import { Injectable } from '@nestjs/common';
import { lookup } from 'dns/promises';
import { isIP } from 'net';

/**
 * Server-side favicon / icon discovery with SSRF protection (T-08-05).
 *
 * Ported from personal-dashboard/src/lib/favorite-icons.ts.
 * Security guards:
 * - DNS resolves every URL (including redirects) and checks for private IP ranges
 * - redirect: 'manual' — follows redirects manually so each hop is re-validated
 * - 4000 ms AbortController timeout per request
 * - 200 000 character HTML cap to prevent memory exhaustion (T-08-09)
 * - Blocked hostnames: localhost, .local, 0.0.0.0
 */

const FALLBACK_ICON_PATH = '/favicon.ico';
const HTML_FETCH_TIMEOUT_MS = 4000;
const ICON_FETCH_TIMEOUT_MS = 4000;
const MAX_REDIRECTS = 2;
const MAX_HTML_CHARS = 200000;
const MAX_ICON_BYTES = 1_000_000;

type FetchHtmlResult = {
  html: string;
  finalUrl: string;
};

function isPrivateIpv4(address: string): boolean {
  const parts = address.split('.').map((part) => Number.parseInt(part, 10));

  if (
    parts.length !== 4 ||
    parts.some(
      (part) => !Number.isInteger(part) || part < 0 || part > 255,
    )
  ) {
    return true;
  }

  const [a, b] = parts;

  return (
    a === 0 ||
    a === 10 ||
    a === 127 ||
    (a === 100 && b !== undefined && b >= 64 && b <= 127) ||
    (a === 169 && b === 254) ||
    (a === 172 && b !== undefined && b >= 16 && b <= 31) ||
    (a === 192 && b === 168) ||
    (a === 192 && b === 0) ||
    (a === 198 && (b === 18 || b === 19)) ||
    a >= 224
  );
}

function isPrivateIpv6(address: string): boolean {
  const lower = address.toLowerCase();

  if (
    lower === '::' ||
    lower === '::1' ||
    lower.startsWith('fc') ||
    lower.startsWith('fd') ||
    lower.startsWith('fe80:') ||
    lower.startsWith('ff')
  ) {
    return true;
  }

  // IPv4-mapped IPv6 (::ffff:<ipv4>) — delegate to isPrivateIpv4 to cover all
  // RFC 1918 ranges (10.x, 172.16-31.x, 192.168.x) and 169.254.x link-local
  const v4MappedMatch = lower.match(/^::ffff:(\d+\.\d+\.\d+\.\d+)$/);
  if (v4MappedMatch) {
    return isPrivateIpv4(v4MappedMatch[1]);
  }

  return false;
}

function isPrivateIpAddress(address: string): boolean {
  const version = isIP(address);

  if (version === 4) return isPrivateIpv4(address);
  if (version === 6) return isPrivateIpv6(address);

  return true; // Unknown format → block by default
}

function isBlockedHostname(hostname: string): boolean {
  const h = hostname.trim().toLowerCase();

  return (
    h === 'localhost' ||
    h.endsWith('.localhost') ||
    h.endsWith('.local') ||
    h === '0.0.0.0'
  );
}

export async function isPublicHttpUrl(url: URL): Promise<boolean> {
  if (url.protocol !== 'http:' && url.protocol !== 'https:') {
    return false;
  }

  if (isBlockedHostname(url.hostname)) {
    return false;
  }

  const directVersion = isIP(url.hostname);

  if (directVersion !== 0) {
    return !isPrivateIpAddress(url.hostname);
  }

  try {
    const addresses = await lookup(url.hostname, { all: true });

    if (addresses.length === 0) return false;

    return addresses.every((a) => !isPrivateIpAddress(a.address));
  } catch {
    return false;
  }
}

function getOriginFaviconUrl(pageUrl: string): string {
  try {
    const url = new URL(pageUrl);

    return new URL(FALLBACK_ICON_PATH, url.origin).toString();
  } catch {
    return FALLBACK_ICON_PATH;
  }
}

function parseAttributes(tag: string): Record<string, string> {
  const attrs: Record<string, string> = {};
  const re = /([a-zA-Z_:.-]+)\s*=\s*("([^"]*)"|'([^']*)'|([^\s"'>]+))/g;
  let m: RegExpExecArray | null;

  while ((m = re.exec(tag)) !== null) {
    const key = m[1].toLowerCase();
    const value = m[3] ?? m[4] ?? m[5] ?? '';

    attrs[key] = value;
  }

  return attrs;
}

function toAbsoluteUrl(value: string | undefined, base: string): string | null {
  if (!value) return null;

  try {
    const url = new URL(value, base);

    if (url.protocol !== 'http:' && url.protocol !== 'https:') return null;

    return url.toString();
  } catch {
    return null;
  }
}

function extractIconFromHtml(html: string, baseUrl: string): string | null {
  const linkTags = html.match(/<link\b[^>]*>/gi) ?? [];
  const metaTags = html.match(/<meta\b[^>]*>/gi) ?? [];

  const linkCandidates = linkTags
    .map((tag) => parseAttributes(tag))
    .map((a) => ({
      rel: (a.rel ?? '').toLowerCase(),
      href: toAbsoluteUrl(a.href, baseUrl),
    }))
    .filter((c) => c.href);

  const appleTouchIcon = linkCandidates.find((c) =>
    c.rel.includes('apple-touch-icon'),
  )?.href;

  if (appleTouchIcon) return appleTouchIcon;

  const icon = linkCandidates.find((c) =>
    c.rel.split(/\s+/).includes('icon'),
  )?.href;

