require "socket"
require "json"

module Maptool
  # A minimal client for gpsd's JSON protocol (https://gpsd.io/gpsd_json.html).
  # Connects over TCP, asks gpsd to watch for reports, and yields a plain
  # position hash for every TPV ("time-position-velocity") report — gpsd's
  # term for a GNSS fix — for as long as the block keeps running.
  class GpsdClient
    RECONNECT_DELAY = 5 # seconds, after gpsd is unreachable or drops us

    def initialize(host: "127.0.0.1", port: 2947)
      @host = host
      @port = port
    end

    # Never returns under normal operation: reconnects on any I/O error
    # (gpsd not running yet, connection dropped, etc.) after a short delay,
    # so a missing/disconnected receiver is not fatal to the caller.
    def each_position
      loop do
        watch { |position| yield position }
      rescue SystemCallError, EOFError, SocketError => e
        warn "[gpsd] #{e.class}: #{e.message} — retrying in #{RECONNECT_DELAY}s"
        sleep RECONNECT_DELAY
      end
    end

    private

    def watch
      socket = TCPSocket.new(@host, @port)
      socket.puts('?WATCH={"enable":true,"json":true};')

      socket.each_line do |line|
        report = JSON.parse(line)
        next unless report["class"] == "TPV"
        next unless report["lat"] && report["lon"]

        yield(
          lat: report["lat"],
          lon: report["lon"],
          # altHAE (height above ellipsoid) is gpsd's modern field; older
          # gpsd versions only send "alt".
          alt: report["altHAE"] || report["alt"],
          speed: report["speed"],
          track: report["track"],
          mode: report["mode"], # 0/1 = no fix, 2 = 2D, 3 = 3D
          eph: report["eph"], # estimated horizontal position error, metres
          time: report["time"],
        )
      end
    ensure
      socket&.close
    end
  end
end