finanzen/convex/lib/amortization.test.ts

/// <reference types="vite/client" />

import { describe, expect, test } from "vitest";
import { buildSchedule, computePaymentFromTerm, paymentsMade } from "./amortization";

describe("amortization", () => {
  test("builds schedule without currentBalance from start", () => {
    const result = buildSchedule({
      principal: 10000,
      annualRate: 3.5,
      startDate: new Date("2024-01-15"),
      termMonths: 24,
    });

    expect(result.schedule).toHaveLength(24);
    expect(result.payment).toBeGreaterThan(0);
    expect(result.totalAmount).toBeGreaterThan(10000);
    expect(result.totalInterest).toBeGreaterThan(0);
    expect(result.paidMonths).toBe(0);
    expect(result.currentBalance).toBe(result.schedule[result.schedule.length - 1].balance);
  });

  test("computes monthly payment for a 60 month loan", () => {
    const payment = computePaymentFromTerm(10000, 3.5, 60);
    expect(payment).toBeCloseTo(181.92, 2);
  });

  test("calculates paid months from currentBalance", () => {
    const principal = 10000;
    const annualRate = 3.5;
    const termMonths = 60;
    const monthlyPayment = computePaymentFromTerm(principal, annualRate, termMonths);

    const paidMonths = paymentsMade(principal, annualRate, monthlyPayment, 8135.81);
    expect(paidMonths).toBe(12);
  });

  test("reconstructs plan from currentBalance and keeps total deviation small", () => {
    const principal = 10000;
    const annualRate = 3.5;
    const termMonths = 60;
    const monthlyPayment = computePaymentFromTerm(principal, annualRate, termMonths);

    const result = buildSchedule({
      principal,
      annualRate,
      startDate: new Date("2024-01-15"),
      monthlyPayment,
      currentBalance: 8135.81,
    });

    expect(result.paidMonths).toBe(12);
    expect(result.schedule[11].balance).toBeCloseTo(8135.81, 2);
    expect(result.schedule).toHaveLength(60);
    expect(result.totalAmount).toBeCloseTo(principal + result.totalInterest, 0);
    expect(result.totalAmount).toBeGreaterThanOrEqual(principal);
  });

  test("builds schedule from termMonths when monthlyPayment is omitted", () => {
    const result = buildSchedule({
      principal: 12000,
      annualRate: 4.5,
      startDate: new Date("2024-01-15"),
      termMonths: 48,
    });

    expect(result.payment).toBeGreaterThan(0);
    expect(result.schedule).toHaveLength(48);
    expect(result.totalInterest).toBeGreaterThan(0);
  });

  test("reconstructs plan from termMonths and currentBalance without monthlyPayment", () => {
    const principal = 12000;
    const annualRate = 4.5;
    const termMonths = 48;

    const result = buildSchedule({
      principal,
      annualRate,
      startDate: new Date("2024-01-15"),
      termMonths,
      currentBalance: 7634.12,
    });

    expect(result.payment).toBeGreaterThan(0);
    expect(result.paidMonths).toBeGreaterThan(0);
    expect(result.paidMonths).toBeLessThan(48);
    expect(result.schedule[result.paidMonths - 1].balance).toBeCloseTo(7634.12, 2);
    expect(result.schedule).toHaveLength(48);
  });

  test("throws when neither monthlyPayment nor termMonths is provided", () => {
    expect(() =>
      buildSchedule({
        principal: 10000,
        annualRate: 3.5,
        startDate: new Date("2024-01-15"),
      }),
    ).toThrow("Entweder monthlyPayment oder termMonths erforderlich");
  });

  test("handles zero interest schedule", () => {
    const result = buildSchedule({
      principal: 12000,
      annualRate: 0,
      startDate: new Date("2024-01-15"),
      monthlyPayment: 200,
    });

    expect(result.schedule).toHaveLength(60);
    expect(result.totalInterest).toBe(0);
    expect(result.totalAmount).toBe(12000);
  });

  test("returns realistic plan for user scenario with currentBalance", () => {
    const principal = 1860;
    const annualRate = 5.99;
    const termMonths = 24;
    const monthlyPayment = computePaymentFromTerm(principal, annualRate, termMonths);

    const result = buildSchedule({
      principal,
      annualRate,
      startDate: new Date("2024-01-01"),
      monthlyPayment,
      currentBalance: 775.25,
      asOf: new Date("2025-06-01"),
    });

    expect(result.paidMonths).toBeGreaterThan(0);
    expect(result.paidMonths).toBeLessThan(24);
    expect(result.schedule[result.paidMonths - 1].balance).toBeCloseTo(775.25, 2);
    expect(result.schedule).toHaveLength(24);
  });
});