設計模式：樣板方法模式 (Template Method)

abstract class ReportGenerator {
  // 樣板方法：固定的流程
  generate(): string {
    const data = this.fetchData();
    const processed = this.processData(data);
    const header = this.renderHeader();
    const body = this.renderBody(processed);
    const footer = this.renderFooter();
    return [header, body, footer].join('\n');
  }

  // 抽象步驟：由子類別實作
  protected abstract fetchData(): Record[];
  protected abstract renderBody(data: Record[]): string;

  // 預設實作，子類別可覆寫
  protected processData(data: Record[]): Record[] {
    return data;
  }

  protected renderHeader(): string {
    return '=== Report ===';
  }

  protected renderFooter(): string {
    return `Generated at ${new Date().toISOString()}`;
  }
}

// 銷售報表
class SalesReport extends ReportGenerator {
  protected fetchData(): Record[] {
    // 實際實作會從 API 或資料庫取得資料
    return [
      { product: 1, revenue: 12000 },
      { product: 2, revenue: 8500 },
    ];
  }

  protected processData(data: Record[]): Record[] {
    // 額外處理：依營收由高到低排序
    return [...data].sort((a, b) => b['revenue'] - a['revenue']);
  }

  protected renderBody(data: Record[]): string {
    return data.map(row => `Product ${row['product']}: $${row['revenue']}`).join('\n');
  }

  protected renderHeader(): string {
    return '=== Sales Report ===';
  }
}

// 庫存報表
class InventoryReport extends ReportGenerator {
  protected fetchData(): Record[] {
    return [
      { sku: 1001, stock: 45 },
      { sku: 1002, stock: 3 },
    ];
  }

  protected renderBody(data: Record[]): string {
    return data
      .map(row => {
        const warning = row['stock'] < 10 ? ' [低庫存警告]' : '';
        return `SKU ${row['sku']}: ${row['stock']} 件${warning}`;
      })
      .join('\n');
  }
}

// 使用
const salesReport = new SalesReport();
console.log(salesReport.generate());

const inventoryReport = new InventoryReport();
console.log(inventoryReport.generate());