# maps/router — route-aware planning primitives

The Go backend for the "search along the route" capability
([DESIGN.md](../DESIGN.md), "Driving trips" section). Everything the LLM
would otherwise *guess* about a trip — route shape, travel times, detour
costs — is computed here and returned with provenance.

## What's in here

```
internal/geo/       dependency-free geodesy (haversine, point-to-line)
internal/route/     Router interface + backends
                    - valhalla.go: hosted/self-hosted Valhalla (works today;
                      no geometry on the hosted instance → chord fallback)
                    - osrm.go:     local OSRM (full geometry; scripts/setup-osrm.sh)
internal/plan/      the primitives:
                    - stopcost.go: stop_cost  = route(A,C via B) − route(A,C)
                    - optimize.go: optimize_stops (exhaustive ≤10 stops, greedy above)
                    - corridor.go: corridor = buffer band around the route
cmd/routectl/       manual CLI: route | stopcost | optimize
cmd/bench/          benchmark runner (bench/tasks.json)
bench/tasks.json    5 real NE-corridor tasks with recorded goldens
scripts/setup-osrm.sh  self-host OSRM over the NH+MA+CT+NY extract
```

Zero third-party Go dependencies (stdlib only).

## Try it (hosted Valhalla, no setup)

```sh
go run ./cmd/routectl route --from 44.054,-71.650 --to 40.729,-73.966
# Lincoln NH → Queens NY: ~5h32m, 531 km, I-93/90/495/290 (NOT via Boston)

go run ./cmd/routectl stopcost --from 44.054,-71.650 --to 40.729,-73.966 \
  --stop 42.2767,-71.806:60:cascade \
  --stop 43.507,-71.548:50:ladd \
  --stop 43.642,-71.156:120:mtmajor
# cascade   detour= 6 min  (on the I-495 corridor)
# ladd      detour=17 min
# mtmajor   detour=80 min  (off-corridor)

go run ./cmd/routectl optimize --from 44.054,-71.650 --to 40.729,-73.966 --k 2 \
  --stop 42.2767,-71.806:60:cascade --stop 43.507,-71.548:50:ladd \
  --stop 43.902,-71.64:60:plymouth --stop 42.425,-71.68:60:brewsters
# order: ladd -> cascade   detour: 23 min
```

## Tests

```sh
go test ./...                                  # unit tests, offline, fast
go test -tags integration ./...                # live router (Valhalla by default)
ROUTER_BACKEND=osrm ROUTER_URL=http://localhost:5000 \
  go test -tags integration ./...              # against local OSRM
```

The integration tests encode the key regression: the fastest
Lincoln→Queens drive stays ≥15 km from Boston (it goes via Worcester,
I-90/I-495/I-290). That is exactly the fact an LLM confidently gets wrong.

## Benchmark

```sh
go run ./cmd/bench          # run all tasks, assert goldens + corridor checks
go run ./cmd/bench --record # re-record goldens (after a backend/extract change)
```

Tasks are real OD pairs across NH/MA/CT/NY with real POI candidates and
dwell times. Check types:

| type | meaning |
|---|---|
| `golden_route_min` | direct route time within a recorded band (±10%) |
| `route_avoids` | route stays ≥ N km from a point (geometry, or bbox when the backend has none) |
| `stop_free` / `stop_detour_at_least` / `stop_detour_band` | computed detour of a candidate vs an expected band |
| `optimize_order` | the exhaustive optimum matches the recorded order |
| `crosscheck` | single multi-waypoint route ≈ matrix-summed prediction (≤ maxPct drift) |

Current state: **26 PASS / 0 FAIL** against hosted Valhalla.

## Self-hosted OSRM (full geometry, exact corridors)

`scripts/setup-osrm.sh` builds OSRM 26.4.1 from source (Boost header-only),
extracts the 4-state PBFs (pre-downloaded in `../osm/`), partitions,
customizes, and serves on `:5000`. Then:

```sh
go run ./cmd/routectl route --backend osrm --from 44.054,-71.650 --to 40.729,-73.966
```

With OSRM, `Route.Geometry` is populated, so the corridor is exact
(distances to the actual routed polyline) instead of the chord fallback
used with the hosted Valhalla. This is what makes
`search_along_route`'s spatial pre-filter (PostGIS `ST_DWithin` on the
corridor polygon in the v1 design) sound.

## Design notes

- **The route is computed, never narrated.** No function in this module
  returns a time estimate from model knowledge; every number is router
  output (`computed` provenance per DESIGN.md R4).
- **Matrix via pairwise route calls.** Valhalla's
  `sources_to_targets` caps at 150 km, so the common path is
  `BuildMatrix` (pairwise `/route`). OSRM's `Table` service is available
  for the local backend when a single-call matrix matters.
- **optimize_stops semantics**: the user asked for *k* stops ("two
  hikes"), so the objective minimizes total = detour + dwell + overhead
  over ordered subsets of **exactly k**, with fallback (a) to the best
  feasible smaller set when a time budget is given, then (b) to the best
  exactly-k ignoring the budget, flagged `Feasible=false`.
  "At most k" without a budget degenerates (zero stops always wins).
- **Stop coordinates are POI centroids.** The router snaps to the
  network; a 200 m centroid error moves a 531 km route's detour by
  seconds, not minutes.