#!/usr/bin/env ruby
# frozen_string_literal: true

# Speaks gpsd's own JSON wire protocol (https://gpsd.io/gpsd_json.html)
# directly on gpsd's default port — GpsdClient (lib/maptool/gpsd_client.rb)
# connects to it exactly as it would to real gpsd, so no app code or config
# changes are needed. Unlike docs/development.adoc's fake-NMEA-source
# example, this doesn't need gpsd itself installed/running at all, and
# actually moves (a fixed point isn't much of a "GNSS position" to look
# at) — drives a loop around a center point at a driving-ish pace, one TPV
# report per second like a real receiver.
#
# Usage: bin/gnss_simulator.rb [lat] [lon] [radius_m] [port]
require "socket"
require "json"

$stdout.sync = true

LAT = (ARGV[0] || 48.1486).to_f
LON = (ARGV[1] || 17.1077).to_f
RADIUS_M = (ARGV[2] || 300).to_f
PORT = (ARGV[3] || 2947).to_i
LOOP_SECONDS = 120 # time to complete one full loop
EARTH_RADIUS_M = 6_371_000

def point_on_loop(center_lat, center_lon, radius_m, angle_rad)
  # Flat-plane offset, not geodesic — fine at this radius (metres, not km),
  # same reasoning as measure.js's own approximations at this scale.
  lat_offset = (radius_m * Math.sin(angle_rad)) / EARTH_RADIUS_M
  lon_offset = (radius_m * Math.cos(angle_rad)) / (EARTH_RADIUS_M * Math.cos(center_lat * Math::PI / 180))
  [center_lat + lat_offset * 180 / Math::PI, center_lon + lon_offset * 180 / Math::PI]
end

server = TCPServer.new("127.0.0.1", PORT)
puts "[gnss-simulator] driving a #{RADIUS_M.to_i}m loop around #{LAT},#{LON} — listening on 127.0.0.1:#{PORT}"

loop do
  socket = server.accept
  puts "[gnss-simulator] client connected"
  start = Time.now

  begin
    loop do
      elapsed = Time.now - start
      angle = (elapsed / LOOP_SECONDS) * 2 * Math::PI
      lat, lon = point_on_loop(LAT, LON, RADIUS_M, angle)

      # Bearing of travel: tangent to the circle, 90 degrees ahead of the
      # radius vector at this point.
      track = (angle * 180 / Math::PI + 90) % 360
      speed = (2 * Math::PI * RADIUS_M) / LOOP_SECONDS # m/s, constant pace

      report = {
        class: "TPV",
        mode: 3,
        lat: lat,
        lon: lon,
        altHAE: 150.0,
        speed: speed,
        track: track,
        eph: 3.0,
        time: Time.now.utc.strftime("%Y-%m-%dT%H:%M:%S.%3NZ"),
      }
      socket.puts(JSON.generate(report))
      sleep 1
    end
  rescue Errno::EPIPE, IOError
    puts "[gnss-simulator] client disconnected"
  ensure
    socket.close
  end
end