interface Migration {
  name: string;
  sql: string;
}

export class MigrationService {
  private static instance: MigrationService;
  private migrations: Migration[] = [];

  private constructor() {}

  static getInstance(): MigrationService {
    if (!MigrationService.instance) {
      MigrationService.instance = new MigrationService();
    }
    return MigrationService.instance;
  }

  registerMigration(migration: Migration) {
    this.migrations.push(migration);
  }

  /**
   * @param sqlExec - A function that executes a SQL statement and returns some update result
   * @param sqlQuery - A function that executes a SQL query and returns the result in some format
   * @param extractMigrationNames - A function that extracts the names (string array) from a "select name from migrations" query
   */
  async runMigrations<T>(
    // note that this does not take parameters because the Capacitor SQLite 'execute' is different
    sqlExec: (sql: string) => Promise<unknown>,
    sqlQuery: (sql: string) => Promise<T>,
    extractMigrationNames: (result: T) => Set<string>,
  ): Promise<void> {

    // Create migrations table if it doesn't exist
    const result0 = await sqlExec(`
      CREATE TABLE IF NOT EXISTS migrations (
        id INTEGER PRIMARY KEY AUTOINCREMENT,
        name TEXT NOT NULL UNIQUE,
        executed_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
      );
    `);

    // Get list of executed migrations
    const result1: T = await sqlQuery("SELECT name FROM migrations;");
    const executedMigrations = extractMigrationNames(result1);


    // Run pending migrations in order
    for (const migration of this.migrations) {
      if (!executedMigrations.has(migration.name)) {
        await sqlExec(migration.sql);

        await sqlExec(
          `INSERT INTO migrations (name) VALUES ('${migration.name}')`,
        );

      }
    }
  }
}

export default MigrationService.getInstance();