const express = require('express');
const crypto = require('crypto');
const { Pool } = require('pg');

const app = express();
app.use(express.json());

const pool = new Pool({
  connectionString: process.env.DATABASE_URL,
});

async function initDB() {
  const client = await pool.connect();
  try {
    await client.query(`
      CREATE TABLE IF NOT EXISTS links (
        hash TEXT PRIMARY KEY,
        url TEXT NOT NULL,
        hits INTEGER NOT NULL DEFAULT 0,
        created_at TIMESTAMP NOT NULL DEFAULT NOW()
      )
    `);
    console.log('Database table ready');
  } finally {
    client.release();
  }
}

function makeHash(url) {
  return crypto.createHash('sha256').update(url).digest('base64url').slice(0, 7);
}

// POST /api/links  {"url": "https://example.com"} -> {"hash": "...", "short_url": "..."}
app.post('/api/links', async (req, res) => {
  const { url } = req.body;
  let parsed;
  try {
    parsed = new URL(url);
  } catch {
    return res.status(400).json({ error: '"url" must be a valid URL' });
  }
  if (parsed.protocol !== 'http:' && parsed.protocol !== 'https:') {
    return res.status(400).json({ error: 'only http/https URLs allowed' });
  }
  const hash = makeHash(parsed.href);
  try {
    await pool.query(
      'INSERT INTO links (hash, url) VALUES ($1, $2) ON CONFLICT (hash) DO NOTHING',
      [hash, parsed.href]
    );
    res.status(201).json({
      hash,
      short_url: `${req.protocol}://${req.get('host')}/${hash}`,
    });
  } catch (err) {
    console.error(err);
    res.status(500).json({ error: 'Failed to create link' });
  }
});

// GET /api/links -> recent links with hit counts
app.get('/api/links', async (req, res) => {
  try {
    const { rows } = await pool.query(
      'SELECT hash, url, hits, created_at FROM links ORDER BY created_at DESC LIMIT 50'
    );
    res.json(rows);
  } catch (err) {
    console.error(err);
    res.status(500).json({ error: 'Failed to list links' });
  }
});

// GET /:hash -> 302 redirect
app.get('/:hash([A-Za-z0-9_-]{7})', async (req, res) => {
  try {
    const { rows } = await pool.query(
      'UPDATE links SET hits = hits + 1 WHERE hash = $1 RETURNING url',
      [req.params.hash]
    );
    if (rows.length === 0) return res.status(404).json({ error: 'Link not found' });
    res.redirect(302, rows[0].url);
  } catch (err) {
    console.error(err);
    res.status(500).json({ error: 'Failed to resolve link' });
  }
});

app.get('/', (req, res) => {
  res.json({
    name: 'url-shortener',
    usage: {
      shorten: 'POST /api/links {"url": "https://example.com"}',
      list: 'GET /api/links',
      follow: 'GET /<hash>',
    },
  });
});

initDB().then(() => {
  const PORT = process.env.PORT || 3000;
  app.listen(PORT, () => console.log(`Server running on port ${PORT}`));
});