/**
 * The article number where a group's posts reach `target` (ms since epoch),
 * by binary search between `first` and `last`. Article numbers only roughly
 * follow posting time, so the answer is within `precision` articles of the
 * first one posted at or after `target`.
 *
 * `dateOf(n)` returns the posting time of the first article at or after `n`
 * that still exists, or null when there is none nearby (expired or removed);
 * a gap counts as older than `target`.
 */
export async function articleAtDate(
  first: bigint,
  last: bigint,
  target: number,
  dateOf: (n: bigint) => Promise<number | null>,
  precision = 10_000n
): Promise<bigint> {
  let lo = first
  let hi = last
  while (hi - lo > precision) {
    const mid = lo + (hi - lo) / 2n
    const date = await dateOf(mid)
    if (date === null || date < target) lo = mid
    else hi = mid
  }
  return lo
}