require "sinatra/base"
require "sinatra/json"
require "oauth2"
require "securerandom"
require "zip"
require "stringio"
require_relative "lib/maptool/gpx"
require_relative "lib/maptool/fit"
require_relative "lib/maptool/gpx_store"
require_relative "lib/maptool/gpsd_client"
require_relative "lib/maptool/livetrack_store"
require_relative "lib/maptool/livetrack_client"
require_relative "lib/maptool/kiwix"
require_relative "lib/maptool/plugins"
require_relative "lib/maptool/pmtiles_builder"

module Maptool
  class App < Sinatra::Base
    set :public_folder, File.join(__dir__, "public")
    set :views, File.join(__dir__, "views")

    # sinatra/json defaults to MultiJson, whose `encode` (what
    # sinatra-contrib 4.2.1 actually calls) is deprecated in newer
    # multi_json versions in favor of a constant this app has no other use
    # for — stdlib JSON already does the same job and every use of the
    # `json` helper below is a plain hash/array, nothing MultiJson-specific.
    set :json_encoder, JSON

    # A persistent secret across restarts/redeploys — otherwise every deploy
    # would invalidate every logged-in session's cookie (see
    # docs/deployment.adoc). Falls back to a random one so the app still
    # loads locally/in CI without it set; that fallback just means sessions
    # don't survive a restart there, which is fine outside production.
    set :sessions, httponly: true, secure: production?, same_site: :lax,
                   secret: ENV.fetch("SESSION_SECRET") { SecureRandom.hex(32) }

    # Cache-busts /js and /css requests: browsers (and a caching proxy in
    # front of production, see docs/deployment.adoc) hang onto static
    # assets for hours, so without this a deploy that changes app.js/css
    # can go unnoticed by an already-open tab until that cache expires.
    # A fresh value every process start (i.e. every deploy, which restarts
    # the container) is enough — it doesn't need to survive restarts.
    ASSET_VERSION = Time.now.to_i

    # Written by the CI deploy job (.gitea/workflows/ci.yml) right before it
    # drops .git from the deployed copy, so this is the only way to tell
    # which commit is actually live short of asking the deploy job itself —
    # handy for exactly the kind of "did my push actually deploy, or am I
    # looking at a stale browser cache" confusion ASSET_VERSION above can't
    # answer by itself (that only proves *a* deploy happened recently, not
    # *which* one). Absent outside a real deploy (local dev, CI's own
    # `check` job), hence the fallback.
    REVISION = begin
      File.read(File.join(__dir__, "REVISION")).strip
    rescue Errno::ENOENT
      "dev"
    end

    # Overridable so the test suite can point this at a throwaway directory
    # instead of the real data/ — nothing else needs to set this.
    DATA_DIR = ENV.fetch("MAPTOOL_DATA_DIR") { File.join(__dir__, "data") }

    OFFLINE_PMTILES_PATH = File.join(DATA_DIR, "offline.pmtiles")
    TERRAIN_PMTILES_PATH = File.join(DATA_DIR, "terrain.pmtiles")
    GPX_STORE = GpxStore.new(File.join(DATA_DIR, "gpx"))
    LIVETRACK_STORE = LiveTrackStore.new(File.join(DATA_DIR, "livetrack.json"))

    # Not under DATA_DIR: unlike everything else there (uploads, PMTiles,
    # the kiwix ZIM), a plugin is code, not local data — plugins/overpass-pois
    # ships checked into this repo as the reference example of the
    # contract. Overridable so tests can point it at a throwaway fixture
    # directory. In production this whole directory is baked into the
    # image (see Dockerfile's `COPY . .`) and wiped/replaced on every
    # deploy along with the rest of the checkout (docs/deployment.adoc) —
    # fine for a plugin checked into this repo, not for a private one.
    PLUGINS_DIR = ENV.fetch("MAPTOOL_PLUGINS_DIR") { File.join(__dir__, "plugins") }

    # A second, optional plugin directory under DATA_DIR — the same
    # persistent bind mount as gpx/kiwix (docs/deployment.adoc), so a
    # private/scraper plugin (one whose ToS forbids living in this public
    # repo) dropped in data/plugins/ survives a redeploy the way PLUGINS_DIR
    # above can't. Checked after PLUGINS_DIR (see Plugins.list/run), so a
    # same-named private plugin never shadows the built-in one.
    PLUGINS_DIRS = [PLUGINS_DIR, File.join(DATA_DIR, "plugins")].freeze

