refactor: organize backend by business responsibility

src/modules/banking/bank-cards/domain/bank-card.calculations.spec.ts

@@ -0,0 +1,153 @@
+import {
+  bankBalance,
+  bankSummary,
+  isCalendarDate,
+  moscowToday,
+} from './bank-card.calculations';
+
+describe('bank card calculations', () => {
+  it('uses Moscow midnight for today and the current month', () => {
+    expect(moscowToday(new Date('2026-08-31T20:59:59.999Z'))).toBe(
+      '2026-08-31',
+    );
+    expect(moscowToday(new Date('2026-08-31T21:00:00.000Z'))).toBe(
+      '2026-09-01',
+    );
+  });
+
+  it.each([
+    '0001-01-01',
+    '9999-12-31',
+    '2024-02-29',
+    '2000-02-29',
+    '2026-09-12',
+  ])('accepts calendar date %s', (value) => {
+    expect(isCalendarDate(value)).toBe(true);
+  });
+
+  it.each([
+    '0000-01-01',
+    '10000-01-01',
+    '2026-02-29',
+    '1900-02-29',
+    '2026-04-31',
+    '2026-00-01',
+    '2026-13-01',
+    '2026-01-00',
+    '2026-01-32',
+    '2026-1-01',
+    '2026-09-12\n',
+    '2026-09-12T00:00:00Z',
+    '',
+  ])('rejects invalid calendar date %s', (value) => {
+    expect(isCalendarDate(value)).toBe(false);
+  });
+
+  it('folds expenses, topups, refunds and both transfer sides exactly', () => {
+    const operations = [
+      { kind: 'expense', cardId: 'a', targetCardId: null, amountKopecks: 10n },
+      { kind: 'expense', cardId: 'a', targetCardId: null, amountKopecks: 20n },
+      { kind: 'topup', cardId: 'a', targetCardId: null, amountKopecks: 15n },
+      { kind: 'refund', cardId: 'a', targetCardId: null, amountKopecks: 5n },
+      { kind: 'transfer', cardId: 'a', targetCardId: 'b', amountKopecks: 120n },
+      { kind: 'expense', cardId: 'c', targetCardId: null, amountKopecks: 300n },
+    ];
+    expect(bankBalance('a', 100n, operations.slice(0, 2))).toBe(70n);
+    expect(bankBalance('a', 100n, operations)).toBe(-30n);
+    expect(bankBalance('b', 0n, operations)).toBe(120n);
+    expect(bankBalance('unused', -1n, operations)).toBe(-1n);
+  });
+
+  it('does not bound computed balances by the storage range', () => {
+    expect(
+      bankBalance('a', 2n ** 63n - 1n, [
+        { kind: 'topup', cardId: 'a', targetCardId: null, amountKopecks: 1n },
+      ]),
+    ).toBe(2n ** 63n);
+    expect(
+      bankBalance('a', -(2n ** 63n), [
+        { kind: 'expense', cardId: 'a', targetCardId: null, amountKopecks: 1n },
+      ]),
+    ).toBe(-(2n ** 63n) - 1n);
+  });
+
+  it('counts refunds in their own month and inherited category, excluding transfers and topups', () => {
+    const operations = [
+      {
+        kind: 'expense',
+        date: '2026-08-31',
+        categoryId: 'food',
+        amountKopecks: 1000n,
+      },
+      {
+        kind: 'refund',
+        date: '2026-09-01',
+        categoryId: 'food',
+        amountKopecks: 500n,
+      },
+      {
+        kind: 'expense',
+        date: '2026-09-30',
+        categoryId: 'food',
+        amountKopecks: 100n,
+      },
+      {
+        kind: 'expense',
+        date: '2026-09-03',
+        categoryId: 'travel',
+        amountKopecks: 20n,
+      },
+      {
+        kind: 'transfer',
+        date: '2026-09-03',
+        categoryId: null,
+        amountKopecks: 2000n,
+      },
+      {
+        kind: 'topup',
+        date: '2026-09-03',
+        categoryId: null,
+        amountKopecks: 3000n,
+      },
+      {
+        kind: 'expense',
+        date: '2026-10-01',
+        categoryId: 'travel',
+        amountKopecks: 5000n,
+      },
+    ];
+    expect(bankSummary('2026-09', operations)).toEqual({
+      expenses: 120n,
+      refunds: 500n,
+      net: -380n,
+      categories: [
+        { categoryId: 'food', expenses: 100n, refunds: 500n, net: -400n },
+        { categoryId: 'travel', expenses: 20n, refunds: 0n, net: 20n },
+      ],
+    });
+    expect(bankSummary('2026-07', operations)).toEqual({
+      expenses: 0n,
+      refunds: 0n,
+      net: 0n,
+      categories: [],
+    });
+  });
+
+  it('keeps summary sums beyond signed 64-bit exact', () => {
+    const operation = {
+      kind: 'expense',
+      date: '2026-09-01',
+      categoryId: 'food',
+      amountKopecks: 2n ** 63n - 1n,
+    };
+    const total = 2n * (2n ** 63n - 1n);
+    expect(bankSummary('2026-09', [operation, operation])).toEqual({
+      expenses: total,
+      refunds: 0n,
+      net: total,
+      categories: [
+        { categoryId: 'food', expenses: total, refunds: 0n, net: total },
+      ],
+    });
+  });
+});

