const UUID_REGEX =
  /^[0-9a-f]{8}-[0-9a-f]{4}-[1-5][0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12}$/i

export type ParticipantDisplayUser = {
  id?: string
  name?: string | null
  username?: string | null
}

export function isUuidLike(value: string): boolean {
  return UUID_REGEX.test(value.trim())
}

function isUsableDisplayValue(
  value: string | null | undefined,
  userId?: string
): value is string {
  const trimmed = value?.trim()
  if (!trimmed) return false
  if (userId && trimmed === userId) return false
  // Followers with no real name were historically provisioned in Stream Chat with a
  // placeholder name of the form `Follower <id>` (the string "Follower " + the user id).
  // Treat it as missing so surfaces fall back to their own label instead of leaking the
  // internal placeholder as a display name (MES-1077).
  if (userId && trimmed === `Follower ${userId}`) return false
  return !isUuidLike(trimmed)
}

/**
 * Resolves a human-readable participant label from Stream user fields.
 * Never falls back to user.id. UUID-like names are treated as missing.
 */
export function resolveParticipantDisplayName(
  user?: ParticipantDisplayUser | null
): string {
  if (isUsableDisplayValue(user?.name, user?.id)) {
    return user.name.trim()
  }

  if (isUsableDisplayValue(user?.username, user?.id)) {
    return user.username.trim()
  }

  return 'Unknown member'
}