import { readFile } from 'node:fs/promises';
import { access } from 'node:fs/promises';
import { join } from 'node:path';

const SOURCE_COUNT = 29;
const DOCS_ROOT = 'src/content/docs/';

export async function loadManifest(path) {
  return JSON.parse(await readFile(path, 'utf8'));
}

export function validateManifest(manifest) {
  if (!Array.isArray(manifest?.entries)) {
    throw new TypeError('Migration manifest entries must be an array.');
  }

  if (manifest.entries.length !== SOURCE_COUNT) {
    throw new Error(`Migration manifest must contain exactly ${SOURCE_COUNT} entries.`);
  }

  const sources = new Set();
  const destinations = new Set();

  for (const entry of manifest.entries) {
    if (!entry || typeof entry !== 'object') {
      throw new TypeError('Each migration manifest entry must be an object.');
    }

    const { source, destination, mode } = entry;
    if (typeof source !== 'string' || typeof destination !== 'string') {
      throw new TypeError('Each migration manifest entry needs string source and destination paths.');
    }
    if (!['page', 'merge'].includes(mode)) {
      throw new TypeError('Each migration manifest entry mode must be page or merge.');
    }
    if (source.startsWith('pages/wiki/')) {
      throw new Error(`Generic DokuWiki source is excluded: ${source}`);
    }
    if (!(source === 'pages/start.txt' || source === 'pages/study.txt' || source.startsWith('pages/study/'))) {
      throw new Error(`Source is outside the selected migration scope: ${source}`);
    }
    if (!destination.startsWith(DOCS_ROOT)) {
      throw new Error(`Destination is outside ${DOCS_ROOT}: ${destination}`);
    }
    if (sources.has(source)) {
      throw new Error(`Duplicate migration source: ${source}`);
    }
    if (destinations.has(destination) && mode !== 'merge') {
      throw new Error(`Duplicate destination requires a later merge entry: ${destination}`);
    }

    sources.add(source);
    destinations.add(destination);
  }

  return { entries: manifest.entries };
}

export async function checkDestinations(manifest, root) {
  const destinations = [...new Set(manifest.entries.map(({ destination }) => destination))];
  const missing = [];

  for (const destination of destinations) {
    try {
      await access(join(root, destination));
    } catch {
      missing.push(destination);
    }
  }

  return {
    total: manifest.entries.length,
    missing,
  };
}