More dev docs

README.md

@@ -24,14 +24,49 @@ You'll need:
 - `npm run check` to see TypeScript errors
 
 The `tests` directory has some diff-based tests. `cd backend; cargo test
---release` will run them.
+--release` will run them. You'll need to follow the instructions below to
+ensure you have `bristol` and `strasbourg` areas set up.
+
+### Faster local development
+
+The tool operates on fixed study areas, generated from clips of OSM data.
+Custom areas imported by the user are always retrieved from the Overpass API,
+with the latest OSM data. For speed and for deterministic tests, there are also
+"built-in" study areas, consisting of pre-clipped osm.pbf files that do not
+automatically use the latest OSM data. These are manually managed and hosted by
+Dustin on assets.od2net.org.
+
+If you're developing locally, you can avoid hitting od2net.org by setting up
+two directories in `web/public/`: `osm` and `boundaries`. If
+`web/public/osm/areas.json` exists, then the Svelte app will load from
+localhost, not od2net.org. You can copy from od2net.org to set this up,
+choosing what study areas to cache:
+
+```
+AREAS="bristol edinburgh strasbourg ut_dallas"
+
+cd web/public
+mkdir boundaries osm
+
+cd osm
+wget https://assets.od2net.org/severance_pbfs/areas.json
+for x in $AREAS; do
+  wget https://assets.od2net.org/severance_pbfs/$x.pbf
+done
+
+cd ../boundaries
+for x in $AREAS; do
+  wget https://assets.od2net.org/boundaries/$x.geojson
+done
+```
 
 ### Architecture
 
 The Rust `backend` crate gets compiled to WASM, with generally type-unsafe APIs
-in `lib.rs`. The Svelte frontend calls these in the main thread (though moving
-to web workers later is a possibility). The "important" state is kept in the
-backend, while the frontend has more ephemeral UI state. The main state is
-serialized as GeoJSON and kept in the user's browser local storage. The
-frontend is effectively a single page app, with the `Mode` toggling between
-different pages.
+in `lib.rs`. On the frontend side, `wasm.ts` wraps these APIs in nicer TS APIs.
+The Svelte frontend calls APIs in the main thread (though moving to web workers
+later is a possibility). The "important" state is kept in the backend, while
+the frontend has more ephemeral UI state. The main state is serialized as
+GeoJSON and kept in the user's browser local storage. The frontend is
+effectively a single page app, with the `Mode` toggling between different
+pages.