export function fileExtension(pathOrName: string): string {
  const clean = pathOrName.split(/[?#]/, 1)[0] ?? ''
  const name = clean.split('/').pop() ?? clean
  const index = name.lastIndexOf('.')
  return index > 0 ? name.slice(index + 1).toLowerCase() : ''
}

export function sanitizeFileName(name: string, fallback = 'file'): string {
  const sanitized = name
    .trim()
    .replace(/[\\/:*?"<>|\u0000-\u001f]/g, '_')
    .replace(/\s+/g, ' ')
  return sanitized || fallback
}

/** Resolve an RFC 5987/6266 filename from a Content-Disposition header. */
export function parseContentDispositionFileName(header: string): string | null {
  const encoded = header.match(/filename\*=(?:[^']*'[^']*')?([^;\s]+)/i)?.[1]
  if (encoded) {
    try {
      return sanitizeFileName(decodeURIComponent(encoded.replace(/['"]/g, '')))
    } catch {
      // Fall through to the non-encoded forms.
    }
  }

  const quoted = header.match(/filename="([^"]+)"/i)?.[1]
  if (quoted?.trim()) return sanitizeFileName(quoted)

  const plain = header.match(/filename=([^;\s"]+)/i)?.[1]
  return plain?.trim() ? sanitizeFileName(plain) : null
}

export function fileNameFromUrl(url: string, fallback = 'download'): string {
  try {
    const pathname = new URL(url).pathname
    const candidate = pathname.split('/').filter(Boolean).at(-1)
    return candidate ? sanitizeFileName(decodeURIComponent(candidate), fallback) : fallback
  } catch {
    const candidate = url.split(/[?#]/, 1)[0]?.split('/').filter(Boolean).at(-1)
    return candidate ? sanitizeFileName(candidate, fallback) : fallback
  }
}