// registry-io.mjs — Atomic read/write for url-registry.json and report files.
// Zero dependencies. Uses rename() for atomic writes.

import { readFileSync, writeFileSync, renameSync, existsSync, mkdirSync } from 'node:fs';
import { join, dirname } from 'node:path';
import { fileURLToPath } from 'node:url';

const __dirname = dirname(fileURLToPath(import.meta.url));
const DEFAULT_DATA_DIR = join(__dirname, '..', 'data');

/**
 * Load the URL registry from disk.
 * @param {string} [dataDir] — defaults to ../data/ relative to lib/
 * @returns {object} parsed registry or empty scaffold
 */
export function loadRegistry(dataDir = DEFAULT_DATA_DIR) {
  const path = join(dataDir, 'url-registry.json');
  if (!existsSync(path)) {
    return {
      version: 1,
      created_at: null,
      last_poll: null,
      sitemap_state: {},
      urls: {},
    };
  }
  return JSON.parse(readFileSync(path, 'utf8'));
}

/**
 * Save the URL registry atomically (write to .tmp, then rename).
 * @param {object} registry
 * @param {string} [dataDir]
 */
export function saveRegistry(registry, dataDir = DEFAULT_DATA_DIR) {
  ensureDir(dataDir);
  const path = join(dataDir, 'url-registry.json');
  const tmp = path + '.tmp';
  writeFileSync(tmp, JSON.stringify(registry, null, 2) + '\n', 'utf8');
  renameSync(tmp, path);
}

/**
 * Load a JSON report file (change-report.json or discovery-report.json).
 * @param {string} name — filename without path (e.g. 'change-report.json')
 * @param {string} [dataDir]
 * @returns {object|null} parsed JSON or null if not found
 */
export function loadReport(name, dataDir = DEFAULT_DATA_DIR) {
  const path = join(dataDir, name);
  if (!existsSync(path)) return null;
  try {
    return JSON.parse(readFileSync(path, 'utf8'));
  } catch {
    return null;
  }
}

/**
 * Save a JSON report file atomically.
 * @param {string} name
 * @param {object} data
 * @param {string} [dataDir]
 */
export function saveReport(name, data, dataDir = DEFAULT_DATA_DIR) {
  ensureDir(dataDir);
  const path = join(dataDir, name);
  const tmp = path + '.tmp';
  writeFileSync(tmp, JSON.stringify(data, null, 2) + '\n', 'utf8');
  renameSync(tmp, path);
}

function ensureDir(dir) {
  if (!existsSync(dir)) mkdirSync(dir, { recursive: true });
}