using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

using static TaxCalculatorInterviewTests.TaxCalculator;

​

//The focus should be on clean, simple and easy to read code 

//Everything but the public interface may be changed

namespace TaxCalculatorInterviewTests

{

    internal class Program

    {

        static void Main(string[] args)

        {

            TaxCalculator taxCalculator = new TaxCalculator();

            taxCalculator.SetCustomTaxRate(Commodity.Transport, 10);

        }

    }

    /// <summary>

    /// This is the public inteface used by our client and may not be changed

    /// </summary>

    public interface ITaxCalculator

    {

        double GetStandardTaxRate(Commodity commodity);

        void SetCustomTaxRate(Commodity commodity, double rate);

        double GetTaxRateForDateTime(Commodity commodity, DateTime date);

        double GetCurrentTaxRate(Commodity commodity);

    }

​

    /// <summary>

    /// Implements a tax calculator for our client.

    /// The calculator has a set of standard tax rates that are hard-coded in the class.

    /// It also allows our client to remotely set new, custom tax rates.

    /// Finally, it allows the fetching of tax rate information for a specific commodity and point in time.

    /// TODO: We know there are a few bugs in the code below, since the calculations look messed up every now and then.

    ///       There are also a number of things that have to be implemented.

    /// </summary>

    public class TaxCalculator : ITaxCalculator

    {

        /// <summary>

        /// Get the standard tax rate for a specific commodity.

        /// </summary>

        public double GetStandardTaxRate(Commodity commodity)

        {

            if (commodity == Commodity.Default)

                return 0.25;

            if (commodity == Commodity.Alcohol)

                return 0.25;

            if (commodity == Commodity.Food)

                return 0.12;

            if (commodity == Commodity.FoodServices)

                return 0.12;

            if (commodity == Commodity.Literature)

                return 0.06;

            if (commodity == Commodity.Transport)

                return 0.06;

            if (commodity == Commodity.CulturalServices)

                return 0.06;

            return 0.25;

        }

​

        private static readonly object _customRatesLock = new object();

        /// <summary>

        /// This method allows the client to remotely set new custom tax rates.

        /// When they do, we save the commodity/rate information as well as the UTC timestamp of when it was done.

        /// NOTE: Each instance of this object supports a different set of custom rates, since we run one thread per customer.

        /// </summary>

        public void SetCustomTaxRate(Commodity commodity, double rate)

        {

            //Lock makes sure only one thread can access at the time. 

            lock (_customRatesLock)

            {

                //TODO: support saving multiple custom rates for different combinations of Commodity/DateTime

                //TODO: make sure we never save duplicates, in case of e.g. clock resets, DST etc - overwrite old values if this happens

                _customRates[commodity] = Tuple.Create(DateTime.Now, rate);

            }

        }

​

        static Dictionary<Commodity, Tuple<DateTime, double>> _customRates = new Dictionary<Commodity, Tuple<DateTime, double>>();

        /// <summary>

        /// Gets the tax rate that is active for a specific point in time (in UTC).

        /// A custom tax rate is seen as the currently active rate for a period from its starting timestamp until a new custom rate is set.

        /// If there is no custom tax rate for the specified date, use the standard tax rate.

        /// </summary>

        public double GetTaxRateForDateTime(Commodity commodity, DateTime date)

        {

            lock (_customRatesLock)

            {

                // Check if there is a custom rate for the commodity

                if (_customRates.ContainsKey(commodity))

                {

                    var customRate = _customRates[commodity];

                    // Checks if the custom rate is valid for the specified date.

                    if (customRate.Item1 <= date)

                    {

                        return customRate.Item2;

                    }

                }

            }

​

            // No custom rate found or the custom rate is not valid for the specified date.

            return GetStandardTaxRate(commodity);

        }

​

        /// <summary>

        /// Gets the tax rate that is active for the current point in time.

        /// A custom tax rate is seen as the currently active rate for a period from its starting timestamp until a new custom rate is set.

        /// If there is no custom tax currently active, use the standard tax rate.

        /// </summary>

        public double GetCurrentTaxRate(Commodity commodity)

        {

            lock (_customRatesLock)

            {

                // Check if there is a custom rate for the commodity

                if (_customRates.ContainsKey(commodity))

                {

                    var customRate = _customRates[commodity];

                    // Check if the custom rate is valid for the current date and time

                    if (customRate.Item1 <= DateTime.UtcNow)

                    {

                        return customRate.Item2;

                    }

                }

​

                // No custom rate found or the custom rate is not valid for the current date and time

                return GetStandardTaxRate(commodity);

            }

        }

​

        public enum Commodity

        {

            //PLEASE NOTE: THESE ARE THE ACTUAL TAX RATES THAT SHOULD APPLY, WE JUST GOT THEM FROM THE CLIENT!

            Default, //25%

            Alcohol, //25%

            Food, //12%

            FoodServices, //12%

            Literature, //6%

            Transport, //6%

            CulturalServices //6%

        }

    }

}