export type ApplyResult = {
    accepted: true;
    outstandingAfter: number;
} | {
    accepted: false;
    reason: 'over-application';
    remaining: number;
};
/**
 * outstanding = totalPaymentDue - sum(payments) - sum(credits).
 *
 * A credit reduces what is owed exactly as a payment does; the two differ in
 * provenance, not arithmetic, which is why they are summed together here.
 */
export declare function computeOutstanding(totalPaymentDue: number, payments: readonly number[], credits: readonly number[]): number;
/**
 * Validates one candidate payment or credit against what is still outstanding.
 *
 * Never clamps. A candidate that would take applied-to-date above the invoice
 * total is rejected outright and the caller reports it, rather than silently
 * recording a figure that does not reconcile. A non-positive candidate is
 * rejected for the same reason: it is not a payment.
 */
export declare function applyPayment(totalPaymentDue: number, payments: readonly number[], credits: readonly number[], candidate: number): ApplyResult;
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