Masuit.Tools/Masuit.Tools.Abstractions/Models/LoanModel.cs

#if NET5_0_OR_GREATER
using System.Collections.Generic;
using System;
using System.ComponentModel;
using System.Linq;
using Microsoft.VisualBasic;

namespace Masuit.Tools.Models;

/// <summary>
/// 贷款模型
/// </summary>
/// <param name="Loan">贷款本金</param>
/// <param name="Rate">贷款初始利率</param>
/// <param name="Start">开始还款日</param>
public record LoanModel(decimal Loan, decimal Rate, int Period, DateTime Start, LoanType LoanType = LoanType.EquivalentInterest)
{
    public Dictionary<DateTime, decimal?> RateAdjustments { get; set; } = new();

    public List<PrepaymentOption> Prepayments { get; set; } = new();

    private static decimal CumIPMT(decimal rate, decimal loan, int period)
    {
        double interest = 0;
        for (int i = 1; i <= period; i++)
        {
            interest += Financial.IPmt((double)(rate / 12), i, period, (double)loan);
        }

        return interest.ToDecimal(2);
    }

    /// <summary>
    /// 生成还款计划
    /// </summary>
    /// <returns></returns>
    public LoanResult Payment()
    {
        var result = LoanType == LoanType.EquivalentPrincipal ? PrepaymentPrincipal() : PrepaymentInterest();
        result.Plans[0].OriginRemainInterest=result.Plans[0].RemainInterest;
        for (var i = 1; i < result.Plans.Count; i++)
        {
            result.Plans[i].Period = i + 1;
            result.Plans[i].OriginRemainInterest = result.Plans[i-1].RemainInterest-result.Plans[i].Interest;
            if (result.Plans[i].LoanType != result.Plans[i - 1].LoanType)
            {
                result.Plans[i].Repayment = result.Plans[i - 1].Balance - result.Plans[i].Balance - result.Plans[i].Amount;
            }
        }

        return result;
    }

    private LoanResult PrepaymentInterest()
    {
        var list = new List<PaymentPlan>()
        {
            new()
            {
                Date = Start,
                LoanType = LoanType.EquivalentInterest
            }
        };
        var pmt = -Financial.Pmt((double)(Rate / 12), Period, (double)Loan);
        list[0].Rate = Rate;
        list[0].Period = 1;
        list[0].RemainPeriod = Period;
        list[0].Payment = pmt.ToDecimal(2);
        list[0].Interest = Math.Round(Loan * Rate / 12, 2, MidpointRounding.AwayFromZero);
        list[0].Amount = list[0].Payment - list[0].Interest;
        list[0].Balance = Loan - list[0].Amount;
        for (var i = 1; i < Period; i++)
        {
            var current = Start.AddMonths(i);
            var adj = RateAdjustments.FirstOrDefault(x => x.Key <= current && x.Key > current.AddMonths(-1));
            var newRate = adj.Value ?? list[i - 1].Rate;
            var prepayment = Prepayments.FirstOrDefault(x => x.Date <= current && x.Date > current.AddMonths(-1));
            if (prepayment?.ChangeType is LoanType.EquivalentPrincipal)
            {
                list.AddRange(new LoanModel(list[i - 1].Balance - prepayment.Amount, newRate, list[i - 1].RemainPeriod - 1, current, LoanType.EquivalentPrincipal)
                {
                    Prepayments = Prepayments,
                    RateAdjustments = RateAdjustments
                }.PrepaymentPrincipal().Plans);
                break;
            }

            list.Add(new PaymentPlan()
            {
                Period = i,
                Date = current,
                LoanType = LoanType.EquivalentInterest
            });
            list[i].Rate = newRate;
            list[i].Repayment = prepayment?.Amount ?? 0;
            if (Prepayments.FirstOrDefault(x => x.Date <= current.AddMonths(-1) && x.Date > current.AddMonths(-2))?.ReducePeriod == true)
            {
                var leftPeriod = (int)Math.Round(-Math.Log((double)(1 - (list[i - 1].Balance * list[i].Rate / 12) / list[i - 1].Payment)) / Math.Log((double)(1 + list[i].Rate / 12)));
                list[i].PeriodReduce = Period - list.Count + 1 - leftPeriod;
                list[i].RemainPeriod = leftPeriod;
            }
            else
            {
                list[i].RemainPeriod = list[i - 1].RemainPeriod - 1;
            }
            list[i].Payment = -Financial.Pmt((double)(list[i].Rate / 12), list[i].RemainPeriod, (double)list[i - 1].Balance).ToDecimal(2);
            if ((current - adj.Key).TotalDays > 0 && (current - adj.Key).TotalDays < 30)
            {
                var days = (decimal)(list[i].Date - list[i - 1].Date).TotalDays;
                list[i].Payment = list[i - 1].Payment / days * (decimal)Math.Abs((adj.Key - list[i - 1].Date).TotalDays) + list[i].Payment / days * (decimal)Math.Abs((current - adj.Key).TotalDays);
            }
            list[i].Interest = Math.Round(list[i - 1].Balance * list[i].Rate / 12, 2);
            list[i].Amount = Math.Round(list[i].Payment - list[i].Interest, 2);
            list[i].Balance = Math.Round(list[i - 1].Balance - list[i].Amount - list[i].Repayment, 2);
            if (list[i].Balance <= 0)
            {
                list[i].Payment += list[i].Balance;
                break;
            }
        }

