/** ISO-shaped time-context timestamp formatting shared by production and replay validation. */

type TimestampPart = 'day' | 'hour' | 'minute' | 'month' | 'second' | 'timeZoneName' | 'year'

/**
 * Create the exact formatter used by durable time-context readings.
 * @param timeZone - Explicit display zone, or `undefined` for the process fallback.
 * @returns A formatter with stable numeric local fields and long numeric offset.
 */
export function createTimestampFormatter(timeZone?: string): Intl.DateTimeFormat {
  return new Intl.DateTimeFormat('en-US', {
    ...(timeZone === undefined ? {} : { timeZone }),
    year: 'numeric',
    month: '2-digit',
    day: '2-digit',
    hour: '2-digit',
    minute: '2-digit',
    second: '2-digit',
    hourCycle: 'h23',
    timeZoneName: 'longOffset',
  })
}

/**
 * Format an epoch millisecond value as an ISO-shaped timestamp with offset and IANA zone.
 * @param now - Epoch milliseconds to display.
 * @param formatter - Formatter created for `timeZone`.
 * @param timeZone - Canonical zone label carried in brackets.
 * @returns The durable timestamp text.
 */
export function formatTimestamp(now: number, formatter: Intl.DateTimeFormat, timeZone: string): string {
  const parts = Object.fromEntries(
    formatter.formatToParts(now).map(part => [part.type, part.value]),
  ) as Record<TimestampPart, string>
  const offset = parts.timeZoneName.replace(/^GMT$/, 'GMT+00:00').slice(3)
  return `${parts['year']}-${parts['month']}-${parts['day']}T${parts['hour']}:${parts['minute']}:${parts['second']}${offset}[${timeZone}]`
}