type RelativeTimeInput = string | number | Date | null | undefined;

const relativeTimeFormatter = new Intl.RelativeTimeFormat("en", { numeric: "auto" });

const parseRelativeTime = (value: RelativeTimeInput) => {
  if (value === null || value === undefined) {
    return null;
  }
  const date = value instanceof Date ? value : new Date(value);
  const timestamp = date.getTime();
  if (Number.isNaN(timestamp)) {
    return null;
  }
  return timestamp;
};

export const formatRelativeTime = (value: RelativeTimeInput) => {
  const timestamp = parseRelativeTime(value);
  if (timestamp === null) {
    return null;
  }

  const diffSeconds = Math.round((timestamp - Date.now()) / 1000);
  const absSeconds = Math.abs(diffSeconds);
  if (absSeconds < 60) {
    return relativeTimeFormatter.format(diffSeconds, "second");
  }

  const diffMinutes = Math.round(diffSeconds / 60);
  if (Math.abs(diffMinutes) < 60) {
    return relativeTimeFormatter.format(diffMinutes, "minute");
  }

  const diffHours = Math.round(diffMinutes / 60);
  if (Math.abs(diffHours) < 24) {
    return relativeTimeFormatter.format(diffHours, "hour");
  }

  const diffDays = Math.round(diffHours / 24);
  if (Math.abs(diffDays) < 30) {
    return relativeTimeFormatter.format(diffDays, "day");
  }

  const diffMonths = Math.round(diffDays / 30);
  if (Math.abs(diffMonths) < 12) {
    return relativeTimeFormatter.format(diffMonths, "month");
  }

  const diffYears = Math.round(diffDays / 365);
  return relativeTimeFormatter.format(diffYears, "year");
};