src/modules/banking/bank-cards/domain/bank-card.calculations.ts

@@ -0,0 +1,91 @@
+export function moscowToday(now = new Date()): string {
+  return new Intl.DateTimeFormat('en-CA', {
+    timeZone: 'Europe/Moscow',
+    year: 'numeric',
+    month: '2-digit',
+    day: '2-digit',
+  }).format(now);
+}
+
+export function isCalendarDate(value: string): boolean {
+  if (value.length !== 10 || !/^\d{4}-\d{2}-\d{2}$/.test(value)) return false;
+  if (value.startsWith('0000')) return false;
+  const date = new Date(`${value}T00:00:00.000Z`);
+  return (
+    !Number.isNaN(date.getTime()) && date.toISOString().slice(0, 10) === value
+  );
+}
+
+export function bankBalance(
+  cardId: string,
+  opening: bigint,
+  operations: Array<{
+    kind: string;
+    cardId: string;
+    targetCardId: string | null;
+    amountKopecks: bigint;
+  }>,
+): bigint {
+  // ponytail: O(N) history fold; optimize only after measuring growing histories.
+  let balance = opening;
+  for (const operation of operations) {
+    if (operation.cardId === cardId) {
+      if (operation.kind === 'expense' || operation.kind === 'transfer')
+        balance -= operation.amountKopecks;
+      else if (operation.kind === 'topup' || operation.kind === 'refund')
+        balance += operation.amountKopecks;
+    }
+    if (operation.kind === 'transfer' && operation.targetCardId === cardId)
+      balance += operation.amountKopecks;
+  }
+  return balance;
+}
+
+export function bankSummary(
+  period: string | { from: string; to: string },
+  operations: Array<{
+    kind: string;
+    date: string;
+    amountKopecks: bigint;
+    categoryId: string | null;
+  }>,
+) {
+  let expenses = 0n;
+  let refunds = 0n;
+  const categories = new Map<
+    string,
+    { categoryId: string; expenses: bigint; refunds: bigint; net: bigint }
+  >();
+  // ponytail: O(N) history fold; optimize only after measuring growing histories.
+  for (const operation of operations) {
+    if (
+      typeof period === 'string'
+        ? operation.date.slice(0, 7) !== period
+        : operation.date < period.from || operation.date > period.to
+    )
+      continue;
+    if (operation.kind !== 'expense' && operation.kind !== 'refund') continue;
+    const expense = operation.kind === 'expense' ? operation.amountKopecks : 0n;
+    const refund = operation.kind === 'refund' ? operation.amountKopecks : 0n;
+    expenses += expense;
+    refunds += refund;
+    if (operation.categoryId !== null) {
+      const category = categories.get(operation.categoryId) ?? {
+        categoryId: operation.categoryId,
+        expenses: 0n,
+        refunds: 0n,
+        net: 0n,
+      };
+      category.expenses += expense;
+      category.refunds += refund;
+      category.net = category.expenses - category.refunds;
+      categories.set(category.categoryId, category);
+    }
+  }
+  return {
+    expenses,
+    refunds,
+    net: expenses - refunds,
+    categories: [...categories.values()],
+  };
+}

