#!/usr/bin/env node
/*
 * find-your-subreddits.mjs
 *
 * Given a few words about your business, find the Reddit communities your customers and
 * competitors are actually in, and how big they are. Ten minutes of this saves you from
 * researching the wrong community for a week.
 *
 * It deliberately uses only the Arctic Shift endpoint I measured as fast and reliable
 * (subreddit metadata, about 380ms). It does not run text searches or aggregates, because
 * those are best effort on a free service and fail under load. See the cookbook for why.
 *
 * No dependencies. Node 18 or later.
 *
 * Usage:
 *   node find-your-subreddits.mjs laundr laundromat washing
 *   node find-your-subreddits.mjs carwash detailing auto --min 1000
 *   node find-your-subreddits.mjs storage selfstorage moving --json > subs.json
 *
 * Options:
 *   --min N        minimum subscribers (default 300)
 *   --limit N      results per term (default 20)
 *   --json         machine-readable output
 *   --delay MS     pause between requests (default 1200, please be kind)
 *
 * From https://jwatte.com/blog/reddit-data-arctic-shift-for-business/
 * Arctic Shift is by Arthur Heitmann: https://github.com/ArthurHeitmann/arctic_shift
 * Free to copy and adapt. No warranty. Read it before you run it.
 */

const BASE = 'https://arctic-shift.photon-reddit.com';
const UA = 'find-your-subreddits.mjs (+https://jwatte.com)';

const argv = process.argv.slice(2);
const flag = (name, dflt) => {
  const i = argv.indexOf(name);
  return i === -1 ? dflt : (Number(argv[i + 1]) || dflt);
};
const MIN = flag('--min', 300);
const LIMIT = flag('--limit', 20);
const DELAY = flag('--delay', 1200);
const asJson = argv.includes('--json');

const terms = argv.filter((a, i) => {
  if (a.startsWith('--')) return false;
  const prev = argv[i - 1];
  if (prev === '--min' || prev === '--limit' || prev === '--delay') return false;
  return true;
});

if (!terms.length) {
  console.error('usage: node find-your-subreddits.mjs <term> [term...] [--min N] [--limit N] [--json]');
  console.error('example: node find-your-subreddits.mjs laundr laundromat washing');
  process.exit(2);
}

const sleep = (ms) => new Promise((r) => setTimeout(r, ms));

async function lookup(term) {
  const url =
    `${BASE}/api/subreddits/search?subreddit_prefix=${encodeURIComponent(term)}` +
    `&min_subscribers=${MIN}&limit=${LIMIT}` +
    `&fields=display_name,subscribers,created_utc,public_description`;
  const ctl = new AbortController();
  const t = setTimeout(() => ctl.abort(), 45000);
  try {
    const r = await fetch(url, { headers: { 'user-agent': UA, accept: 'application/json' }, signal: ctl.signal });
    clearTimeout(t);
    if (!r.ok) return { term, error: `HTTP ${r.status}` };
    const j = await r.json();
    // The API reports its own soft failures in the body with a 200 status.
    if (j && j.error) return { term, error: j.error };
    return { term, rows: j.data || [] };
  } catch (e) {
    clearTimeout(t);
    return { term, error: String((e && (e.cause?.code || e.name)) || 'ERR') };
  }
}

const seen = new Map();
const problems = [];

for (const term of terms) {
  const res = await lookup(term);
  if (res.error) {
    problems.push(`${term}: ${res.error}`);
    if (!asJson) console.error(`  ! ${term}: ${res.error}`);
  } else {
    for (const row of res.rows) {
      const name = row.display_name;
      if (!name) continue;
      const subs = Number(row.subscribers) || 0;
      if (!seen.has(name) || seen.get(name).subscribers < subs) {
        seen.set(name, {
          name,
          subscribers: subs,
          created: row.created_utc ? new Date(Number(row.created_utc) * 1000).toISOString().slice(0, 7) : null,
          description: (row.public_description || '').replace(/\s+/g, ' ').slice(0, 100),
          matched: term,
        });
      }
    }
  }
  await sleep(DELAY);
}

const all = [...seen.values()].sort((a, b) => b.subscribers - a.subscribers);

if (asJson) {
  console.log(JSON.stringify({ terms, min: MIN, found: all.length, problems, subreddits: all }, null, 2));
} else {
  if (!all.length) {
    console.log('\nNothing found. Try shorter prefixes: "laundr" matches more than "laundromat".\n');
    process.exit(1);
  }
  const pad = Math.min(28, Math.max(...all.map((s) => s.name.length)) + 2);
  console.log(`\n  ${all.length} communities, largest first\n`);
  for (const s of all) {
    console.log(
      `  r/${s.name.padEnd(pad)} ${String(s.subscribers.toLocaleString()).padStart(11)}   ${s.created || '       '}   ${s.description}`
    );
  }

  // The point of the whole exercise: the biggest one is rarely the one named after the industry.
  const biggest = all[0];
  const named = all.find((s) => terms.some((t) => s.name.toLowerCase() === t.toLowerCase()));
  console.log('');
  if (named && named.name !== biggest.name) {
    const ratio = named.subscribers ? Math.round(biggest.subscribers / named.subscribers) : 0;
    console.log(`  Note: r/${named.name} has ${named.subscribers.toLocaleString()} members,`);
    console.log(`        but r/${biggest.name} has ${biggest.subscribers.toLocaleString()}${ratio > 1 ? `, about ${ratio}x more` : ''}.`);
    console.log(`        The community named after the industry is usually not the one to study.`);
  } else {
    console.log(`  Largest: r/${biggest.name} at ${biggest.subscribers.toLocaleString()} members.`);
  }
  console.log(`\n  Next: read fifty posts in the top two or three by hand before writing any code.\n`);
  if (problems.length) console.log(`  ${problems.length} term(s) failed: ${problems.join('; ')}\n`);
}