/**
 * Config-Audit Rollback Engine
 * Restores configuration from backup with checksum verification.
 * Zero external dependencies.
 */

import { readFile, writeFile, readdir, stat, rm } from 'node:fs/promises';
import { join } from 'node:path';
import { getBackupDir, getLegacyBackupDir, parseManifest, checksum } from './lib/backup.mjs';

/**
 * Resolve a backup id to its directory, canonical root first, then the
 * pre-v2.2.0 root. Returns null when the id exists in neither.
 * @param {string} backupId
 * @returns {Promise<{ path: string, legacy: boolean } | null>}
 */
async function resolveBackupPath(backupId) {
  for (const [root, legacy] of [[getBackupDir(), false], [getLegacyBackupDir(), true]]) {
    const candidate = join(root, backupId);
    try {
      await stat(join(candidate, 'manifest.yaml'));
      return { path: candidate, legacy };
    } catch {
      // try the next root
    }
  }
  return null;
}

/**
 * List all available backups.
 * @returns {Promise<{ backups: object[] }>}
 */
export async function listBackups() {
  const backups = [];
  const seen = new Set();

  // Canonical root first; a legacy backup with the same id must not shadow it.
  for (const [backupRoot, legacy] of [[getBackupDir(), false], [getLegacyBackupDir(), true]]) {
    let entries;
    try {
      entries = await readdir(backupRoot, { withFileTypes: true });
    } catch {
      continue;
    }

    for (const entry of entries) {
      if (!entry.isDirectory() || seen.has(entry.name)) continue;

      const backupPath = join(backupRoot, entry.name);
      const manifestPath = join(backupPath, 'manifest.yaml');

      try {
        const manifestContent = await readFile(manifestPath, 'utf-8');
        const manifest = parseManifest(manifestContent);

        seen.add(entry.name);
        backups.push({
          id: entry.name,
          createdAt: manifest.created_at,
          legacy,
          files: manifest.files.map(f => ({
            originalPath: f.originalPath,
            backupPath: f.backupPath,
            checksum: f.checksum,
            sizeBytes: f.sizeBytes,
          })),
          created: manifest.created,
        });
      } catch {
        // Skip backups without valid manifest
        continue;
      }
    }
  }

  // Sort newest first
  backups.sort((a, b) => b.id.localeCompare(a.id));

  return { backups };
}

/**
 * Restore files from a backup.
 * @param {string} backupId
 * @param {object} [opts]
 * @param {boolean} [opts.dryRun=false]
 * @param {boolean} [opts.verify=true]
 * @returns {Promise<{ restored: object[], failed: object[] }>}
 */
export async function restoreBackup(backupId, opts = {}) {
  const verify = opts.verify !== false;
  const resolved = await resolveBackupPath(backupId);
  if (!resolved) throw new Error(`Backup not found: ${backupId}`);

  const backupPath = resolved.path;
  const manifestContent = await readFile(join(backupPath, 'manifest.yaml'), 'utf-8');

  const manifest = parseManifest(manifestContent);
  const restored = [];
  const failed = [];

  // A manifest with entries that parsed to nothing would restore nothing while
  // reporting success. Fail loudly instead.
  if (manifest.files.length === 0 && /^\s+-\s/m.test(manifestContent)) {
    throw new Error(`Unreadable manifest for backup ${backupId}: entries present but none parsed`);
  }

  for (const fileEntry of manifest.files) {
    const backupFilePath = join(backupPath, fileEntry.backupPath);

    if (opts.dryRun) {
      restored.push({
        originalPath: fileEntry.originalPath,
        status: 'dry-run',
      });
      continue;
    }

    try {
      // Read backup file
      const content = await readFile(backupFilePath);

      // Verify checksum before restoring
      if (verify) {
        const hash = checksum(content);
        if (hash !== fileEntry.checksum) {
          failed.push({
            originalPath: fileEntry.originalPath,
            status: 'checksum-mismatch',
            error: `Expected ${fileEntry.checksum}, got ${hash}`,
          });
          continue;
        }
      }

      // Write to original path
      await writeFile(fileEntry.originalPath, content);

      // Verify after write
      if (verify) {
        const written = await readFile(fileEntry.originalPath);
        const writtenHash = checksum(written);
        if (writtenHash !== fileEntry.checksum) {
          failed.push({
            originalPath: fileEntry.originalPath,
            status: 'checksum-mismatch',
            error: 'Checksum mismatch after write',
          });
          continue;
        }
      }

      restored.push({
        originalPath: fileEntry.originalPath,
        status: 'restored',
      });
    } catch (err) {
      failed.push({
        originalPath: fileEntry.originalPath,
        status: 'failed',
        error: err.message,
      });
    }
  }

  // Files implement CREATED are absent from the backup by definition, so they
  // survive the restore. Report them — a half-restored target is only dangerous
  // when it is also silent.
  return { restored, failed, createdNotRemoved: manifest.created, legacy: resolved.legacy };
}

/**
 * Delete a backup directory.
 * @param {string} backupId
 * @returns {Promise<{ deleted: boolean, error?: string }>}
 */
export async function deleteBackup(backupId) {
  const resolved = await resolveBackupPath(backupId);
  if (!resolved) return { deleted: false, error: `Backup not found: ${backupId}` };

  try {
    await rm(resolved.path, { recursive: true, force: true });
    return { deleted: true };
  } catch (err) {
    return { deleted: false, error: err.message };
  }
}