Dev docs

Author: dabreegster

README.md

@@ -1,3 +1,37 @@
 # Low-traffic neighbourhood tool v2
 
-This is a WIP next iteration of <ltn.abstreet.org>
+This is a new version of [ltn.abstreet.org](https://ltn.abstreet.org). It
+doesn't have all features from v1 yet, but it solves many problems with the
+original. Please try both versions and give feedback.
+
+## Developer docs
+
+### Installation
+
+You'll need:
+[npm](https://docs.npmjs.com/downloading-and-installing-node-js-and-npm),
+[wasm-pack](https://github.com/rustwasm/wasm-pack), and
+[cargo](https://www.rust-lang.org/tools/install).
+
+`cd web`, and then:
+
+- `npm i` to install dependencies
+- `npm run wasm` to rebuild the Rust backend
+  - vite doesn't automatically rebuild when you edit things
+- `npm run dev` to run locally
+  - Changes to the Svelte/CSS usually auto-reload in your browser
+- `npm run fmt` to auto-format code
+- `npm run check` to see TypeScript errors
+
+The `tests` directory has some diff-based tests. `cd backend; cargo test
+--release` will run them.
+
+### 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.