feat(new tool): implement amortization-calculator tool (#245)

This PR adds a new tool in the finance category called Amortization Calculator. It's used for calculating the amount of interest and principle that are a part of a payment to an amortized loan, like a car-loan or home-mortgage.

---------

Co-authored-by: Jacob Alford <github.scouting378@passmail.net>
Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>

Author: 3 people

locales/en.yml

@@ -1210,6 +1210,30 @@ tools:
       placeholder-put-your-full-prompt-here: Put your full prompt here...
       label-character-length-for-each-chunk: Character length for each chunk
       title-divided-prompts: Divided prompts
+  amortization-calculator:
+    title: Amortization Calculator
+    description: Calculate loan amortization schedules with monthly payment breakdowns
+    footer: Amortization Calculator
+    texts:
+      title-loan-parameters: Loan Parameters
+      title-overview: Overview
+      title-amortization-schedule: Amortization Schedule
+      label-loan-amount: Loan Amount
+      label-loan-term: Loan Term
+      label-interest-rate: Interest Rate
+      label-displayed-currency: Displayed Currency
+      label-end-of-year: "End of Year {year}"
+      label-year: Year
+      label-month: Month
+      label-interest: Interest
+      label-principal: Principal
+      label-ending-balance: Ending Balance
+      label-monthly-payment: Monthly Payment
+      label-total-of-payments: Total of Payments
+      label-total-interest: Total Interest
+      message-loan-amount-must-be-positive: Loan amount must be a positive value
+      message-loan-term-must-be-positive-integer: Loan term must be a positive integer
+      message-interest-rate-must-be-between-1-and-100: Interest rate must be a positive value between 0 and 100
   angle-converter:
     title: Angle Units Converter
     description: Convert values between angle units

