# Task 3 Report: Final Mileage Election Projector

## Outcome

Implemented `ProjectDailyMileage` in `go/vehicle-gateway/internal/stats/source_mileage.go` and added focused projector coverage in `go/vehicle-gateway/internal/stats/source_mileage_test.go`.

## RED

Before implementation, the new projector test failed as expected because the function did not exist yet:

```text
internal/stats/source_mileage_test.go:87:9: undefined: ProjectDailyMileage
```

That confirmed the test was exercising the missing task-3 surface rather than passing accidentally.

## GREEN

After implementation:

- `go test ./internal/stats -run TestProjectDailyMileageSelectsCandidateAndMarksSource -count=1`
- `go test ./internal/stats -count=1`

Both passed.

## What Changed

- Added `maxSelectedDailyMileageKM = 1000`.
- Added `ProjectDailyMileage(...)` with three SQL steps:
  - clear `is_selected` for the VIN/date/protocol slice,
  - project the elected row from `vehicle_daily_mileage_source` into `vehicle_daily_mileage`,
  - mark the winning source row as selected.
- Kept the candidate identity intact: `source_key` still carries protocol + phone/device + source IP, while the data-source join uses protocol + source IP.
- Added a focused test that asserts the clear/project/mark sequence and the key SQL fragments.

## Files Changed

- `go/vehicle-gateway/internal/stats/source_mileage.go`
- `go/vehicle-gateway/internal/stats/source_mileage_test.go`

## Self-Review

- The projector stays aligned with the existing stats schema and helper conventions.
- The SQL ordering and join logic preserve the intended source identity split; there is no IP-only simplification.
- I did not add an unrelated schema migration because the current repository state already contained the needed `vehicle_daily_mileage_source` and `vehicle_daily_mileage` columns/indexes for this projector.
- The only notable follow-up risk is that the final selection query relies on MySQL-style `INSERT ... SELECT ... LIMIT 1 ON DUPLICATE KEY UPDATE` behavior, which matches the rest of the stats package but should still be exercised against the target DB in an integration run.

## Commit

`aa12317b5` - `feat(stats): project elected mileage source`