    # Shared secret for the Garmin LiveTrack webhook (a Google Apps Script
    # watching Gmail for LiveTrack emails, see docs/deployment.adoc) — not a
    # Gitea-authenticated user, so it can't use the OAuth2 login flow above.
    # Blank means the endpoint is disabled, same fallback story as the
    # OAuth2 secrets: fine outside production, nothing works without it set.
    LIVETRACK_WEBHOOK_SECRET = ENV.fetch("LIVETRACK_WEBHOOK_SECRET", "")

    # An external kiwix-serve instance (see docs/development.adoc), not
    # something this app runs itself — blank disables the "Nearby on
    # Wikipedia" popup link the same way the other optional-integration
    # blanks above do. Only ever reached server-side (the Docker-network
    # service name in production, e.g. "http://kiwix:8080") — production
    # sits behind a reverse proxy (map.priesol.net) that only forwards to
    # this app, so a browser can't reach kiwix-serve directly by any
    # hostname/port of its own. GET /kiwix/* below proxies to it instead.
    KIWIX_URL = ENV.fetch("KIWIX_URL", "")

    # Login delegates to the Gitea instance the user already has an account
    # on, rather than maintaining a local username/password — see
    # docs/architecture.adoc. Only users listed in ALLOWED_GITEA_USERS may
    # write; anyone else can authenticate against Gitea but is still
    # refused. No defaults for the OAuth2 client id/secret/user list: blank
    # values just mean login (and thus writing) doesn't work, which is fine
    # outside production — nothing here is eagerly validated at load time.
    GITEA_URL = ENV.fetch("GITEA_URL", "https://git.priesol.net")
    ALLOWED_GITEA_USERS = ENV.fetch("MAPTOOL_ALLOWED_GITEA_USER", "").split(",").map(&:strip)
    OAUTH_CLIENT = OAuth2::Client.new(
      ENV.fetch("GITEA_OAUTH_CLIENT_ID", ""),
      ENV.fetch("GITEA_OAUTH_CLIENT_SECRET", ""),
      site: GITEA_URL,
      authorize_url: "/login/oauth/authorize",
      token_url: "/login/oauth/access_token"
    )

    helpers do
      def logged_in?
        !!session[:user]
      end

      def require_login!
        halt 401, json(error: "login required", login_url: "/login") unless logged_in?
      end

      def pmtiles_file_status(path)
        return { available: false } unless File.exist?(path)

        { available: true, size: File.size(path), mtime: File.mtime(path).utc.iso8601 }
      end

      def callback_url
        "#{request.base_url}/auth/callback"
      end

      def save_zip_entries(folder:, io:)
        paths = []
        Zip::File.open_buffer(io) do |zip|
          zip.each do |entry|
            next unless entry.file? && GpxStore::TRACK_EXTENSIONS.any? { |ext| entry.name.downcase.end_with?(ext) }

            paths << GPX_STORE.save(folder: folder, filename: File.basename(entry.name), io: StringIO.new(entry.get_input_stream.read))
          end
        end
        paths
      end
    end

    get "/login" do
      session[:oauth_state] = SecureRandom.hex(16)
      redirect OAUTH_CLIENT.auth_code.authorize_url(
        redirect_uri: callback_url,
        state: session[:oauth_state],
        scope: "read:user"
      )
    end

    get "/auth/callback" do
      halt 400, "invalid state" unless params[:state] && params[:state] == session.delete(:oauth_state)

      token = OAUTH_CLIENT.auth_code.get_token(params[:code], redirect_uri: callback_url)
      user = token.get("/api/v1/user").parsed
      halt 403, "not authorized" unless ALLOWED_GITEA_USERS.include?(user["login"])

      session[:user] = user["login"]
      redirect "/"
    end

    get "/logout" do
      session.clear
      redirect "/"
    end

    # Live GNSS position, polled by the frontend from /api/gnss/position. A
    # single background thread reads gpsd (if it's running — harmless retry
    # loop if not, see GpsdClient) and keeps this updated; polling instead
    # of push (SSE/WebSocket) is deliberate — gpsd itself only reports at
    # ~1Hz, so there is nothing to gain from a push channel, and it avoids
    # Sinatra's `stream(:keep_open)` helper, which — absent an EventMachine
    # reactor (i.e. under plain threaded Puma, as here) — busy-loops calling
    # its block repeatedly instead of waiting for external writes; fine for
    # a single blocking per-message write, not for "register once, get
    # pushed to later" like this.
    # A fix is only reported back as "available" for this long after it
    # last arrived — otherwise a receiver that's lost its fix, or been
    # unplugged, would keep showing its last position forever as if it
    # were still live.
    GNSS_STALE_AFTER = 10 # seconds

