using System;
using System.Collections.Generic;
using System.Linq;
using Pos.Models;

namespace Pos.Service
{
    /// <summary>
    /// Holder styr på den igangværende handel og dens beregninger.
    /// Servicen har ingen afhængighed til WPF eller web.
    /// </summary>
    public class SalesTransactionService
    {
        private readonly List<PriceLineModel> _saleLines = new List<PriceLineModel>();
        private readonly List<AmountGridModel> _payments = new List<AmountGridModel>();

        public IReadOnlyList<PriceLineModel> SaleLines => _saleLines;
        public IReadOnlyList<AmountGridModel> Payments => _payments;

        public decimal TotalPrice =>
            _saleLines.Sum(line =>
                CalculateLineTotal(line.Price, line.NumberProducts));

        public decimal AmountPaid =>
            _payments.Sum(payment => payment.Amount);

        public decimal RemainingAmount =>
            Math.Max(TotalPrice - AmountPaid, 0);

        public decimal Change =>
            Math.Max(AmountPaid - TotalPrice, 0);

        public bool IsFullyPaid =>
            TotalPrice > 0 && AmountPaid >= TotalPrice;

        /// <summary>
        /// Beregner totalen for en enkelt salgslinje.
        /// Parsing og visning af resultatet er fortsat UI-ansvar.
        /// </summary>
        public decimal CalculateLineTotal(decimal price, int numberOfProducts)
        {
            ValidatePriceAndQuantity(price, numberOfProducts);
            return price * numberOfProducts;
        }

        public PriceLineModel AddSaleLine(
            int id,
            string name,
            int numberOfProducts,
            decimal price,
            bool isProductGroup)
        {
            if (id <= 0)
                throw new ArgumentOutOfRangeException(
                    nameof(id),
                    "Id skal være større end 0.");

            if (string.IsNullOrWhiteSpace(name))
                throw new ArgumentException(
                    "Navn skal være udfyldt.",
                    nameof(name));

            ValidatePriceAndQuantity(price, numberOfProducts);

            PriceLineModel saleLine = new PriceLineModel
            {
                Id = id,
                Name = name,
                NumberProducts = numberOfProducts,
                Price = price,
                IsProductGroup = isProductGroup
            };

            _saleLines.Add(saleLine);
            return saleLine;
        }

        public void RemoveSaleLineAt(int index)
        {
            if (index < 0 || index >= _saleLines.Count)
                throw new ArgumentOutOfRangeException(nameof(index));

            _saleLines.RemoveAt(index);
        }

        /// <summary>
        /// Registrerer en betaling. Hvis amount er null, betales restbeløbet.
        /// </summary>
        public AmountGridModel AddPayment(
            PaymentMethod paymentMethod,
            decimal? amount = null)
        {
            if (_saleLines.Count == 0)
                throw new InvalidOperationException(
                    "Der kan ikke registreres betaling på et tomt salg.");

            decimal paymentAmount = amount ?? RemainingAmount;

            if (paymentAmount <= 0)
                throw new ArgumentOutOfRangeException(
                    nameof(amount),
                    "Betalingsbeløbet skal være større end 0.");

            AmountGridModel payment = new AmountGridModel
            {
                Amount = paymentAmount,
                PaymentMethodText = GetPaymentMethodText(paymentMethod)
            };

            _payments.Add(payment);
            return payment;
        }

        public void RemovePaymentAt(int index)
        {
            if (index < 0 || index >= _payments.Count)
                throw new ArgumentOutOfRangeException(nameof(index));

            _payments.RemoveAt(index);
        }

        public void EnsureCanComplete()
        {
            if (_saleLines.Count == 0)
                throw new InvalidOperationException(
                    "Et tomt salg kan ikke afsluttes.");

            if (!IsFullyPaid)
                throw new InvalidOperationException(
                    "Salget er ikke fuldt betalt.");
        }

        public void Reset()
        {
            _saleLines.Clear();
            _payments.Clear();
        }

        private static void ValidatePriceAndQuantity(
            decimal price,
            int numberOfProducts)
        {
            if (price < 0)
                throw new ArgumentOutOfRangeException(
                    nameof(price),
                    "Prisen må ikke være negativ.");

            if (numberOfProducts <= 0)
                throw new ArgumentOutOfRangeException(
                    nameof(numberOfProducts),
                    "Antal skal være større end 0.");
        }

        private static string GetPaymentMethodText(PaymentMethod paymentMethod)
        {
            switch (paymentMethod)
            {
                case PaymentMethod.Card:
                    return "Kort";
                case PaymentMethod.Cash:
                    return "Kontant";
                case PaymentMethod.GiftCard:
                    return "Gavekort";
                case PaymentMethod.MobilePay:
                    return "MobilePay";
                default:
                    throw new ArgumentOutOfRangeException(nameof(paymentMethod));
            }
        }
    }
}