Core_Be/Domain/DiscountCalculator.cs

namespace Fitness_core_BE.Domain
{
    public static class DiscountCalculator
    {
        public static decimal ComputeDiscountAmount(decimal subtotal, string? discountType, decimal? discountValue)
        {
            if (string.IsNullOrEmpty(discountType) || discountValue is not decimal value || value <= 0 || subtotal <= 0)
                return 0m;

            var discount = discountType switch
            {
                "Percentage" => subtotal * Math.Clamp(value, 0m, 100m) / 100m,
                "Flat" => Math.Clamp(value, 0m, subtotal),
                _ => 0m
            };
            return Math.Round(discount, 2);
        }
    }
}