    GNSS_LATEST = {}
    GNSS_LATEST_MUTEX = Mutex.new

    Thread.new do
      GpsdClient.new.each_position do |position|
        GNSS_LATEST_MUTEX.synchronize do
          GNSS_LATEST.replace(position)
          GNSS_LATEST[:received_at] = Time.now.to_f
        end
      end
    end

    get "/" do
      erb :index, locals: { logged_in: logged_in?, user: session[:user], revision: REVISION }
    end

    get "/api/health" do
      json status: "ok", revision: REVISION
    end

    get "/api/offline-tiles" do
      json pmtiles_file_status(OFFLINE_PMTILES_PATH)
    end

    # Served as a plain range-request-capable file, not decoded server-side:
    # pmtiles.js reads the archive's directory and fetches only the byte
    # ranges it needs for the tiles currently on screen.
    get "/tiles/offline.pmtiles" do
      halt 404 unless File.exist?(OFFLINE_PMTILES_PATH)

      send_file OFFLINE_PMTILES_PATH, type: "application/octet-stream"
    end

    # Lists the selectable countries and which of them are currently
    # selected (data/offline_regions.json) — read-only, no login needed,
    # same tier as the plain availability checks above.
    get "/api/offline-regions" do
      json available: PmtilesBuilder.available_regions, selected: PmtilesBuilder.selected_regions(DATA_DIR)
    end

    # Just updates the selection — does not itself start a build, so
    # ticking several countries doesn't fire off several rebuilds. The
    # frontend follows this with the two build routes below once the user
    # is done choosing.
    post "/api/offline-regions" do
      require_login!
      codes = Array(params[:codes])

      begin
        PmtilesBuilder.set_selected_regions(DATA_DIR, codes)
      rescue PmtilesBuilder::BuildError => e
        halt 400, json(error: e.message)
      end
      json ok: true
    end

    # Regenerates offline.pmtiles for the currently selected regions (see
    # POST /api/offline-regions above) by extracting them from Protomaps'
    # own daily full-planet basemap build — see
    # lib/maptool/pmtiles_builder.rb. Runs in the background; poll
    # build-status below for progress.
    post "/api/offline-tiles/build" do
      require_login!
      begin
        PmtilesBuilder.build_offline(data_dir: DATA_DIR)
      rescue PmtilesBuilder::BuildError => e
        halt 409, json(error: e.message)
      end
      json status: "started"
    end

    get "/api/offline-tiles/build-status" do
      json PmtilesBuilder.offline_status
    end

    # Same optional-local-file story as offline.pmtiles above — see
    # docs/development.adoc for how to generate one, until then hillshade
    # just stays unavailable while the offline basemap is active.
    get "/api/terrain-tiles" do
      json pmtiles_file_status(TERRAIN_PMTILES_PATH)
    end

    get "/tiles/terrain.pmtiles" do
      halt 404 unless File.exist?(TERRAIN_PMTILES_PATH)

      send_file TERRAIN_PMTILES_PATH, type: "application/octet-stream"
    end

    post "/api/terrain-tiles/build" do
      require_login!
      begin
        PmtilesBuilder.build_terrain(data_dir: DATA_DIR)
      rescue PmtilesBuilder::BuildError => e
        halt 409, json(error: e.message)
      end
      json status: "started"
    end

    get "/api/terrain-tiles/build-status" do
      json PmtilesBuilder.terrain_status
    end

    # Pinged once per page load (public/js/map.js) — url is a same-origin
    # path (not kiwix-serve's own address, which the browser can't reach
    # directly behind the reverse proxy in front of this app), proxied by
    # GET /kiwix/* below.
    get "/api/kiwix" do
      reachable = Kiwix.reachable?(KIWIX_URL)
      json available: reachable, url: (reachable ? "/kiwix" : "")
    end