        var totalInterest = -CumIPMT(Rate, Loan, Period);
        return new LoanResult(totalInterest, list);
    }

    private LoanResult PrepaymentPrincipal()
    {
        var list = new List<PaymentPlan>()
        {
            new()
            {
                Date = Start,
                LoanType = LoanType.EquivalentPrincipal,
                RemainPeriod = Period
            }
        };
        list[0].Rate = Rate;
        list[0].Period = 1;
        list[0].Interest = Math.Round(Loan * Rate / 12, 2, MidpointRounding.AwayFromZero);
        list[0].Amount = Math.Round(Loan / Period, 2, MidpointRounding.AwayFromZero);
        list[0].Payment = Math.Round(list[0].Amount + list[0].Interest, 2, MidpointRounding.AwayFromZero);
        list[0].Balance = Math.Round(Loan - list[0].Amount, 2, MidpointRounding.AwayFromZero);
        for (var i = 1; i < Period; i++)
        {
            var current = Start.AddMonths(i);
            var adj = RateAdjustments.FirstOrDefault(x => x.Key <= current && x.Key > current.AddMonths(-1));
            var newRate = adj.Value ?? list[i - 1].Rate;
            var prepayment = Prepayments.FirstOrDefault(x => x.Date <= current && x.Date > current.AddMonths(-1));
            if (prepayment?.ChangeType is LoanType.EquivalentInterest)
            {
                list.AddRange(new LoanModel(list[i - 1].Balance - prepayment.Amount, newRate, list[i - 1].RemainPeriod, current)
                {
                    Prepayments = Prepayments,
                    RateAdjustments = RateAdjustments
                }.PrepaymentInterest().Plans);
                break;
            }

            list.Add(new PaymentPlan()
            {
                Period = i,
                Date = current,
                LoanType = LoanType.EquivalentPrincipal
            });
            list[i].Rate = newRate;
            list[i].Repayment = prepayment?.Amount ?? 0;
            list[i].Interest = Math.Round(list[i - 1].Balance * list[i].Rate / 12, 2, MidpointRounding.AwayFromZero);
            if ((current - adj.Key).TotalDays > 0 && (current - adj.Key).TotalDays < 30)
            {
                var days = (decimal)(list[i].Date - list[i - 1].Date).TotalDays;
                list[i].Interest = list[i - 1].Interest / days * (decimal)Math.Abs((adj.Key - list[i - 1].Date).TotalDays) + list[i].Interest / days * (decimal)Math.Abs((current - adj.Key).TotalDays);
            }

            if (prepayment?.ReducePeriod == true)
            {
                list[i].PeriodReduce = (int)Math.Round(list[i].Repayment / (Loan / Period));
                list[i].RemainPeriod = list[i - 1].RemainPeriod - list[i].PeriodReduce - 1;
            }
            else
            {
                list[i].RemainPeriod = list[i - 1].RemainPeriod - 1;
            }

            list[i].Amount = Math.Round(list[i - 1].Balance / (Period - i - list.Sum(p => p.PeriodReduce)), 2, MidpointRounding.AwayFromZero);
            list[i].Payment = Math.Round(list[i].Amount + list[i].Interest, 2, MidpointRounding.AwayFromZero);
            list[i].Balance = Math.Round(list[i - 1].Balance - list[i].Amount - list[i].Repayment, 2, MidpointRounding.AwayFromZero);
            if (list[i].Balance <= 0)
            {
                list[i].Payment += list[i].Balance;
                break;
            }
        }

        var totalInterest = Loan * Rate / 12 * (Period + 1) / 2;
        return new LoanResult(totalInterest, list);
    }
}

