// Load everything eagerly in a large app
@Module({
  imports: [
    UsersModule,
    OrdersModule,
    PaymentsModule,
    ReportsModule, // Heavy, rarely used
    AnalyticsModule, // Heavy, rarely used
    AdminModule, // Only admins use this
    LegacyModule, // Migration module, rarely used
    BulkImportModule, // Used once a month
  ],
})
export class AppModule {}

// All modules initialize at startup, even if never used
// Slow cold starts in serverless
// Memory wasted on unused modules

// Use LazyModuleLoader for optional modules
import { LazyModuleLoader } from '@nestjs/core';

@Injectable()
export class ReportsService {
  constructor(private lazyModuleLoader: LazyModuleLoader) {}

  async generateReport(type: string): Promise<Report> {
    // Load module only when needed
    const { ReportsModule } = await import('./reports/reports.module');
    const moduleRef = await this.lazyModuleLoader.load(() => ReportsModule);

    const reportsService = moduleRef.get(ReportsGeneratorService);
    return reportsService.generate(type);
  }
}

// Lazy load admin features with caching
@Injectable()
export class AdminService {
  private adminModule: ModuleRef | null = null;

  constructor(private lazyModuleLoader: LazyModuleLoader) {}

  private async getAdminModule(): Promise<ModuleRef> {
    if (!this.adminModule) {
      const { AdminModule } = await import('./admin/admin.module');
      this.adminModule = await this.lazyModuleLoader.load(() => AdminModule);
    }
    return this.adminModule;
  }

  async runAdminTask(task: string): Promise<void> {
    const moduleRef = await this.getAdminModule();
    const taskRunner = moduleRef.get(AdminTaskRunner);
    await taskRunner.run(task);
  }
}

// Reusable lazy loader service
@Injectable()
export class ModuleLoaderService {
  private loadedModules = new Map<string, ModuleRef>();

  constructor(private lazyModuleLoader: LazyModuleLoader) {}

  async load<T>(
    key: string,
    importFn: () => Promise<{ default: Type<T> } | Type<T>>,
  ): Promise<ModuleRef> {
    if (!this.loadedModules.has(key)) {
      const module = await importFn();
      const moduleType = 'default' in module ? module.default : module;
      const moduleRef = await this.lazyModuleLoader.load(() => moduleType);
      this.loadedModules.set(key, moduleRef);
    }
    return this.loadedModules.get(key)!;
  }
}

// Preload modules in background after startup
@Injectable()
export class ModulePreloader implements OnApplicationBootstrap {
  constructor(private lazyModuleLoader: LazyModuleLoader) {}

  async onApplicationBootstrap(): Promise<void> {
    setTimeout(async () => {
      await this.preloadModule(() => import('./reports/reports.module'));
    }, 5000); // 5 seconds after startup
  }

  private async preloadModule(importFn: () => Promise<any>): Promise<void> {
    try {
      const module = await importFn();
      const moduleType = module.default || Object.values(module)[0];
      await this.lazyModuleLoader.load(() => moduleType);
    } catch (error) {
      console.warn('Failed to preload module', error);
    }
  }
}