# Batch 50 — Implementation Report (Cycle 1)

**Tasks**: AZ-339 (C2 MegaLoc + MixVPR Secondary Backbones — Research-only)
**Date**: 2026-05-13
**Cycle**: 1
**Status**: COMPLETE (review verdict: PASS_WITH_WARNINGS, one Medium + one Low finding)

## What was done

Added two secondary `VprStrategy` implementations for IT-12 comparative-study: `MegaLocStrategy` (D=2048, 322×322 input) and `MixVprStrategy` (D=4096, 320×320 input). Both run via the C7 TensorRT runtime (or ONNX-Runtime fallback), apply ImageNet mean/std preprocessing + single-stage L2 normalisation, and delegate retrieval to `FaissBridge`. Both are gated OFF for airborne and operator-tooling per ADR-002 — `BUILD_VPR_MEGALOC` and `BUILD_VPR_MIXVPR` ON only for the research binary and replay-cli.

### Files added (5)

| File | Purpose |
|------|---------|
| `src/gps_denied_onboard/components/c2_vpr/mega_loc.py` | `MegaLocStrategy` class + `create()` factory + `_assert_engine_output_dim` helper |
| `src/gps_denied_onboard/components/c2_vpr/_preprocessor_mega_loc.py` | `MegaLocBackbonePreprocessor` (centre-crop + 322×322 resize + ImageNet normalise + FP16 NCHW) |
| `src/gps_denied_onboard/components/c2_vpr/mix_vpr.py` | `MixVprStrategy` class + `create()` factory + `_assert_engine_output_dim` helper |
| `src/gps_denied_onboard/components/c2_vpr/_preprocessor_mix_vpr.py` | `MixVprBackbonePreprocessor` (centre-crop + 320×320 resize + ImageNet normalise + FP16 NCHW) |
| `tests/unit/c2_vpr/test_az339_mega_loc_mix_vpr.py` | 36 parametrised AC tests across both strategies |

### Files changed
- _None._ The composition-root factory (`runtime_root/vpr_factory.py`) was already wired for `mega_loc` and `mix_vpr` strategy names at AZ-336 land time — `_STRATEGY_TO_BUILD_FLAG` and `_STRATEGY_TO_MODULE` tables already include the rows. The `KNOWN_STRATEGIES` frozenset in `c2_vpr/config.py` already includes both. The `module-layout.md` `Component: c2_vpr` § Internal list already names `mega_loc.py` and `mix_vpr.py` (pre-declared by AZ-336). No CMake change required — `BUILD_VPR_*` gating is environment-variable-based per `_is_build_flag_on` in `vpr_factory.py`.

## AC coverage

All 11 ACs verified per strategy via the parametrised test suite. See `_docs/03_implementation/reviews/batch_50_review.md` § Phase 2 for the AC ↔ test mapping table.

| AC | Status | Notes |
|----|--------|-------|
| AC-1..AC-9 + AC-11 | PASS | Each AC parametrised over both strategies (36 test cases total) |
| AC-10 | PASS with drift | Implementation raises `StrategyNotAvailableError` (env-flag OFF path) and `ConfigError` (runtime-label mismatch path); the spec literally names `ConfigurationError`. Mirrors the established AZ-337 / AZ-338 precedent. Logged as Low finding F2. |

## Test results

- `tests/unit/c2_vpr/test_az339_mega_loc_mix_vpr.py` — **36 / 36 PASS**.
- `tests/unit/c2_vpr/test_protocol_conformance.py` — **25 / 25 PASS** (auto-extends across all 7 strategies; the two new ones are picked up by the parametrised `_STRATEGY_MODULES` table without test changes).
- `tests/unit/c2_vpr/` (full directory: faiss_bridge + net_vlad + ultra_vpr + new AZ-339 file) — **126 / 126 PASS**.
- `tests/unit/test_az508_iso_timestamps.py` — **18 / 18 PASS** (AZ-526 regression guard confirms no new `_iso_ts_from_clock` duplicates introduced by the AZ-339 strategies).
- `tests/unit/test_az270_compose_root.py` — **8 / 8 PASS**.
- `tests/unit/test_az272_fdr_record_schema.py` — **33 / 33 PASS** (unmodified; the new strategies emit FDR records that match the existing schema).
- Full unit suite: **1690 passed, 80 skipped (TRT/CUDA/actionlint), 1 pre-existing failure** (`test_cold_start_under_500ms_p99` — subprocess timeout on cold-start latency budget, unrelated; confirmed by stashing AZ-339 changes and re-running).
- `ruff check` on all 5 new files — clean.

## Architectural decisions

1. **Single parametrised test file `test_az339_mega_loc_mix_vpr.py`** — rather than two near-identical files mirroring `test_ultra_vpr.py` / `test_net_vlad.py`. The two strategies share byte-identical behavioural contracts (same Protocol, same FDR record kinds, same log kinds, same error envelope) and differ only on three values (`DESCRIPTOR_DIM`, `_BACKBONE_LABEL`, preprocessor `input_shape()`). A parametrised approach keeps any future drift visible at the assertion level and reduces the test surface from ~1500 lines (two copies of test_ultra_vpr.py) to ~700 lines.
2. **Preprocessor duplication preserved** (mega_loc vs mix_vpr vs ultra_vpr) — per `components/02_c2_vpr/description.md` § 6 and the task spec § Constraints. Each preprocessor owns its own input-shape constants so a future code drop can change a backbone's preprocessing without coupling other strategies' weights-versions.
3. **`_assert_engine_output_dim` duplicated, NOT extracted** — see Spec Drift / Review Finding F1 below. The cleaner path is a dedicated AZ-527 hygiene PBI mirroring AZ-508 → AZ-526.
4. **`iso_ts_from_clock` imported from the AZ-526 helper from day 1** — neither new strategy introduces a local `_iso_ts_from_clock` body. The AZ-526 regression guard test confirms this.
5. **Runtime-label guard placed inside `create()`** (not in `__init__`) — runtime selection is a composition-time concern; once the strategy is constructed it's expected to work. Matches the UltraVPR / NetVLAD precedent.

## Spec drift noted (carried into review F2)

AZ-339 § AC-10 literally specifies `ConfigurationError` for the build-flag-OFF case. The existing AZ-336 composition-root factory raises `StrategyNotAvailableError` for this case (per its own contract and test coverage at `test_protocol_conformance.py:268-274`). The strategy module's own runtime-label guard raises `ConfigError` for the related "wrong C7 runtime" case. AZ-337 (UltraVPR) and AZ-338 (NetVLAD) followed this same pattern; AZ-339 mirrors them. AC-10 wording should be amended in a future spec pass; no code change required.

## Cumulative review obligation

This batch is mid-window (batch 50, next cumulative review at batch 51 / batches 49-51). The new finding F1 (`_assert_engine_output_dim` 4-way duplication) will surface in that cumulative review, and AZ-527 (the planned hygiene PBI) will close it. The AZ-526 regression guard test confirmed that neither AZ-526's F1+F3 closure regressed in AZ-339.

## Follow-on PBI

**AZ-527** (Hygiene — consolidate `_assert_engine_output_dim` into a c2-internal helper). 2 points. Depends on AZ-339. To be created and prioritised as Batch 51 or 52.
