/**
 * Shared payment/settlement math for invoices. Both the global
 * `BillingService.recordPayment` and the warehouse fee invoice flow apply a
 * payment the same way — accumulate `paidAmount`, derive the outstanding
 * `balanceAmount`, and decide whether the invoice is now fully settled. Keeping
 * it here means the two flows can never drift on rounding or the
 * partial-vs-full threshold.
 */

/** Round to 2 decimals, avoiding binary float drift. */
export const round2 = (n: number): number => Math.round(n * 100) / 100;

export interface SettlementResult {
  /** New cumulative amount paid. */
  paidAmount: number;
  /** Remaining balance (0 once fully paid). */
  balanceAmount: number;
  /** True once the balance reaches zero. */
  fullyPaid: boolean;
}

/**
 * Apply a single payment of `amount` to an invoice with `totalAmount` already
 * carrying `currentPaid`. Caller is responsible for validating `amount > 0` and
 * the invoice being in a payable state.
 */
export function applySettlement(
  totalAmount: number,
  currentPaid: number,
  amount: number,
): SettlementResult {
  const total = Number(totalAmount);
  const paidAmount = round2(Number(currentPaid) + Number(amount));
  const balanceAmount = Math.max(0, round2(total - paidAmount));
  return { paidAmount, balanceAmount, fullyPaid: paidAmount >= total };
}