src/tools/amortization-calculator/amortization-calculator.e2e.spec.ts

@@ -0,0 +1,192 @@
+import { expect, test } from '@playwright/test';
+
+test.describe('Tool - Amortization Calculator', () => {
+  test.beforeEach(async ({ page }) => {
+    await page.goto('/amortization-calculator');
+  });
+
+  test('Has correct title', async ({ page }) => {
+    await expect(page).toHaveTitle('Amortization Calculator - IT Tools');
+  });
+
+  test('Calculates amortization schedule with default values', async ({ page }) => {
+    const loanAmountInput = page.getByLabel('Loan Amount');
+    const loanTermInput = page.getByLabel('Loan Term');
+    const interestRateInput = page.getByLabel('Interest Rate');
+
+    await expect(loanAmountInput).toHaveValue('200000');
+    await expect(loanTermInput).toHaveValue('360');
+    await expect(interestRateInput).toHaveValue('6');
+
+    const monthlyPaymentCell = page.locator('tbody tr td').first();
+    await expect(monthlyPaymentCell).toContainText('$1,199.10');
+
+    const totalPaymentsCell = page.locator('tbody tr td').nth(1);
+    await expect(totalPaymentsCell).toContainText('$431,676.38');
+  });
+
+  test('Updates calculation when loan amount changes', async ({ page }) => {
+    const loanAmountInput = page.getByLabel('Loan Amount');
+
+    await loanAmountInput.clear();
+    await loanAmountInput.fill('100000');
+
+    const monthlyPaymentCell = page.locator('tbody tr td').first();
+    await expect(monthlyPaymentCell).toContainText('$599.55');
+
+    const totalPaymentsCell = page.locator('tbody tr td').nth(1);
+    await expect(totalPaymentsCell).toContainText('$215,838.19');
+  });
+
+  test('Updates calculation when loan term changes', async ({ page }) => {
+    const loanTermInput = page.getByLabel('Loan Term');
+
+    await loanTermInput.clear();
+    await loanTermInput.fill('180');
+
+    const monthlyPaymentCell = page.locator('tbody tr td').first();
+    await expect(monthlyPaymentCell).toContainText('$1,687.71');
+  });
+
+  test('Updates calculation when interest rate changes', async ({ page }) => {
+    const interestRateInput = page.getByLabel('Interest Rate');
+
+    await interestRateInput.clear();
+    await interestRateInput.fill('4.5');
+
+    const monthlyPaymentCell = page.locator('tbody tr td').first();
+    await expect(monthlyPaymentCell).toContainText('$1,013.37');
+  });
+
+  test('Displays amortization schedule table', async ({ page }) => {
+    const scheduleTable = page.locator('table').nth(1);
+
+    await expect(scheduleTable.locator('thead th').nth(0)).toContainText('Month');
+    await expect(scheduleTable.locator('thead th').nth(1)).toContainText('Interest');
+    await expect(scheduleTable.locator('thead th').nth(2)).toContainText('Principal');
+    await expect(scheduleTable.locator('thead th').nth(3)).toContainText('Ending Balance');
+
+    const firstMonthRow = scheduleTable.locator('tbody tr').first();
+    await expect(firstMonthRow.locator('td').nth(0)).toContainText('1');
+    await expect(firstMonthRow.locator('td').nth(1)).toContainText('$1,000.00');
+    await expect(firstMonthRow.locator('td').nth(2)).toContainText('$199.10');
+    await expect(firstMonthRow.locator('td').nth(3)).toContainText('$199,800.90');
+  });
+
+  test('Shows validation error for invalid loan amount', async ({ page }) => {
+    const loanAmountInput = page.getByLabel('Loan Amount');
+
+    await loanAmountInput.clear();
+    await loanAmountInput.fill('-1000');
+    await loanAmountInput.blur();
+
+    await expect(page.locator('text=Loan amount must be a positive value')).toBeVisible();
+  });
+
+  test('Shows validation error for zero loan amount', async ({ page }) => {
+    const loanAmountInput = page.getByLabel('Loan Amount');
+
+    await loanAmountInput.clear();
+    await loanAmountInput.fill('0');
+    await loanAmountInput.blur();
+
+    await expect(page.locator('text=Loan amount must be a positive value')).toBeVisible();
+  });
+
+  test('Shows validation error for non-integer loan term', async ({ page }) => {
+    const loanTermInput = page.getByLabel('Loan Term');
+
+    await loanTermInput.clear();
+    await loanTermInput.fill('12.5');
+    await loanTermInput.blur();
+
+    await expect(page.locator('text=Loan term must be a positive integer')).toBeVisible();
+  });
+
+  test('Shows validation error for negative loan term', async ({ page }) => {
+    const loanTermInput = page.getByLabel('Loan Term');
+
+    await loanTermInput.clear();
+    await loanTermInput.fill('-12');
+    await loanTermInput.blur();
+
+    await expect(page.locator('text=Loan term must be a positive integer')).toBeVisible();
+  });
+
+  test('Shows validation error for interest rate above 100', async ({ page }) => {
+    const interestRateInput = page.getByLabel('Interest Rate');
+
+    await interestRateInput.clear();
+    await interestRateInput.fill('101');
+    await interestRateInput.blur();
+
+    await expect(page.locator('text=Interest rate must be a positive value between 0 and 100')).toBeVisible();
+  });
+
+  test('Shows validation error for zero interest rate', async ({ page }) => {
+    const interestRateInput = page.getByLabel('Interest Rate');
+
+    await interestRateInput.clear();
+    await interestRateInput.fill('0');
+    await interestRateInput.blur();
+
+    await expect(page.locator('text=Interest rate must be a positive value between 0 and 100')).toBeVisible();
+  });
+
+  test('Hides results when inputs are invalid', async ({ page }) => {
+    const loanAmountInput = page.getByLabel('Loan Amount');
+
+    await loanAmountInput.clear();
+    await loanAmountInput.fill('invalid');
+
+    const resultsTable = page.locator('table').first();
+    await expect(resultsTable).not.toBeVisible();
+  });
+
+  test('Calculates correctly for short-term loan', async ({ page }) => {
+    const loanAmountInput = page.getByLabel('Loan Amount');
+    const loanTermInput = page.getByLabel('Loan Term');
+    const interestRateInput = page.getByLabel('Interest Rate');
+
+    await loanAmountInput.clear();
+    await loanAmountInput.fill('10000');
+    await loanTermInput.clear();
+    await loanTermInput.fill('12');
+    await interestRateInput.clear();
+    await interestRateInput.fill('12');
+
+    const monthlyPaymentCell = page.locator('tbody tr td').first();
+    await expect(monthlyPaymentCell).toContainText('$888.49');
+
+    const scheduleTable = page.locator('table').nth(1);
+    // Find the last month row (before the year summary row)
+    const lastMonthRow = scheduleTable.locator('tbody tr').nth(11); // 12th month (0-indexed)
+    await expect(lastMonthRow.locator('td').nth(0)).toContainText('12');
+    await expect(lastMonthRow.locator('td').nth(3)).toContainText('$0.00');
+  });
+
+  test('Schedule shows decreasing balance over time', async ({ page }) => {
+    const loanAmountInput = page.getByLabel('Loan Amount');
+    const loanTermInput = page.getByLabel('Loan Term');
+
+    await loanAmountInput.clear();
+    await loanAmountInput.fill('50000');
+    await loanTermInput.clear();
+    await loanTermInput.fill('12');
+
+    const scheduleTable = page.locator('table').nth(1);
+    const firstBalance = await scheduleTable.locator('tbody tr').nth(0).locator('td').nth(3).textContent();
+    const sixthBalance = await scheduleTable.locator('tbody tr').nth(5).locator('td').nth(3).textContent();
+    // Get the last month row (12th month, index 11) before the year summary row
+    const lastMonthBalance = await scheduleTable.locator('tbody tr').nth(11).locator('td').nth(3).textContent();
+
+    // The values may contain country code in e2e tests
+    const firstBalanceValue = Number.parseFloat(firstBalance!.replace(/[A-Za-z$,]/g, ''));
+    const sixthBalanceValue = Number.parseFloat(sixthBalance!.replace(/[A-Za-z$,]/g, ''));
+    const lastBalanceValue = Number.parseFloat(lastMonthBalance!.replace(/[A-Za-z$,]/g, ''));
+
+    expect(firstBalanceValue).toBeGreaterThan(sixthBalanceValue);
+    expect(sixthBalanceValue).toBeGreaterThan(lastBalanceValue);
+    expect(lastBalanceValue).toBeCloseTo(0, 0);
+  });
+});