const MAX_HTML_BYTES = 512 * 1024

function metaContent(html: string, keys: string[]): string | null {
  for (const key of keys) {
    const escaped = key.replace(/[.*+?^${}()|[\]\\]/g, '\\$&')
    const before = new RegExp(`<meta[^>]+(?:property|name)=["']${escaped}["'][^>]+content=["']([^"']+)["']`, 'i')
    const after = new RegExp(`<meta[^>]+content=["']([^"']+)["'][^>]+(?:property|name)=["']${escaped}["']`, 'i')
    const match = before.exec(html) ?? after.exec(html)
    if (match?.[1]?.trim()) return match[1].trim()
  }
  return null
}

export interface LinkPreview {
  image: string | null
  title: string | null
}

const BLOCKED_HOST = /^(localhost|127\.|0\.0\.0\.0|::1|\[::1\]|169\.254\.|10\.|192\.168\.|172\.(1[6-9]|2\d|3[01])\.|.*\.(local|internal|lan)$)/i

export function isBlockedHost(hostname: string): boolean {
  return BLOCKED_HOST.test(hostname)
}

export async function fetchLinkPreview(url: string): Promise<LinkPreview> {
  try {
    const hostname = new URL(url).hostname
    if (isBlockedHost(hostname)) return { image: null, title: null }
    const res = await fetch(url, {
      signal: AbortSignal.timeout(5000),
      redirect: 'follow',
      headers: {
        'user-agent': 'Mozilla/5.0 (compatible; 4Brett-LinkPreview/1.0)',
        'accept': 'text/html,application/xhtml+xml',
      },
    })
    if (!res.ok) return { image: null, title: null }
    const contentType = res.headers.get('content-type') ?? ''
    if (!contentType.includes('text/html') && !contentType.includes('application/xhtml')) {
      return { image: null, title: null }
    }
    const html = (await res.text()).slice(0, MAX_HTML_BYTES)
    const base = res.url || url

    const image = metaContent(html, ['og:image', 'og:image:url', 'og:image:secure_url', 'twitter:image', 'twitter:image:src'])
    const title = metaContent(html, ['og:title', 'twitter:title'])
      ?? /<title[^>]*>([^<]+)<\/title>/i.exec(html)?.[1]?.trim()
      ?? null

    return {
      image: image ? new URL(image, base).href.slice(0, 2048) : null,
      title: title?.slice(0, 500) ?? null,
    }
  }
  catch {
    return { image: null, title: null }
  }
}