import * as fs from 'fs';
import * as path from 'path';
import { migrateV0ToV1 } from './migrators/v0_to_v1.js';

/**
 * Detected storage format version.
 * - v0: Legacy JSON format ({db}/{coll}.json files)
 * - v1: Bitcask binary format ({db}/{coll}/data.rdb directories)
 */
type TStorageVersion = 0 | 1;

/**
 * StorageMigrator — runs before the Rust engine starts.
 *
 * Detects the current storage format version and runs the appropriate
 * migration chain. The Rust engine only knows the current format (v1).
 *
 * Migration is safe: original files are never modified or deleted.
 * On success, a console hint is printed about which old files can be removed.
 */
export class StorageMigrator {
  private storagePath: string;

  constructor(storagePath: string) {
    this.storagePath = storagePath;
  }

  /**
   * Run any needed migrations. Safe to call even if storage is already current.
   */
  async run(): Promise<void> {
    if (!fs.existsSync(this.storagePath)) {
      return; // No data yet — nothing to migrate
    }

    const version = this.detectVersion();

    if (version === 1) {
      return; // Already current
    }

    if (version === 0) {
      console.log(`[smartdb] Detected v0 (JSON) storage format at ${this.storagePath}`);
      console.log(`[smartdb] Running migration v0 → v1 (Bitcask binary format)...`);

      const deletableFiles = await migrateV0ToV1(this.storagePath);

      if (deletableFiles.length > 0) {
        console.log(`[smartdb] Migration v0 → v1 complete.`);
        console.log(`[smartdb] The following old files can be safely deleted:`);
        for (const f of deletableFiles) {
          console.log(`[smartdb]   ${f}`);
        }
      } else {
        console.log(`[smartdb] Migration v0 → v1 complete. No old files to clean up.`);
      }
    }
  }

  /**
   * Detect the storage format version by inspecting the directory structure.
   *
   * v0: {db}/{coll}.json files exist
   * v1: {db}/{coll}/data.rdb directories exist
   */
  private detectVersion(): TStorageVersion {
    const entries = fs.readdirSync(this.storagePath, { withFileTypes: true });

    for (const entry of entries) {
      if (!entry.isDirectory()) continue;

      const dbDir = path.join(this.storagePath, entry.name);
      const dbEntries = fs.readdirSync(dbDir, { withFileTypes: true });

      for (const dbEntry of dbEntries) {
        // v1: subdirectory with data.rdb
        if (dbEntry.isDirectory()) {
          const dataRdb = path.join(dbDir, dbEntry.name, 'data.rdb');
          if (fs.existsSync(dataRdb)) {
            return 1;
          }
        }
        // v0: .json file (not .indexes.json)
        if (dbEntry.isFile() && dbEntry.name.endsWith('.json') && !dbEntry.name.endsWith('.indexes.json')) {
          return 0;
        }
      }
    }

    // Empty or unrecognized — treat as v1 (fresh start)
    return 1;
  }
}