= Architecture
:toc:
:toclevels: 2

== Overview

MapTool is a Sinatra web application, served locally by Puma and
opened in a browser.

== Directory layout

[cols="1,3"]
|===
| Path | Purpose

| `app.rb`, `config.ru`
| Sinatra application and Rack entry point.

| `config/puma.rb`
| Puma server configuration (binds to `127.0.0.1` only — the app is
  local-only, not exposed on the network).

| `public/`, `views/`
| Frontend assets and Sinatra templates. The map view uses MapLibre GL
  JS, rendering vector tiles.

| `public/vendor/`
| Third-party frontend libraries, vendored (not loaded from a CDN) so
  the app also works with no internet access. See
  `docs/third-party-notices.adoc`.

| `data/`
| Local, gitignored data files, e.g. `offline.pmtiles`.

| `docs/`
| Project documentation, in AsciiDoc.
|===

== Offline maps

Map data is vector, not raster: the app ships
https://maplibre.org[MapLibre GL JS] (vendored, see below) and the map
view (`public/js/map.js`) offers two styles:

* *Online* — a full hosted vector style,
  https://tiles.openfreemap.org/styles/liberty[OpenFreeMap "liberty"],
  requires internet access.
* *Offline* — built at runtime from a local
  https://github.com/protomaps/PMTiles[PMTiles] archive at
  `data/offline.pmtiles`, served as-is (see below) at
  `/tiles/offline.pmtiles`, decoded client-side by
  https://github.com/protomaps/PMTiles[pmtiles.js] registered as a
  `pmtiles://` protocol handler, and styled with base layers from
  https://github.com/protomaps/basemaps[protomaps-themes-base].

`GET /api/offline-tiles` reports whether `data/offline.pmtiles` is
present (`{"available": true|false}`); the frontend uses it once, on
load, to default to the offline style when local tile data exists —
otherwise it defaults to the online style. Either way, the layer
toggle in the top-left of the map lets the user switch manually.

PMTiles is a single, self-indexed file, which is why this needs no
tile-decoding logic on the server: `app.rb`'s `/tiles/offline.pmtiles`
route (`send_file`, which Sinatra serves with HTTP range-request
support) just returns byte ranges of the file; pmtiles.js in the
browser reads the archive's own directory and requests only the bytes
it needs for the tiles on screen.

=== Known limitation: no offline text labels

The offline style only renders `protomaps-themes-base`'s non-label
layers (roads, buildings, land use, water — no place names or street
labels). Label rendering in MapLibre needs a "glyphs" server serving
font PBF ranges, which would mean vendoring a font glyph set locally
too; that has not been done, to keep the initial scope manageable.
Revisit this if labelled offline maps turn out to matter in practice.

No PMTiles file ships with the repository — it is expected to be
supplied locally (see `docs/development.adoc`) and stays gitignored
because such extracts are typically large binary files.

== Weather radar overlay

`public/js/radar.js` adds a "Radar" map control that overlays recent
precipitation radar from https://www.rainviewer.com[RainViewer]'s free
public API (no API key required). Enabling it:

. Fetches `https://api.rainviewer.com/public/weather-maps.json`,
  which lists RainViewer's past radar frames (fixed 10-minute
  cadence, at most 13 covering the last 120 minutes).
. Takes the most recent 7 (last ~60 minutes — the same default as
  the https://github.com/jpettitt/weather-radar-card[Home Assistant
  weather-radar-card], for a consistent feel with that setup) and
  adds one MapLibre raster source/layer per frame, all pre-loaded but
  only the current frame's `raster-opacity` non-zero.
. Animates by toggling that opacity across frames on an interval
  (play/pause + a scrub slider), rather than swapping tiles in and out
  — avoids reloading tiles on every step.

Tile URL: `{host}{frame.path}/256/{z}/{x}/{y}/2/1_0.png` — `2` is
RainViewer's "Universal Blue" color scheme, `1` enables their
smoothing, `0` disables the snow overlay. `host` and each frame's
`path` come from the API response.

Since `map.setStyle()` (switching Online/Offline) tears down all
custom sources and layers, the control listens for `style.load` and
re-adds the radar layers if the overlay was on.

This is a live, online-only overlay — it always needs internet access
and is unrelated to the offline PMTiles basemap above. Per RainViewer's
terms, the API is free for personal/non-commercial use.

== Networking

The Sinatra server only binds to `127.0.0.1`. It is not meant to be
reachable from other machines.