impl/typescript.ts

/**
 * How fast an SLO's error budget is burning, in thousandths: the window's
 * error rate divided by the error rate the SLO allows. 1000 spends exactly
 * the budget over the SLO period; 14400 (14.4x) spends 2% of a 30-day budget
 * in one hour.
 *
 * Exact BigInt arithmetic: bad x 10000 x 1000 passes 2^53 at under a billion
 * bad events.
 */
export function burnRate(targetBasisPoints: number, totalEvents: number, badEvents: number): number {
  if (!Number.isSafeInteger(targetBasisPoints) || targetBasisPoints < 1 || targetBasisPoints > 9999) {
    throw new RangeError(`targetBasisPoints must be a whole number from 1 to 9999 (10000 leaves no error budget), received ${targetBasisPoints}`);
  }
  if (!Number.isSafeInteger(totalEvents) || totalEvents < 0) {
    throw new RangeError(`totalEvents must be a whole number of at least 0, received ${totalEvents}`);
  }
  if (!Number.isSafeInteger(badEvents) || badEvents < 0) {
    throw new RangeError(`badEvents must be a whole number of at least 0, received ${badEvents}`);
  }
  if (badEvents > totalEvents) throw new RangeError(`badEvents must not exceed totalEvents: ${badEvents} > ${totalEvents}`);
  // An empty window has no error rate; 0 keeps an alert quiet rather than failing it.
  if (totalEvents === 0) return 0;
  const n = BigInt(badEvents) * 10000n * 1000n;
  const d = BigInt(totalEvents) * BigInt(10000 - targetBasisPoints);
  return Number((2n * n + d) / (2n * d));
}