/**
 * Builds a proper UTC HttpDate timestamp from a Date object
 * since not all environments will have this as the expected
 * format.
 *
 * See: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/toUTCString
 * > Prior to ECMAScript 2018, the format of the return value
 * > varied according to the platform. The most common return
 * > value was an RFC-1123 formatted date stamp, which is a
 * > slightly updated version of RFC-822 date stamps.
 */

// Build indexes outside so we allocate them once.
const days: Array<String> = ["Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"];
// prettier-ignore
const months: Array<String> = ["Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"];

export function dateToUtcString(date: Date): string {
  const year = date.getUTCFullYear();
  const month = date.getUTCMonth();
  const dayOfWeek = date.getUTCDay();
  const dayOfMonthInt = date.getUTCDate();
  const hoursInt = date.getUTCHours();
  const minutesInt = date.getUTCMinutes();
  const secondsInt = date.getUTCSeconds();

  // Build 0 prefixed strings for contents that need to be
  // two digits and where we get an integer back.
  const dayOfMonthString = dayOfMonthInt < 10 ? `0${dayOfMonthInt}` : `${dayOfMonthInt}`;
  const hoursString = hoursInt < 10 ? `0${hoursInt}` : `${hoursInt}`;
  const minutesString = minutesInt < 10 ? `0${minutesInt}` : `${minutesInt}`;
  const secondsString = secondsInt < 10 ? `0${secondsInt}` : `${secondsInt}`;

  return `${days[dayOfWeek]}, ${dayOfMonthString} ${months[month]} ${year} ${hoursString}:${minutesString}:${secondsString} GMT`;
}