/// <summary>
/// 贷款方式
/// </summary>
public enum LoanType
{
    /// <summary>
    /// 等额本息
    /// </summary>
    [Description("等额本息")]
    EquivalentInterest,

    /// <summary>
    /// 等额本金
    /// </summary>
    [Description("等额本金")]
    EquivalentPrincipal,
}

/// <summary>
/// 提前还款选项
/// </summary>
/// <param name="Date">提前还款时间</param>
/// <param name="Amount">提前还款金额</param>
/// <param name="ReducePeriod">是否减少期数</param>
/// <param name="ChangeType">新还款方式(若还款方式改变，不支持减少期数)</param>
public record PrepaymentOption(DateTime Date, decimal Amount, bool ReducePeriod = false, LoanType? ChangeType = null);

/// <summary>
/// 贷款结果
/// </summary>
/// <param name="TotalInterest">总利息</param>
/// <param name="Plans">还款计划</param>
public record LoanResult(decimal TotalInterest, List<PaymentPlan> Plans)
{
    /// <summary>
    /// 总提前还款额
    /// </summary>
    public decimal TotalRepayment => Plans.Sum(e => e.Repayment);

    /// <summary>
    /// 实际总利息
    /// </summary>
    public decimal ActualInterest => Plans.Sum(e => e.Interest);

    /// <summary>
    /// 实际还款总额
    /// </summary>
    public decimal ActualPayment => Plans.Sum(e => e.Payment + e.Repayment);

    /// <summary>
    /// 节省利息
    /// </summary>
    public decimal SavedInterest => TotalInterest - ActualInterest;

    public void Deconstruct(out decimal totalInterest, out decimal actualInterest, out decimal totalRepayment, out List<PaymentPlan> paymentPlans)
    {
        totalInterest = TotalInterest;
        actualInterest = ActualInterest;
        totalRepayment = TotalRepayment;
        paymentPlans = Plans;
    }

    public void Deconstruct(out decimal totalInterest, out decimal actualInterest, out decimal savedInterest, out decimal totalRepayment, out List<PaymentPlan> paymentPlans)
    {
        totalInterest = TotalInterest;
        actualInterest = ActualInterest;
        totalRepayment = TotalRepayment;
        paymentPlans = Plans;
        savedInterest = SavedInterest;
    }

    public void Deconstruct(out decimal totalInterest, out decimal actualInterest, out decimal savedInterest, out decimal totalRepayment, out decimal actualPayment, out List<PaymentPlan> paymentPlans)
    {
        totalInterest = TotalInterest;
        actualInterest = ActualInterest;
        totalRepayment = TotalRepayment;
        paymentPlans = Plans;
        savedInterest = SavedInterest;
        actualPayment = ActualPayment;
    }
}

public record PaymentPlan
{
    /// <summary>
    /// 期数
    /// </summary>
    public int Period { get; internal set; } = 12;

    /// <summary>
    /// 还款日
    /// </summary>
    public DateTime Date { get; internal set; } = DateTime.Now;

    /// <summary>
    /// 月供
    /// </summary>
    public decimal Payment { get; internal set; }

    /// <summary>
    /// 年利率
    /// </summary>
    public decimal Rate { get; internal set; }

    /// <summary>
    /// 月还利息
    /// </summary>
    public decimal Interest { get; internal set; }

    /// <summary>
    /// 月还本金
    /// </summary>
    public decimal Amount { get; internal set; }

    /// <summary>
    /// 当期提前还款额
    /// </summary>
    public decimal Repayment { get; internal set; }

    /// <summary>
    /// 当期剩余本金
    /// </summary>
    public decimal Balance { get; internal set; }

    /// <summary>
    /// 当期剩余利息
    /// </summary>
    public decimal RemainInterest => LoanType switch
    {
        LoanType.EquivalentInterest => Payment * (RemainPeriod-1) - Balance-Repayment,
        LoanType.EquivalentPrincipal => RemainPeriod * Interest - (RemainPeriod - 1) * RemainPeriod * Amount * (Rate / 12) / 2-Interest,
        _ => 0
    };

    /// <summary>
    /// 当期剩余利息(提前还款/利率调整前)
    /// </summary>
    public decimal OriginRemainInterest { get; internal set; }

    /// <summary>
    /// 贷款类型（默认等额本息）
    /// </summary>
    public LoanType LoanType { get; internal set; }

    /// <summary>
    /// 期数减少
    /// </summary>
    internal int PeriodReduce { get; set; }

    /// <summary>
    /// 剩余期数
    /// </summary>
    internal int RemainPeriod { get; set; }
}

#endif