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

import { BankOperationKind } from './bank-operation-kind';

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 === BankOperationKind.Expense ||
        operation.kind === BankOperationKind.Transfer
      )
        balance -= operation.amountKopecks;
      else if (
        operation.kind === BankOperationKind.Topup ||
        operation.kind === BankOperationKind.Refund
      )
        balance += operation.amountKopecks;
    }
    if (
      operation.kind === BankOperationKind.Transfer &&
      operation.targetCardId === cardId
    )
      balance += operation.amountKopecks;
  }
  return balance;
}

export function bankSummary(
  period: { 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 (operation.date < period.from || operation.date > period.to) continue;
    if (
      operation.kind !== BankOperationKind.Expense &&
      operation.kind !== BankOperationKind.Refund
    )
      continue;
    const expense =
      operation.kind === BankOperationKind.Expense
        ? operation.amountKopecks
        : 0n;
    const refund =
      operation.kind === BankOperationKind.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()],
  };
}