    # Backs the "Wikipedia" map layer (public/js/gpx.js) — replaces the old
    # per-popup "Nearby on Wikipedia" link, which only ever pointed at one
    # POI at a time. Read-only and login-free, same as the tree/plugin
    # listing routes below: viewing the map needs no account, only writing
    # to it does.
    get "/api/kiwix/nearby" do
      halt 404 if KIWIX_URL.empty?
      bbox = %w[south west north east].map { |key| params[key].to_s }
      halt 400, json(error: "south, west, north, east are required") if bbox.any?(&:empty?)

      json Kiwix.nearby(KIWIX_URL, bbox.map(&:to_f))
    end

    # Proxies kiwix-serve's own search UI and content pages through this
    # app instead of linking the browser to it directly (see KIWIX_URL) —
    # kiwix-serve is started with --urlRootLocation=/kiwix
    # (docker-compose.yaml), so every link/asset it emits already carries
    # this same /kiwix prefix and Just Works through this passthrough. GET
    # only: browsing a read-only ZIM archive never needs to write anything.
    get "/kiwix/*" do
      halt 404 if KIWIX_URL.empty?
      response = Kiwix.proxy(KIWIX_URL, "/kiwix/#{params[:splat].first}", request.query_string)
      halt 502 unless response

      content_type response["content-type"] || "application/octet-stream"
      status response.code.to_i
      body response.body
    end

    get "/api/gpx/tree" do
      json GPX_STORE.tree
    end

    # A .zip (Garmin's bulk/FIT export format) is expanded into its
    # .gpx/.fit/.geojson entries instead of being stored itself — nothing
    # downstream (tree listing, map rendering) knows what to do with a zip.
    post "/api/gpx" do
      require_login!
      file = params[:file]
      halt 400, json(error: "file is required") unless file && file[:tempfile]

      begin
        paths = if file[:filename].to_s.downcase.end_with?(".zip")
                  save_zip_entries(folder: params[:folder].to_s, io: file[:tempfile])
                else
                  [GPX_STORE.save(folder: params[:folder].to_s, filename: file[:filename], io: file[:tempfile])]
                end
      rescue GpxStore::InvalidPath
        halt 400, json(error: "invalid folder")
      rescue Zip::Error
        halt 400, json(error: "invalid zip file")
      end

      json paths: paths, tree: GPX_STORE.tree
    end

    # A hand-drawn point/note (see notes.js) — unlike the upload above, this
    # grows one existing/new .geojson "layer" file in place (GpxStore#
    # append_feature) rather than always landing as its own new file.
    post "/api/gpx/notes" do
      require_login!
      begin
        feature = JSON.parse(params[:feature].to_s)
      rescue JSON::ParserError
        halt 400, json(error: "invalid feature")
      end
      halt 400, json(error: "invalid feature") unless feature.is_a?(Hash) && feature["geometry"]

      begin
        path = GPX_STORE.append_feature(
          feature: feature,
          path: params[:path],
          folder: params[:folder].to_s,
          name: params[:name].to_s
        )
      rescue GpxStore::InvalidPath
        halt 400, json(error: "invalid path")
      rescue Errno::ENOENT
        halt 404, json(error: "file not found")
      end

      json path: path, tree: GPX_STORE.tree
    end

    # Editing/deleting one note (see the popup's Edit/Delete actions,
    # gpx.js) — targets a feature by properties["id"], not array position
    # (see GpxStore#update_feature/#delete_feature for why).
    put "/api/gpx/notes" do
      require_login!
      begin
        feature = JSON.parse(params[:feature].to_s)
      rescue JSON::ParserError
        halt 400, json(error: "invalid feature")
      end
      halt 400, json(error: "invalid feature") unless feature.is_a?(Hash) && feature["geometry"]
      halt 400, json(error: "path and id are required") if params[:path].to_s.empty? || params[:id].to_s.empty?

      begin
        GPX_STORE.update_feature(path: params[:path], id: params[:id], feature: feature)
      rescue GpxStore::InvalidPath
        halt 400, json(error: "invalid path")
      rescue Errno::ENOENT
        halt 404, json(error: "note not found")
      end

      json ok: true
    end

    delete "/api/gpx/notes" do
      require_login!
      halt 400, json(error: "path and id are required") if params[:path].to_s.empty? || params[:id].to_s.empty?

