/**
 * Flag — renders a country flag image from an ISO 3166-1 alpha-2 code.
 * Images served from flagcdn.com (free, no auth required).
 */

const COUNTRY_NAMES: Record<string, string> = {
  RU: "Россия",
  DE: "Германия",
  NL: "Нидерланды",
  SG: "Сингапур",
  US: "США",
  GB: "Великобритания",
  FR: "Франция",
  FI: "Финляндия",
  SE: "Швеция",
  PL: "Польша",
  UA: "Украина",
  TR: "Турция",
  JP: "Япония",
  HK: "Гонконг",
}

let regionNames: Intl.DisplayNames | null | undefined

function regionDisplayName(iso: string): string | undefined {
  try {
    if (regionNames === undefined) {
      regionNames = typeof Intl !== "undefined" && "DisplayNames" in Intl
        ? new Intl.DisplayNames(["ru"], { type: "region" })
        : null
    }
    return regionNames?.of(iso) ?? undefined
  } catch {
    return undefined
  }
}

/** Country name in Russian (fallback to ISO code) */
export function countryName(code: string): string {
  const iso = code.toUpperCase()
  if (!iso) return code
  if (COUNTRY_NAMES[iso]) return COUNTRY_NAMES[iso]
  const intl = regionDisplayName(iso)
  if (intl && intl !== iso) return intl
  return iso
}

interface FlagProps {
  code: string
  /** px size — width of the flag image (height auto-scales 3:2 ratio) */
  size?: number
  className?: string
}

// flagcdn.com supports only these widths
const CDN_SIZES = [20, 40, 80, 160, 320, 640, 1280, 2560]

function nearestCdnSize(px: number): number {
  return CDN_SIZES.find(s => s >= px) ?? CDN_SIZES[CDN_SIZES.length - 1]
}

/** CDN URL for SVG `<image href>` (flagcdn widths only). */
export function flagCdnUrl(code: string, size = 40): string | null {
  const lower = code.toLowerCase()
  if (!/^[a-z]{2}$/.test(lower)) return null
  return `https://flagcdn.com/w${nearestCdnSize(size)}/${lower}.png`
}

/**
 * Renders a flag <img> for a given ISO 3166-1 alpha-2 country code.
 * Source: https://flagcdn.com — free CDN, no API key needed.
 */
export function Flag({ code, size = 20, className }: FlagProps) {
  if (!code) return null
  const lower  = code.toLowerCase()
  if (!/^[a-z]{2}$/.test(lower)) return null
  const name   = countryName(code.toUpperCase())
  const cdnSrc = nearestCdnSize(size)
  const cdnSrc2x = nearestCdnSize(size * 2)
  return (
    // Внешний CDN (динамический URL) — next/image без remotePatterns не подходит
    // eslint-disable-next-line @next/next/no-img-element -- flagcdn.com, размеры задаём явно
    <img
      src={`https://flagcdn.com/w${cdnSrc}/${lower}.png`}
      srcSet={`https://flagcdn.com/w${cdnSrc2x}/${lower}.png 2x`}
      width={size}
      height={Math.round(size * 0.75)}
      alt={name}
      title={name}
      className={className}
      style={{ display: "inline-block", verticalAlign: "middle", borderRadius: 2 }}
    />
  )
}