src/modules/banking/bank-cards/domain/rub-money.spec.ts

@@ -0,0 +1,57 @@
+import { parseRubMoney, rubMoney } from './rub-money';
+
+describe('RUB money', () => {
+  it.each([
+    ['0', 0n],
+    ['-100', -10000n],
+    ['0.01', 1n],
+    ['123.4', 12340n],
+    ['90071992547409.93', 9007199254740993n],
+    ['92233720368547758.07', 2n ** 63n - 1n],
+    ['-92233720368547758.08', -(2n ** 63n)],
+  ])('roundtrips %s exactly', (value, kopecks) => {
+    expect(parseRubMoney({ currency: 'RUB', value })).toBe(kopecks);
+    expect(rubMoney(kopecks)).toEqual({ currency: 'RUB', value });
+  });
+
+  it.each([
+    '92233720368547758.08',
+    '-92233720368547758.09',
+    '1.001',
+    '1.000',
+    'NaN',
+    'Infinity',
+    '1e2',
+    '1,23',
+    '',
+    ' 1',
+    '1\n',
+    '01',
+  ])('rejects invalid input %s without rounding', (value) => {
+    expect(() => parseRubMoney({ currency: 'RUB', value })).toThrow();
+  });
+
+  it('rejects other currencies and non-string values', () => {
+    expect(() => parseRubMoney({ currency: 'USD', value: '1' })).toThrow();
+    expect(() =>
+      parseRubMoney({ currency: 'RUB', value: 1 as unknown as string }),
+    ).toThrow();
+  });
+
+  it('requires positive operation amounts, including at the maximum', () => {
+    for (const value of ['0', '-0.00', '-0.01'])
+      expect(() => parseRubMoney({ currency: 'RUB', value }, true)).toThrow();
+    expect(parseRubMoney({ currency: 'RUB', value: '0.01' }, true)).toBe(1n);
+    expect(
+      parseRubMoney({ currency: 'RUB', value: '92233720368547758.07' }, true),
+    ).toBe(2n ** 63n - 1n);
+  });
+
+  it('formats computed sums beyond the storage range without losing precision', () => {
+    expect(rubMoney(2n * (2n ** 63n - 1n))).toEqual({
+      currency: 'RUB',
+      value: '184467440737095516.14',
+    });
+    expect(rubMoney(-1n)).toEqual({ currency: 'RUB', value: '-0.01' });
+  });
+});

src/modules/banking/bank-cards/domain/rub-money.ts

@@ -0,0 +1,27 @@
+import { decimalFromString, formatDecimal } from '../../../../common/decimal';
+import type { Money } from '../../../../common/money.types';
+
+export function parseRubMoney(money: Money, positive = false): bigint {
+  if (
+    money?.currency !== 'RUB' ||
+    typeof money.value !== 'string' ||
+    money.value !== money.value.trim()
+  )
+    throw new TypeError('Expected RUB money with a decimal string');
+  const amount = decimalFromString(money.value);
+  if (amount.scale > 2)
+    throw new RangeError('RUB money supports at most two decimal places');
+  const kopecks = amount.value * 10n ** BigInt(2 - amount.scale);
+  if (kopecks < -(2n ** 63n) || kopecks > 2n ** 63n - 1n)
+    throw new RangeError('RUB money is outside the signed 64-bit range');
+  if (positive && kopecks <= 0n)
+    throw new RangeError('Operation amount must be positive');
+  return kopecks;
+}
+
+export function rubMoney(kopecks: bigint): Money {
+  return {
+    currency: 'RUB',
+    value: formatDecimal({ value: kopecks, scale: 2 }),
+  };
+}