert on decoded output or written register state — works correctly when going through pub use re-exports.

Coverage is complementary: unit tests cover all branches and error paths; integration tests confirm end-to-end bit patterns for each register group with realistic multi-field values.

plumbob feature tests (src/client/plumbob_client.rs)

When compiled with --features plumbob, additional tests exercise the ScdcClient implementation. These call methods through the plumbob::ScdcClient trait and assert that the type conversions between culvert and plumbob's owned types are correct. Error propagation through the trait boundary is also covered.

Coverage

CI measures line coverage with cargo-llvm-cov. The baseline is stored in .coverage-baseline (currently 100%); CI fails if coverage drops more than 0.1% below it. New register coverage without tests will trip this.

Philosophy

Scdc<T> runs identically against simulated and real ScdcTransport implementations. A test that cannot run with an in-memory transport does not belong in this repository. Hardware is never a test dependency.