import { readFile, stat } from 'node:fs/promises';
import { isAbsolute, relative, resolve, sep } from 'node:path';

const SOURCE_COUNT = 29;
const DOCS_ROOT = 'src/content/docs/';
const CANONICAL_ENTRIES = new Map([
  ['pages/start.txt', ['src/content/docs/index.mdx', 'page']],
  ['pages/study.txt', ['src/content/docs/index.mdx', 'merge']],
  ['pages/study/msv_incognito.txt', ['src/content/docs/about-incognito.md', 'page']],
  ['pages/study/bachelor.txt', ['src/content/docs/data-science-and-ai/index.md', 'page']],
  ['pages/study/bachelor/year_1.txt', ['src/content/docs/data-science-and-ai/year-1/index.md', 'page']],
  ['pages/study/bachelor/year_1/project_1-1.txt', ['src/content/docs/data-science-and-ai/year-1/project-1-1.md', 'page']],
  ['pages/study/bachelor/year_1/project_1-2.txt', ['src/content/docs/data-science-and-ai/year-1/project-1-2.md', 'page']],
  ['pages/study/bachelor/year_2.txt', ['src/content/docs/data-science-and-ai/year-2/index.md', 'page']],
  ['pages/study/bachelor/year_2/honours_programme.txt', ['src/content/docs/data-science-and-ai/year-2/honours-programme.md', 'page']],
  ['pages/study/bachelor/year_2/project_2-1.txt', ['src/content/docs/data-science-and-ai/year-2/project-2-1.md', 'page']],
  ['pages/study/bachelor/year_2/project_2-2.txt', ['src/content/docs/data-science-and-ai/year-2/project-2-2.md', 'page']],
  ['pages/study/bachelor/year_3.txt', ['src/content/docs/data-science-and-ai/year-3/index.md', 'page']],
  ['pages/study/bachelor/year_3/bachelors_thesis.txt', ['src/content/docs/data-science-and-ai/year-3/bachelors-thesis.md', 'page']],
  ['pages/study/bachelor/year_3/honours_programme.txt', ['src/content/docs/data-science-and-ai/year-3/honours-programme.md', 'page']],
  ['pages/study/bachelor/year_3/project_3-1.txt', ['src/content/docs/data-science-and-ai/year-3/project-3-1.md', 'page']],
  ['pages/study/bachelor/year_3/study_abroad.txt', ['src/content/docs/data-science-and-ai/year-3/study-abroad.md', 'page']],
  ['pages/study/master_ai.txt', ['src/content/docs/master-ai/index.md', 'page']],
  ['pages/study/master_ai/year_1.txt', ['src/content/docs/master-ai/year-1/index.md', 'page']],
  ['pages/study/master_ai/year_1/project_mai_1.txt', ['src/content/docs/master-ai/year-1/research-project-1.md', 'page']],
  ['pages/study/master_ai/year_1/project_mai_2.txt', ['src/content/docs/master-ai/year-1/research-project-2.md', 'page']],
  ['pages/study/master_ai/year_2.txt', ['src/content/docs/master-ai/year-2/index.md', 'page']],
  ['pages/study/master_dsdm.txt', ['src/content/docs/master-dsdm/index.md', 'page']],
  ['pages/study/master_dsdm/year_1.txt', ['src/content/docs/master-dsdm/year-1/index.md', 'page']],
  ['pages/study/master_dsdm/year_2.txt', ['src/content/docs/master-dsdm/year-2/index.md', 'page']],
  ['pages/study/useful_information.txt', ['src/content/docs/useful-information/index.md', 'page']],
  ['pages/study/useful_information/pages/dke_locations.txt', ['src/content/docs/useful-information/dke-locations.md', 'page']],
  ['pages/study/useful_information/pages/handy_locations.txt', ['src/content/docs/useful-information/handy-locations.md', 'page']],
  ['pages/study/useful_information/pages/it_services.txt', ['src/content/docs/useful-information/it-services.md', 'page']],
  ['pages/study/useful_information/pages/laptop_buy_advice.txt', ['src/content/docs/useful-information/laptop-buying-advice.md', 'page']],
]);

function normalizeSeparators(path) {
  return path.replaceAll('\\', '/');
}

function hasParentTraversal(path) {
  return normalizeSeparators(path).split('/').includes('..');
}

function isContained(root, candidate) {
  const path = relative(root, candidate);
  return path === '' || (!isAbsolute(path) && path !== '..' && !path.startsWith(`..${sep}`));
}

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.');
    }
    const normalizedSource = normalizeSeparators(source);
    const normalizedDestination = normalizeSeparators(destination);
    const virtualProjectRoot = resolve(sep, 'migration-project');
    const virtualDocsRoot = resolve(virtualProjectRoot, 'src/content/docs');
    const resolvedDestination = resolve(virtualProjectRoot, normalizedDestination);

    if (normalizedSource.startsWith('pages/wiki/')) {
      throw new Error(`Generic DokuWiki source is excluded: ${source}`);
    }
    if (hasParentTraversal(source) || !(normalizedSource === 'pages/start.txt' || normalizedSource === 'pages/study.txt' || normalizedSource.startsWith('pages/study/'))) {
      throw new Error(`Source is outside the selected migration scope: ${source}`);
    }
    if (hasParentTraversal(destination) || !normalizedDestination.startsWith(DOCS_ROOT) || !isContained(virtualDocsRoot, resolvedDestination)) {
      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);
  }

  for (const { source, destination, mode } of manifest.entries) {
    const [canonicalDestination, canonicalMode] = CANONICAL_ENTRIES.get(source) ?? [];
    if (canonicalDestination !== destination || canonicalMode !== mode) {
      throw new Error(`Entry differs from the canonical migration contract: ${source} -> ${destination} (${mode}).`);
    }
  }

  return { entries: manifest.entries };
}

export async function checkDestinations(manifest, root) {
  const destinations = [...new Set(manifest.entries.map(({ destination }) => destination))];
  const missing = [];
  const notFiles = [];
  const projectRoot = resolve(root);
  const docsRoot = resolve(projectRoot, 'src/content/docs');

  for (const destination of destinations) {
    const candidate = resolve(projectRoot, normalizeSeparators(destination));
    if (!isContained(docsRoot, candidate)) {
      throw new Error(`Destination resolves outside ${DOCS_ROOT}: ${destination}`);
    }
    try {
      const details = await stat(candidate);
      if (!details.isFile()) notFiles.push(destination);
    } catch {
      missing.push(destination);
    }
  }

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