Purrse/src/Purrse.Api/Services/LoanService.cs

using Microsoft.EntityFrameworkCore;
using Purrse.Core.DTOs;
using Purrse.Core.Interfaces.Services;
using Purrse.Core.Models;
using Purrse.Data;

namespace Purrse.Api.Services;

public class LoanService : ILoanService
{
    private readonly PurrseDbContext _db;

    public LoanService(PurrseDbContext db) => _db = db;

    public async Task<LoanDetailResponse?> GetLoanDetailAsync(Guid userId, Guid accountId)
    {
        var account = await _db.Accounts.Include(a => a.LoanDetail).ThenInclude(l => l!.AmortizationEntries)
            .FirstOrDefaultAsync(a => a.Id == accountId && a.UserId == userId);

        if (account?.LoanDetail == null) return null;

        var l = account.LoanDetail;
        var schedule = l.AmortizationEntries.OrderBy(e => e.PaymentNumber)
            .Select(e => new AmortizationEntryResponse(e.PaymentNumber, e.PaymentDate, e.PaymentAmount, e.PrincipalAmount, e.InterestAmount, e.EscrowAmount, e.RemainingBalance))
            .ToList();

        return new LoanDetailResponse(l.Id, l.AccountId, account.Name, l.OriginalBalance, account.Balance,
            l.InterestRate, l.TermMonths, l.MonthlyPayment, l.OriginationDate, l.MaturityDate,
            l.EscrowAmount, l.ExtraPayment, schedule);
    }

    public async Task<LoanDetailResponse> CreateLoanDetailAsync(Guid userId, Guid accountId, CreateLoanDetailRequest request)
    {
        var account = await _db.Accounts.FirstOrDefaultAsync(a => a.Id == accountId && a.UserId == userId)
            ?? throw new KeyNotFoundException("Account not found");

        var loanDetail = new LoanDetail
        {
            Id = Guid.NewGuid(),
            AccountId = accountId,
            OriginalBalance = request.OriginalBalance,
            InterestRate = request.InterestRate,
            TermMonths = request.TermMonths,
            MonthlyPayment = request.MonthlyPayment,
            OriginationDate = request.OriginationDate,
            MaturityDate = request.MaturityDate,
            EscrowAmount = request.EscrowAmount,
            ExtraPayment = request.ExtraPayment
        };

        var entries = GenerateAmortizationSchedule(loanDetail);
        loanDetail.AmortizationEntries = entries;

        _db.LoanDetails.Add(loanDetail);
        await _db.SaveChangesAsync();

        var schedule = entries.Select(e => new AmortizationEntryResponse(e.PaymentNumber, e.PaymentDate, e.PaymentAmount, e.PrincipalAmount, e.InterestAmount, e.EscrowAmount, e.RemainingBalance)).ToList();

        return new LoanDetailResponse(loanDetail.Id, accountId, account.Name, loanDetail.OriginalBalance,
            account.Balance, loanDetail.InterestRate, loanDetail.TermMonths, loanDetail.MonthlyPayment,
            loanDetail.OriginationDate, loanDetail.MaturityDate, loanDetail.EscrowAmount, loanDetail.ExtraPayment, schedule);
    }

    public async Task<LoanDetailResponse> UpdateLoanDetailAsync(Guid userId, Guid accountId, CreateLoanDetailRequest request)
    {
        var account = await _db.Accounts.Include(a => a.LoanDetail).ThenInclude(l => l!.AmortizationEntries)
            .FirstOrDefaultAsync(a => a.Id == accountId && a.UserId == userId)
            ?? throw new KeyNotFoundException("Account not found");

        var l = account.LoanDetail ?? throw new KeyNotFoundException("No loan detail found");

        _db.AmortizationEntries.RemoveRange(l.AmortizationEntries);

        l.OriginalBalance = request.OriginalBalance;
        l.InterestRate = request.InterestRate;
        l.TermMonths = request.TermMonths;
        l.MonthlyPayment = request.MonthlyPayment;
        l.OriginationDate = request.OriginationDate;
        l.MaturityDate = request.MaturityDate;
        l.EscrowAmount = request.EscrowAmount;
        l.ExtraPayment = request.ExtraPayment;

        var entries = GenerateAmortizationSchedule(l);
        l.AmortizationEntries = entries;
        await _db.SaveChangesAsync();

        var schedule = entries.Select(e => new AmortizationEntryResponse(e.PaymentNumber, e.PaymentDate, e.PaymentAmount, e.PrincipalAmount, e.InterestAmount, e.EscrowAmount, e.RemainingBalance)).ToList();

