import path from 'path';
import { Worker } from 'worker_threads';

const WORKER_PATH = path.resolve(process.cwd(), 'assets/app/workers/unionWorker.js');

// The app's own npm dependencies (installed by `meteor build`) live in
// programs/server/npm/node_modules — a directory that's a SIBLING of
// programs/server/assets, not an ancestor of it. Node's require() only walks
// up parent directories looking for node_modules, so a plain
// require('@turf/circle') from the worker script (a raw asset, not part of
// Meteor's own module system that knows about this layout) can never find it
// and fails with "Cannot find module '@turf/circle'". Resolving the absolute
// path here, where Meteor's own module system already knows how to find it,
// and handing it to the worker sidesteps that entirely.
const NPM_MODULES = path.resolve(process.cwd(), 'npm/node_modules');
const turfPaths = {
  circle: path.join(NPM_MODULES, '@turf/circle'),
  union: path.join(NPM_MODULES, '@turf/union'),
  truncate: path.join(NPM_MODULES, '@turf/truncate')
};

/**
 * Circle-union of already-validated, already-decorated subscriptions, run in a
 * worker thread so the main event loop (DDP/HTTP) stays free regardless of
 * how long any single turf.union call takes. subs must be an array of plain
 * { location: { lat, lon }, distance } objects. If baseUnion is given, subs
 * are merged into it instead of a union computed from scratch.
 */
const calcUnionAsync = (subs, baseUnion = null) => new Promise((resolve, reject) => {
  const worker = new Worker(WORKER_PATH, { workerData: { subs, baseUnion, turfPaths } });
  worker.once('message', (msg) => {
    worker.terminate();
    if (msg.error) {
      reject(new Error(msg.error));
    } else {
      resolve(msg.union);
    }
  });
  worker.once('error', (err) => {
    worker.terminate();
    reject(err);
  });
});

export default calcUnionAsync;