  if (icon) return icon;

  const shortcutIcon = linkCandidates.find((c) =>
    c.rel.includes('shortcut icon'),
  )?.href;

  if (shortcutIcon) return shortcutIcon;

  const imageSrc = linkCandidates.find((c) =>
    c.rel.includes('image_src'),
  )?.href;

  if (imageSrc) return imageSrc;

  const metaImage = metaTags
    .map((tag) => parseAttributes(tag))
    .map((a) => ({
      property: (a.property ?? a.name ?? '').toLowerCase(),
      content: toAbsoluteUrl(a.content, baseUrl),
    }))
    .find(
      (c) =>
        c.content &&
        (c.property === 'og:image' ||
          c.property === 'og:logo' ||
          c.property === 'twitter:image'),
    )?.content;

  return metaImage ?? null;
}

/**
 * Shared SSRF-guarded fetch used by every outbound request this service makes.
 * Follows redirects manually (up to MAX_REDIRECTS) so each hop is re-validated
 * against isPublicHttpUrl before being requested — a redirect target must not
 * be able to bypass the private-IP/blocked-hostname guard.
 *
 * Returns the final non-redirect Response, or null if the guard blocks any
 * hop, the request errors, or the redirect budget is exhausted.
 */
async function fetchWithRedirectGuard(
  pageUrl: URL,
  options: { accept: string; timeoutMs: number },
): Promise<{ response: Response; finalUrl: URL } | null> {
  let currentUrl = pageUrl;

  for (let redirectCount = 0; redirectCount <= MAX_REDIRECTS; redirectCount++) {
    const isPublic = await isPublicHttpUrl(currentUrl);

    if (!isPublic) return null;

    const controller = new AbortController();
    const timeout = setTimeout(() => controller.abort(), options.timeoutMs);

    try {
      const response = await fetch(currentUrl.toString(), {
        redirect: 'manual', // SSRF: follow manually so each hop is re-validated
        signal: controller.signal,
        headers: {
          Accept: options.accept,
          'User-Agent': 'tessera/1.0',
        },
      });

      if (response.status >= 300 && response.status < 400) {
        const location = response.headers.get('location');

        if (!location) return null;

        currentUrl = new URL(location, currentUrl);
        continue;
      }

      if (!response.ok) return null;

      return { response, finalUrl: currentUrl };
    } catch {
      return null;
    } finally {
      clearTimeout(timeout);
    }
  }

  return null;
}

async function fetchHtml(pageUrl: URL): Promise<FetchHtmlResult | null> {
  const result = await fetchWithRedirectGuard(pageUrl, {
    accept: 'text/html,application/xhtml+xml,*/*',
    timeoutMs: HTML_FETCH_TIMEOUT_MS,
  });

  if (!result) return null;

  const contentType = result.response.headers.get('content-type') ?? '';

  if (!contentType.toLowerCase().includes('text/html')) return null;

  const html = await result.response.text();

  return {
    html: html.slice(0, MAX_HTML_CHARS), // T-08-09: HTML cap
    finalUrl: result.finalUrl.toString(),
  };
}

@Injectable()
export class IconDiscoveryService {
  /**
   * Discover the best icon URL for a given web page URL.
   * Falls back to <origin>/favicon.ico when discovery fails or URL is private.
   *
   * SSRF protection: every URL and redirect target is validated against
   * private IP ranges, blocked hostnames, and forced-proxy vectors (T-08-05).
   */
  async discoverFavoriteIconUrl(pageUrl: string): Promise<string> {
    const fallback = getOriginFaviconUrl(pageUrl);

    try {
      const url = new URL(pageUrl);
      const htmlResult = await fetchHtml(url);

      if (!htmlResult) return fallback;

      return extractIconFromHtml(htmlResult.html, htmlResult.finalUrl) ?? fallback;
    } catch {
      return fallback;
    }
  }

  /**
   * Fetch the raw bytes of a stored icon URL, SSRF-guarded, for streaming
   * back to the browser from Tessera's own origin (avoids Cross-Origin-
   * Resource-Policy blocks on hotlinked cross-origin <img> loads).
   *
   * Throws on any failure — blocked target, timeout, non-image content-type,
   * or an oversized body. Callers must not return a placeholder image; let
   * the caller map the failure to an HTTP error status instead.
   */
  async fetchIconBytes(
    iconUrl: string,
  ): Promise<{ contentType: string; body: Buffer }> {
    const url = new URL(iconUrl);

    const result = await fetchWithRedirectGuard(url, {
      // A bare "image/*" Accept header (paired with our non-browser
      // User-Agent) trips bot-mitigation WAFs on some sites (observed:
      // chatgpt.com/favicon.ico returns 403 with this combo) — a realistic
      // browser-style image Accept list avoids that false positive.
      accept: 'image/avif,image/webp,image/apng,image/svg+xml,image/*,*/*;q=0.8',
      timeoutMs: ICON_FETCH_TIMEOUT_MS,
    });

    if (!result) {
      throw new Error('Icon fetch blocked or failed');
    }

    const contentType = result.response.headers.get('content-type') ?? '';

    if (!contentType.toLowerCase().startsWith('image/')) {
      throw new Error(`Icon response is not an image (${contentType})`);
    }

    const arrayBuffer = await result.response.arrayBuffer();

    if (arrayBuffer.byteLength > MAX_ICON_BYTES) {
      throw new Error('Icon response exceeds size limit');
    }

    return { contentType, body: Buffer.from(arrayBuffer) };
  }
}