        return new LoanDetailResponse(l.Id, accountId, account.Name, l.OriginalBalance, account.Balance,
            l.InterestRate, l.TermMonths, l.MonthlyPayment, l.OriginationDate, l.MaturityDate,
            l.EscrowAmount, l.ExtraPayment, schedule);
    }

    public async Task<List<AmortizationEntryResponse>> GetAmortizationScheduleAsync(Guid userId, Guid accountId)
    {
        var detail = await GetLoanDetailAsync(userId, accountId)
            ?? throw new KeyNotFoundException("Loan detail not found");
        return detail.AmortizationSchedule;
    }

    public async Task<PayoffScenarioResponse> CalculatePayoffScenarioAsync(Guid userId, Guid accountId, PayoffScenarioRequest request)
    {
        var account = await _db.Accounts.Include(a => a.LoanDetail).ThenInclude(l => l!.AmortizationEntries)
            .FirstOrDefaultAsync(a => a.Id == accountId && a.UserId == userId)
            ?? throw new KeyNotFoundException("Account not found");

        var l = account.LoanDetail ?? throw new KeyNotFoundException("No loan detail found");

        var originalSchedule = l.AmortizationEntries.OrderBy(e => e.PaymentNumber).ToList();
        var totalInterestOriginal = originalSchedule.Sum(e => e.InterestAmount);
        var originalPayoffDate = originalSchedule.Last().PaymentDate;

        // Build new schedule with extra payments
        var balance = Math.Abs(account.Balance);
        var monthlyRate = l.InterestRate / 100 / 12;
        var extraMonthly = request.ExtraMonthlyPayment ?? 0;
        var basePayment = l.MonthlyPayment - (l.EscrowAmount ?? 0);
        var now = DateTime.UtcNow;
        var paymentDate = new DateTime(now.Year, now.Month, l.OriginationDate.Day, 0, 0, 0, DateTimeKind.Utc);

        var newEntries = new List<AmortizationEntryResponse>();
        int paymentNum = 0;

        // Apply lump sum if specified
        if (request.LumpSumPayment.HasValue && request.LumpSumDate.HasValue)
        {
            balance -= request.LumpSumPayment.Value;
            if (balance < 0) balance = 0;
        }

        while (balance > 0 && paymentNum < 600)
        {
            paymentNum++;
            paymentDate = paymentDate.AddMonths(1);

            var interest = balance * monthlyRate;
            var principal = Math.Min(basePayment + extraMonthly - interest, balance);
            if (principal < 0) principal = 0;

            balance -= principal;
            if (balance < 0.01m) balance = 0;

            newEntries.Add(new AmortizationEntryResponse(
                paymentNum, paymentDate, principal + interest + (l.EscrowAmount ?? 0),
                principal, interest, l.EscrowAmount, balance));
        }

        var totalInterestNew = newEntries.Sum(e => e.InterestAmount);
        var newPayoffDate = newEntries.LastOrDefault()?.PaymentDate ?? originalPayoffDate;
        var monthsSaved = ((originalPayoffDate.Year - newPayoffDate.Year) * 12) + (originalPayoffDate.Month - newPayoffDate.Month);

        return new PayoffScenarioResponse(
            originalPayoffDate, newPayoffDate, totalInterestOriginal, totalInterestNew,
            totalInterestOriginal - totalInterestNew, monthsSaved, newEntries);
    }

    private static List<AmortizationEntry> GenerateAmortizationSchedule(LoanDetail loan)
    {
        var entries = new List<AmortizationEntry>();
        var balance = loan.OriginalBalance;
        var monthlyRate = loan.InterestRate / 100 / 12;
        var payment = loan.MonthlyPayment - (loan.EscrowAmount ?? 0);
        var extra = loan.ExtraPayment ?? 0;
        var paymentDate = loan.OriginationDate;

        for (int i = 1; i <= loan.TermMonths && balance > 0; i++)
        {
            paymentDate = paymentDate.AddMonths(1);
            var interest = balance * monthlyRate;
            var principal = Math.Min(payment + extra - interest, balance);
            balance -= principal;
            if (balance < 0.01m) balance = 0;

            entries.Add(new AmortizationEntry
            {
                Id = Guid.NewGuid(),
                PaymentNumber = i,
                PaymentDate = paymentDate,
                PaymentAmount = principal + interest + (loan.EscrowAmount ?? 0),
                PrincipalAmount = principal,
                InterestAmount = interest,
                EscrowAmount = loan.EscrowAmount,
                RemainingBalance = balance
            });

            if (balance == 0) break;
        }

        return entries;
    }
}