public async Task Validate()

in src/Modules/SimplCommerce.Module.Pricing/Services/CouponService.cs [26:140]
99 lines of code
29 McCabe index (conditional complexity)

        public async Task<CouponValidationResult> Validate(long customerId, string couponCode, CartInfoForCoupon cart)
        {
            var coupon = await _couponRepository.Query()
                .Include(x => x.CartRule).ThenInclude(c => c.Products)
                .Include(x => x.CartRule).ThenInclude(c => c.Categories)
                .FirstOrDefaultAsync(x => x.Code == couponCode);
            var validationResult = new CouponValidationResult { Succeeded = false };

            if(coupon == null || !coupon.CartRule.IsActive)
            {
                validationResult.ErrorMessage = $"The coupon {couponCode} is not exist.";
                return validationResult;
            }

            if (coupon.CartRule.StartOn.HasValue && coupon.CartRule.StartOn > DateTimeOffset.Now)
            {
                validationResult.ErrorMessage = $"The coupon {couponCode} should be used after {coupon.CartRule.StartOn}.";
                return validationResult;
            }

            if (coupon.CartRule.EndOn.HasValue && coupon.CartRule.EndOn <= DateTimeOffset.Now)
            {
                validationResult.ErrorMessage = $"The coupon {couponCode} is expired.";
                return validationResult;
            }

            var couponUsageCount = _cartRuleUsageRepository.Query().Count(x => x.CouponId == coupon.Id);
            if(coupon.CartRule.UsageLimitPerCoupon.HasValue && couponUsageCount >= coupon.CartRule.UsageLimitPerCoupon)
            {
                validationResult.ErrorMessage = $"The coupon {couponCode} is all used.";
                return validationResult;
            }

            var couponUsageByCustomerCount = _cartRuleUsageRepository.Query().Count(x => x.CouponId == coupon.Id && x.UserId == customerId);
            if (coupon.CartRule.UsageLimitPerCustomer.HasValue && couponUsageByCustomerCount >= coupon.CartRule.UsageLimitPerCustomer)
            {
                validationResult.ErrorMessage = $"You can use the coupon {couponCode} only {coupon.CartRule.UsageLimitPerCustomer} times";
                return validationResult;
            }

            IList<DiscountableProduct> discountableProducts = new List<DiscountableProduct>();
            if(!coupon.CartRule.Products.Any() && !coupon.CartRule.Categories.Any())
            {
                var productIds = cart.Items.Select(x => x.ProductId);
                discountableProducts = _productRepository.Query()
                   .Where(x => productIds.Contains(x.Id))
                   .Select(x => new DiscountableProduct { Id = x.Id, Name = x.Name, Price = x.Price }).ToList();
            }
            else
            {
                discountableProducts = GetDiscountableProduct(coupon.CartRule.Products, coupon.CartRule.Categories);
            }

            foreach (var item in cart.Items)
            {
                if ((coupon.CartRule.UsageLimitPerCoupon.HasValue && couponUsageCount >= coupon.CartRule.UsageLimitPerCoupon) ||
                    (coupon.CartRule.UsageLimitPerCustomer.HasValue && couponUsageByCustomerCount >= coupon.CartRule.UsageLimitPerCustomer))
                {
                    break;
                }

                var discountableProduct = discountableProducts.FirstOrDefault(x => x.Id == item.ProductId);
                if (discountableProduct != null)
                {
                    var discountedProduct = new DiscountedProduct { Id = discountableProduct.Id, Name = discountableProduct.Name, Price = discountableProduct.Price, Quantity = 1 };
                    couponUsageCount = couponUsageCount + 1;
                    couponUsageByCustomerCount = couponUsageByCustomerCount + 1;
                    for (var i = 1; i < item.Quantity; i++)
                    {
                        if ((coupon.CartRule.UsageLimitPerCoupon.HasValue && couponUsageCount >= coupon.CartRule.UsageLimitPerCoupon) ||
                            (coupon.CartRule.UsageLimitPerCustomer.HasValue && couponUsageByCustomerCount >= coupon.CartRule.UsageLimitPerCustomer))
                        {
                            break;
                        }

                        discountedProduct.Quantity = discountedProduct.Quantity + 1;
                        couponUsageCount = couponUsageCount + 1;
                        couponUsageByCustomerCount = couponUsageByCustomerCount + 1;
                    }

                    validationResult.DiscountedProducts.Add(discountedProduct);
                }
            }

            if (!validationResult.DiscountedProducts.Any())
            {
                validationResult.ErrorMessage = $"The coupon {couponCode} doesn't apply to any products in your cart";
                return validationResult;
            }

            validationResult.Succeeded = true;
            validationResult.CouponId = coupon.Id;
            validationResult.CouponCode = coupon.Code;
            validationResult.CouponRuleName = coupon.CartRule.Name;
            validationResult.CartRule = coupon.CartRule;

            switch (coupon.CartRule.RuleToApply)
            {
                case "cart_fixed":
                    validationResult.DiscountAmount = coupon.CartRule.DiscountAmount;
                    return validationResult;

                case "by_percent":
                    foreach(var item in validationResult.DiscountedProducts)
                    {
                        item.DiscountAmount = (item.Price * coupon.CartRule.DiscountAmount / 100) * item.Quantity;
                    }

                    validationResult.DiscountAmount = validationResult.DiscountedProducts.Sum(x => x.DiscountAmount);
                    return validationResult;

                default:
                    throw new InvalidOperationException($"{coupon.CartRule.RuleToApply} is not supported");
            }
        }