/** Helpers for displaying geo database keys. */

const displayNames = new Map<string, Intl.DisplayNames>()

function names(lang: string): Intl.DisplayNames | null {
  const locale = lang === 'zh' ? 'zh-CN' : 'en'
  if (!displayNames.has(locale)) {
    try {
      displayNames.set(locale, new Intl.DisplayNames([locale], { type: 'region' }))
    } catch {
      return null
    }
  }
  return displayNames.get(locale) ?? null
}

/**
 * Localized country name for a two letter code, or null when the code is not a
 * country. GeoIP databases also contain provider groups such as "cloudflare".
 */
export function regionName(code: string, lang: string): string | null {
  if (!/^[a-z]{2}$/i.test(code)) return null
  const upper = code.toUpperCase()
  try {
    const name = names(lang)?.of(upper)
    return name && name !== upper ? name : null
  } catch {
    return null
  }
}

/** Flag emoji for a two letter country code, empty for anything else. */
export function flagOf(code: string): string {
  if (!/^[a-z]{2}$/i.test(code)) return ''
  const base = 0x1f1e6
  const upper = code.toUpperCase()
  return String.fromCodePoint(
    base + upper.charCodeAt(0) - 65,
    base + upper.charCodeAt(1) - 65,
  )
}