/**
 * Kilometre decimals for a distance: one below 10 km, none from there on.
 * The cut is taken on the tenth that prints, so 9.96 km reads 10 km, not 10.0.
 * @param {number} meters
 * @returns {0|1}
 */
export function kilometreDigits(meters) {
  return Math.round(meters / 100) < 100 ? 1 : 0;
}

/**
 * The unit and precision a distance prints with: metres below 1 km, then
 * kilometres with `kilometreDigits`. Metres round to `metreStep` (10 for tens),
 * except below `exactBelowM`, where they stay whole. Each cut is taken after
 * rounding, so 999.6 m reads 1.0 km rather than 1000 m.
 * @param {number} meters
 * @param {{metreStep?: number, exactBelowM?: number}} [options]
 * @returns {{value: number, unit: 'm'|'km', digits: 0|1}|null} null for a
 *   distance that is negative or not finite. The value is already rounded to
 *   `digits`, so toFixed and Intl render it the same way.
 */
export function distanceParts(meters, { metreStep = 1, exactBelowM = 0 } = {}) {
  if (!Number.isFinite(meters) || meters < 0) return null;
  const step = meters < exactBelowM ? 1 : metreStep;
  const shownMeters = Math.round(meters / step) * step;
  if (shownMeters < 1000) return { value: shownMeters, unit: 'm', digits: 0 };
  const digits = kilometreDigits(meters);
  const value =
    digits === 1 ? Math.round(meters / 100) / 10 : Math.round(meters / 1000);
  return { value, unit: 'km', digits };
}

/**
 * Plain `850 m` / `4.6 km` / `12 km` text for labels that are not localized.
 * @param {number} meters
 * @param {{metreStep?: number, exactBelowM?: number}} [options]
 * @returns {string|null}
 */
export function formatDistanceText(meters, options) {
  const parts = distanceParts(meters, options);
  if (!parts) return null;
  return `${parts.value.toFixed(parts.digits)} ${parts.unit}`;
}