feat: add manual bank card ledger

Store exact RUB balances, atomic transfers, linked refunds and versioned edits independently of T-Bank. Add migration, HTTP and durability verification.

src/common/decimal.ts

@@ -0,0 +1,62 @@
+export type Decimal = { value: bigint; scale: number };
+
+export function decimalFromString(value: string): Decimal {
+  if (!/^-?(?:0|[1-9]\d*)(?:\.\d+)?$/.test(value))
+    throw new Error('Invalid decimal string');
+  const negative = value.startsWith('-');
+  const [whole, fraction = ''] = (negative ? value.slice(1) : value).split('.');
+  return {
+    value: (negative ? -1n : 1n) * BigInt(`${whole}${fraction}`),
+    scale: fraction.length,
+  };
+}
+
+export function add(left: Decimal, right: Decimal): Decimal {
+  const scale = Math.max(left.scale, right.scale);
+  return {
+    value:
+      left.value * 10n ** BigInt(scale - left.scale) +
+      right.value * 10n ** BigInt(scale - right.scale),
+    scale,
+  };
+}
+
+export function subtract(left: Decimal, right: Decimal): Decimal {
+  return add(left, { value: -right.value, scale: right.scale });
+}
+
+export function multiply(left: Decimal, right: Decimal): Decimal {
+  return { value: left.value * right.value, scale: left.scale + right.scale };
+}
+
+export function isZero(value: Decimal): boolean {
+  return value.value === 0n;
+}
+
+export function compare(left: Decimal, right: Decimal): number {
+  const difference = subtract(left, right).value;
+  return difference === 0n ? 0 : difference > 0n ? 1 : -1;
+}
+
+export function formatDecimal({ value, scale }: Decimal): string {
+  if (value === 0n) return '0';
+  const sign = value < 0n ? '-' : '';
+  if (scale === 0) return `${sign}${value < 0n ? -value : value}`;
+  const digits = (value < 0n ? -value : value)
+    .toString()
+    .padStart(scale + 1, '0');
+  const whole = digits.slice(0, -scale) || '0';
+  const fraction = digits.slice(-scale).replace(/0+$/, '');
+  return `${sign}${whole}${fraction ? `.${fraction}` : ''}`;
+}
+
+// Round half away from zero; callers choose presentation precision explicitly.
+export function divide(left: Decimal, right: Decimal, scale: number): Decimal {
+  if (right.value === 0n) throw new Error('Division by zero');
+  const numerator = left.value * 10n ** BigInt(right.scale + scale);
+  const denominator = right.value * 10n ** BigInt(left.scale);
+  const sign = numerator < 0n !== denominator < 0n ? -1n : 1n;
+  const n = numerator < 0n ? -numerator : numerator;
+  const d = denominator < 0n ? -denominator : denominator;
+  return { value: sign * ((n + d / 2n) / d), scale };
+}