physics.js

const satellite = require('satellite.js');
const fs = require('fs');
const path = require('path');

const TLE_CACHE_PATH = path.join(__dirname, 'TLE_cache.json');
const TLE_URL = 'https://celestrak.org/NORAD/elements/gp.php?GROUP=active&FORMAT=tle';
const TLE_REFRESH_INTERVAL = 60 * 60 * 1000;

let tleCache = {};
let lastFetchTime = 0;

function deg2rad(deg) {
  return deg * (Math.PI / 180);
}

function rad2deg(rad) {
  return rad * (180 / Math.PI);
}

function loadTleCache() {
  try {
    if (fs.existsSync(TLE_CACHE_PATH)) {
      const data = fs.readFileSync(TLE_CACHE_PATH, 'utf-8');
      tleCache = JSON.parse(data);
    }
  } catch (e) {
    console.error('Failed to load TLE cache:', e.message);
    tleCache = {};
  }
}

function saveTleCache() {
  try {
    fs.writeFileSync(TLE_CACHE_PATH, JSON.stringify(tleCache, null, 2));
  } catch (e) {
    console.error('Failed to save TLE cache:', e.message);
  }
}

loadTleCache();

async function fetchTleData() {
  try {
    const response = await fetch(TLE_URL);
    if (!response.ok) throw new Error(`HTTP ${response.status}`);
    const text = await response.text();
    const lines = text.split('\n').filter(l => l.trim());
    const newCache = {};

    for (let i = 0; i + 2 < lines.length; i += 3) {
      const name = lines[i].trim().replace(/^0 /, '');
      const line1 = lines[i + 1].trim();
      const line2 = lines[i + 2].trim();
      if (line1.startsWith('1 ') && line2.startsWith('2 ')) {
        newCache[name] = { name, line1, line2 };
      }
    }

    tleCache = newCache;
    lastFetchTime = Date.now();
    saveTleCache();
    console.log(`Fetched ${Object.keys(newCache).length} TLE records`);
    return newCache;
  } catch (e) {
    console.error('TLE fetch failed:', e.message);
    return tleCache;
  }
}

async function ensureTleFresh() {
  if (Date.now() - lastFetchTime > TLE_REFRESH_INTERVAL) {
    await fetchTleData();
  }
}

function propagateSatellite(tleRecord, date) {
  try {
    const satrec = satellite.twoline2satrec(tleRecord.line1, tleRecord.line2);
    const positionAndVelocity = satellite.propagate(satrec, date);

    if (!positionAndVelocity.position) return null;

    const positionEci = positionAndVelocity.position;
    const gmst = satellite.gstime(date);
    const geodetic = satellite.eciToGeodetic(positionEci, gmst);

    const lat = rad2deg(geodetic.latitude);
    const lng = rad2deg(geodetic.longitude);
    const alt = geodetic.height;

    const earthRadius = 6371;
    const footprintRadius = earthRadius * Math.acos(earthRadius / (earthRadius + alt));

    return { lat, lng, alt: alt * 1000, footprintRadius: footprintRadius * 1000 };
  } catch (e) {
    return null;
  }
}

function getActiveSatellitesState(date) {
  const names = Object.keys(tleCache);
  const state = {};

  for (const name of names) {
    const result = propagateSatellite(tleCache[name], date);
    if (result) {
      state[name] = result;
    }
  }

  return state;
}

function getRandomSatelliteState(date) {
  const names = Object.keys(tleCache);
  if (names.length === 0) return null;

  const name = names[Math.floor(Math.random() * names.length)];
  const result = propagateSatellite(tleCache[name], date);
  if (!result) return null;

  return { name, ...result };
}

function getSatelliteState(name, date) {
  if (!tleCache[name]) return null;
  const result = propagateSatellite(tleCache[name], date);
  if (!result) return null;

  return { name, ...result };
}

function haversineDistance(lat1, lng1, lat2, lng2) {
  const R = 6371000;
  const dLat = deg2rad(lat2 - lat1);
  const dLng = deg2rad(lng2 - lng1);
  const a = Math.sin(dLat / 2) ** 2 +
    Math.cos(deg2rad(lat1)) * Math.cos(deg2rad(lat2)) *
    Math.sin(dLng / 2) ** 2;
  const c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1 - a));
  return R * c;
}

function scoreFromDistance(distanceMeters) {
  if (distanceMeters < 1000) return 10000;
  if (distanceMeters < 10000) return Math.round(10000 - (distanceMeters / 10000) * 2000);
  if (distanceMeters < 100000) return Math.round(8000 - (distanceMeters / 100000) * 3000);
  if (distanceMeters < 500000) return Math.round(5000 - (distanceMeters / 500000) * 3000);
  return Math.max(0, Math.round(2000 - (distanceMeters / 20000000) * 2000));
}

module.exports = {
  fetchTleData,
  ensureTleFresh,
  propagateSatellite,
  getActiveSatellitesState,
  getRandomSatelliteState,
  getSatelliteState,
  haversineDistance,
  scoreFromDistance,
  tleCache,
};