      begin
        GPX_STORE.delete_feature(path: params[:path], id: params[:id])
      rescue GpxStore::InvalidPath
        halt 400, json(error: "invalid path")
      rescue Errno::ENOENT
        halt 404, json(error: "note not found")
      end

      json ok: true
    end

    # Lists whatever import plugins are on disk (see lib/maptool/plugins.rb
    # for the contract) — read-only, doesn't need require_login! the way
    # actually running one does below.
    get "/api/plugins" do
      json Plugins.list(PLUGINS_DIRS)
    end

    # Runs one plugin for the given bbox (the current map viewport, from
    # the frontend's "Run for this area" button) and saves its GeoJSON
    # output as a new file the same way an uploaded .geojson would be
    # saved — reusing GpxStore#save rather than #append_feature since this
    # writes a whole FeatureCollection at once, not one feature into an
    # existing one.
    post "/api/plugins/:name/run" do
      require_login!
      bbox = %w[south west north east].map { |key| params[key].to_s }
      halt 400, json(error: "south, west, north, east are required") if bbox.any?(&:empty?)

      south, west, north, east = bbox.map(&:to_f)
      begin
        geojson = Plugins.run(PLUGINS_DIRS, params[:name], [south, west, north, east])
      rescue Plugins::RunError => e
        halt 502, json(error: e.message)
      end

      # Filename carries the area's center and the run's timestamp (same
      # idea as a p4n2gpx export's own "p4n_area_<lat>_<lon>_<date>_<time>"
      # naming) — without it, running this again for a different area just
      # overwrote/renamed-with-a-counter the same "overpass-pois.geojson",
      # with nothing in the file tree to tell one run's POIs from another's.
      center_lat = format("%.2f", (south + north) / 2)
      center_lon = format("%.2f", (west + east) / 2)
      timestamp = Time.now.strftime("%y%m%d_%H%M")
      filename = "#{params[:name]}_#{center_lat}_#{center_lon}_#{timestamp}.geojson"

      path = GPX_STORE.save(folder: "", filename: filename, io: StringIO.new(JSON.generate(geojson)))
      json path: path, tree: GPX_STORE.tree
    end

    # ?download=geojson exports the file's GeoJSON as an attachment instead
    # of rendering it on the map — the only way to get a .gpx/.fit's
    # converted form out of the app, since GpxStore#geojson's cache is
    # otherwise an internal implementation detail with no filename of its
    # own to expose. ?download=original exports the untouched, as-uploaded
    # file instead (a .geojson source's "original" is that same file, so
    # the two downloads are identical there — harmless, not special-cased).
    # Both gated the same as the other write/manage actions below (upload,
    # move, create folder) — viewing a file on the map stays open to
    # anyone, exporting it as a file does not.
    get "/api/gpx/file/*" do
      require_login! if params[:download]

      if params[:download] == "original"
        begin
          path = GPX_STORE.resolve_file(params[:splat].first)
        rescue GpxStore::InvalidPath
          halt 400
        end
        halt 404 unless File.file?(path)
        return send_file(path, filename: File.basename(path), disposition: "attachment")
      end

      begin
        body = GPX_STORE.geojson(params[:splat].first)
      rescue GpxStore::InvalidPath
        halt 400
      rescue Errno::ENOENT
        halt 404
      rescue Gpx::ParseError, Fit::ParseError
        halt 422, json(error: "could not parse file")
      end

      content_type :json
      if params[:download]
        # Matches the tree's own display name for a cached .gpx/.fit
        # (public/js/gpx.js's _renderFile) — "ride.gpx" downloads as
        # "ride.gpx.geojson", not "ride.geojson", so the two never disagree.
        # A .geojson source is already named right and is left alone.
        original_name = File.basename(params[:splat].first)
        download_name = original_name.downcase.end_with?(".geojson") ? original_name : "#{original_name}.geojson"
        attachment download_name
      end
      body
    end

    post "/api/gpx/folder" do
      require_login!
      begin
        GPX_STORE.create_folder(params[:path].to_s)
      rescue GpxStore::InvalidPath
        halt 400, json(error: "invalid path")
      end

      json tree: GPX_STORE.tree
    end

    delete "/api/gpx/file/*" do
      require_login!
      begin
        GPX_STORE.delete_file(params[:splat].first)
      rescue GpxStore::InvalidPath
        halt 400
      rescue Errno::ENOENT
        halt 404
      end

