/**
 * Config loading — merges JSON config file with CLI args.
 * CLI args always win. Required paths are validated here.
 */
import { readFile } from 'fs/promises';
import { join } from 'path';

const REQUIRED_PATHS = ['source', 'prev', 'pend', 'deltas'];
const DEFAULTS = {
  backend: 'zstd',
};

export async function loadConfig(args) {
  let fileConfig = {};

  if (args.config) {
    try {
      const raw = await readFile(args.config, 'utf8');
      fileConfig = JSON.parse(raw);
    } catch (err) {
      console.error(`Error reading config file ${args.config}: ${err.message}`);
      process.exit(1);
    }
  }

  // CLI args override file config, file config overrides defaults
  const config = { ...DEFAULTS, ...fileConfig, ...filterDefined(args) };

  // Expand --base into --prev/--pend/--deltas, explicit flags take priority
  if (config.base) {
    config.prev   ??= join(config.base, 'previous');
    config.pend   ??= join(config.base, 'pending');
    config.deltas ??= join(config.base, 'deltas');
  }

  // Guard: refuse to run if any required path is missing
  if (config.command === 'run') {
    const missing = REQUIRED_PATHS.filter(k => !config[k]);
    if (missing.length > 0) {
      console.error(`Error: missing required options: ${missing.map(k => `--${k}`).join(', ')}`);
      console.error('Provide them as CLI flags or in a --config JSON file.');
      process.exit(1);
    }
  }

  return config;
}

function filterDefined(obj) {
  return Object.fromEntries(Object.entries(obj).filter(([, v]) => v !== undefined));
}