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src/modules/cart/providers/cart-item.service.ts

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+import { Injectable, NotFoundException } from '@nestjs/common';
+import { Food } from '../../foods/entities/food.entity';
+import { Cart, CartItem } from '../interfaces/cart.interface';
+import { CartValidationService } from './cart-validation.service';
+import { CartCalculationService } from './cart-calculation.service';
+import { CartMessage } from 'src/common/enums/message.enum';
+
+@Injectable()
+export class CartItemService {
+  constructor(
+    private readonly validationService: CartValidationService,
+    private readonly calculationService: CartCalculationService,
+  ) {}
+
+  /**
+   * Create a new cart item from food
+   */
+  createCartItem(food: Food, quantity: number): CartItem {
+    const itemPrice = food.price || 0;
+    const itemDiscount = food.discount || 0;
+
+    return {
+      foodId: food.id,
+      foodTitle: food.title,
+      quantity,
+      price: itemPrice,
+      discount: itemDiscount,
+      totalPrice: this.calculationService.calculateItemTotalPrice(itemPrice, itemDiscount, quantity),
+    };
+  }
+
+  /**
+   * Build cart item from food (updating existing item if provided)
+   */
+  buildCartItemFromFood(food: Food, quantity: number, previous?: CartItem): CartItem {
+    const itemPrice = food.price || 0;
+    const itemDiscount = food.discount || 0;
+    return {
+      ...(previous ?? ({} as CartItem)),
+      foodId: food.id,
+      foodTitle: food.title,
+      quantity,
+      price: itemPrice,
+      discount: itemDiscount,
+      totalPrice: this.calculationService.calculateItemTotalPrice(itemPrice, itemDiscount, quantity),
+    };
+  }
+
+  /**
+   * Get item index in cart
+   */
+  getItemIndex(cart: Cart, foodId: string): number {
+    return cart.items.findIndex(item => item.foodId === foodId);
+  }
+
+  /**
+   * Add or increment item in cart
+   */
+  async addOrIncrementItem(cart: Cart, foodId: string, restaurantId: string, quantityToAdd: number): Promise<void> {
+    const food = await this.validationService.validateAndGetFood(foodId, restaurantId, quantityToAdd);
+    
+    // Validate meal type compatibility
+    this.validationService.assertMealTypeCompatibility(food);
+    
+    // Validate weekday compatibility
+    this.validationService.assertWeekdayCompatibility(food);
+    
+    const index = this.getItemIndex(cart, food.id);
+
+    if (index < 0) {
+      cart.items.push(this.createCartItem(food, quantityToAdd));
+      return;
+    }
+
+    const existingItem = cart.items[index];
+    const newQuantity = existingItem.quantity + quantityToAdd;
+    this.validationService.validateStock(food, newQuantity);
+    cart.items[index] = this.buildCartItemFromFood(food, newQuantity, existingItem);
+  }
+
+  /**
+   * Remove item from cart
+   */
+  removeItemOrFail(cart: Cart, foodId: string): void {
+    const itemIndex = this.getItemIndex(cart, foodId);
+    if (itemIndex < 0) {
+      throw new NotFoundException(CartMessage.ITEM_NOT_FOUND);
+    }
+    cart.items.splice(itemIndex, 1);
+  }
+
+  /**
+   * Decrement item quantity or remove if quantity reaches 0
+   */
+  async decrementOrRemoveItem(cart: Cart, foodId: string): Promise<void> {
+    const itemIndex = this.getItemIndex(cart, foodId);
+    if (itemIndex < 0) {
+      throw new NotFoundException(CartMessage.ITEM_NOT_FOUND);
+    }
+
+    const existingItem = cart.items[itemIndex];
+    const newQuantity = existingItem.quantity - 1;
+
+    if (newQuantity <= 0) {
+      cart.items.splice(itemIndex, 1);
+      return;
+    }
+
+    const food = await this.validationService.getFoodOrFail(foodId);
+    cart.items[itemIndex] = this.buildCartItemFromFood(food, newQuantity, existingItem);
+  }
+}
+