      json tree: GPX_STORE.tree
    end

    delete "/api/gpx/folder/*" do
      require_login!
      begin
        GPX_STORE.delete_folder(params[:splat].first)
      rescue GpxStore::InvalidPath
        halt 400
      rescue Errno::ENOENT
        halt 404
      end

      json tree: GPX_STORE.tree
    end

    post "/api/gpx/move" do
      require_login!
      begin
        path = GPX_STORE.move(from: params[:from].to_s, to_folder: params[:to_folder].to_s)
      rescue GpxStore::InvalidPath
        halt 400, json(error: "invalid path")
      rescue Errno::ENOENT
        halt 404
      end

      json path: path, tree: GPX_STORE.tree
    end

    # Called by the Apps Script watching Gmail for Garmin LiveTrack emails —
    # it only extracts and forwards the session id/token, nothing else.
    post "/api/livetrack/session" do
      halt 403 if LIVETRACK_WEBHOOK_SECRET.empty? || params[:secret] != LIVETRACK_WEBHOOK_SECRET

      session_id = params[:session_id].to_s.strip
      session_token = params[:session_token].to_s.strip
      halt 400, json(error: "session_id is required") if session_id.empty?

      LIVETRACK_STORE.add(session_id, session_token)
      json ok: true
    end

    get "/api/livetrack/sessions" do
      json sessions: LIVETRACK_STORE.list
    end

    # Garmin stops serving a session's live-track page some while after the
    # activity ends (its own session/token URL, not the API — see
    # LiveTrackClient) — the exact cutoff isn't documented, ~24h is what's
    # been observed. Ended sessions old enough to likely be past that are
    # verified once at startup (LiveTrackStore#stale_ended already skips
    # ones already flagged, so this doesn't re-hit Garmin for the same
    # dead session on every restart), so the panel can grey them out up
    # front instead of only failing after a user clicks one and waits on
    # a fetch.
    LIVETRACK_MAP_STALE_AFTER = 24 * 60 * 60 # seconds

    Thread.new do
      LIVETRACK_STORE.stale_ended(LIVETRACK_MAP_STALE_AFTER).each do |session|
        LiveTrackClient.fetch(session["id"], session["token"])
      rescue LiveTrackClient::FetchError
        LIVETRACK_STORE.mark_unavailable(session["id"])
      end
    end

    # Garmin's own API (livetrack.garmin.com/api/v2/...) 403s a plain server
    # request no matter what headers it gets — see LiveTrackClient — so this
    # fetches the session page itself and pulls the track out of that.
    get "/api/livetrack/sessions/:id/track" do
      session = LIVETRACK_STORE.find(params[:id])
      halt 404, json(error: "unknown session") unless session
      halt 404, json(error: "session has no token") if session["token"].to_s.empty?

      begin
        result = LiveTrackClient.fetch(session["id"], session["token"])
      rescue LiveTrackClient::FetchError => e
        halt 502, json(error: "could not fetch track: #{e.message}")
      end
      summary = LiveTrackClient.summarize(result)
      LIVETRACK_STORE.set_summary(session["id"], summary)

      points = result.points
      coordinates = points.filter_map { |p| p.dig("position", "lon") && p.dig("position", "lat") && [p["position"]["lon"], p["position"]["lat"]] }
      halt 502, json(error: "track has no positions") if coordinates.empty?
      last = points.last

      json(
        type: "FeatureCollection",
        features: [
          { type: "Feature", geometry: { type: "LineString", coordinates: coordinates }, properties: {} },
          {
            type: "Feature",
            geometry: { type: "Point", coordinates: coordinates.last },
            # Merges the session summary in too — the point layer's hover
            # popup (public/js/livetrack.js) reads it straight off here
            # rather than needing a second fetch against /sessions.
            properties: summary.merge(
              "time" => last["dateTime"],
              "speed_meters_per_sec" => last["speedMetersPerSec"],
              "heart_rate_beats_per_min" => last["heartRateBeatsPerMin"]
            ),
          },
        ]
      )
    end

    get "/api/gnss/position" do
      require_login!
      GNSS_LATEST_MUTEX.synchronize do
        received_at = GNSS_LATEST[:received_at]
        fresh = received_at && (Time.now.to_f - received_at) < GNSS_STALE_AFTER
        json(fresh ? GNSS_LATEST.merge(available: true) : { available: false })
      end
    end
  end
end