Build a test suite that catches regressions

import { describe, it, expect } from 'vitest';
import { computeLineTotalCents, applyTaxCents, computeInvoiceTotalCents, canTransitionStatus } from '@/lib/invoice-math';

describe('computeLineTotalCents', () => {
  it.each([
    { qty: 3, unitPriceCents: 333, expected: 999 },
    { qty: 7, unitPriceCents: 1, expected: 7 },
    { qty: 1, unitPriceCents: 999999, expected: 999999 },
  ])('$qty x $unitPriceCents cents = $expected cents', ({ qty, unitPriceCents, expected }) => {
    expect(computeLineTotalCents(qty, unitPriceCents)).toBe(expected);
  });
});

describe('applyTaxCents', () => {
  it.each([
    { subtotalCents: 10000, taxRateBps: 825, expected: 825 },
    { subtotalCents: 333, taxRateBps: 1000, expected: 33 },
    { subtotalCents: 1, taxRateBps: 50, expected: 0 },
  ])('rounds $subtotalCents cents at $taxRateBps bps to $expected cents', ({ subtotalCents, taxRateBps, expected }) => {
    expect(applyTaxCents(subtotalCents, taxRateBps)).toBe(expected);
  });
});

describe('computeInvoiceTotalCents', () => {
  it('sums line items and adds tax without floating point drift', () => {
    const items = [
      { qty: 3, unitPriceCents: 333 },
      { qty: 2, unitPriceCents: 150 },
    ];
    expect(computeInvoiceTotalCents(items, 825)).toBe(1406);
  });
});

describe('canTransitionStatus', () => {
  it.each([
    { from: 'draft', to: 'sent', allowed: true },
    { from: 'sent', to: 'paid', allowed: true },
    { from: 'sent', to: 'draft', allowed: false },
    { from: 'paid', to: 'sent', allowed: false },
    { from: 'draft', to: 'paid', allowed: false },
  ])('$from -> $to allowed: $allowed', ({ from, to, allowed }) => {
    expect(canTransitionStatus(from, to)).toBe(allowed);
  });
});