UHD_Media_Tool/apps/server/src/modules/file-manager/preview.ts

import { open } from "node:fs/promises";
import { basename } from "node:path";
import { BencodeError, type BencodeValue, bdecode, parseTorrent } from "@umt/adapters";
import type { FilePreview, FilesPreviewQuery } from "@umt/contracts";
import type { Principal } from "../../http/auth.ts";
import { FilesError, type FmDeps, isAdminSession, recheck, resolveIn } from "./common.ts";
import {
  CUE_LIMIT,
  decodeText,
  extOf,
  IMAGE_MIME,
  imageSize,
  maskTracker,
  parseAss,
  parseSrtLike,
  parseSup,
  previewKindOf,
} from "./preview-parse.ts";

// GET /files/preview(FM-2):仅管理员会话;路径守卫同下载(realpath 在根内、符号链接目标也须在根内,打开前复核)。
// 只读文件头:文本/字幕 ≤ 1 MiB、图片头 512 KiB;.torrent 整个读但上限 4 MiB。只回 JSON,不回原始内容。

export const TEXT_PREVIEW_MAX = 1024 * 1024;
const IMAGE_HEAD = 512 * 1024;
const TORRENT_MAX = 4 * 1024 * 1024;
const TORRENT_FILES = 500;

async function readHead(abs: string, max: number): Promise<{ buf: Uint8Array; size: number }> {
  const fh = await open(abs, "r");
  try {
    const st = await fh.stat();
    if (!st.isFile()) throw new FilesError("bad_request", "不是普通文件");
    const n = Math.min(st.size, max);
    const buf = new Uint8Array(n);
    let off = 0;
    while (off < n) {
      const { bytesRead } = await fh.read(buf, off, n - off, off);
      if (!bytesRead) break;
      off += bytesRead;
    }
    return { buf: buf.subarray(0, off), size: st.size };
  } finally {
    await fh.close();
  }
}

const str = (v: BencodeValue | undefined) => (v instanceof Uint8Array ? new TextDecoder().decode(v) : null);

function trackersOf(root: { [k: string]: BencodeValue }): string[] {
  const out: string[] = [];
  const tiers = root["announce-list"];
  if (Array.isArray(tiers))
    for (const tier of tiers) for (const t of Array.isArray(tier) ? tier : [tier]) out.push(str(t) ?? "");
  out.push(str(root.announce) ?? "");
  return [...new Set(out.filter(Boolean).map(maskTracker))];
}

export async function previewFile(
  d: FmDeps,
  principal: Principal | undefined,
  q: FilesPreviewQuery,
): Promise<FilePreview> {
  if (!isAdminSession(principal)) throw new FilesError("forbidden", "预览仅限控制台管理员会话");
  const r = await resolveIn(d, q.root, q.path);
  if (r.rel === "" || r.lst?.isDirectory()) throw new FilesError("bad_request", "只能预览文件，不能预览目录");
  const name = basename(r.abs);
  const kind = previewKindOf(name);
  const ext = extOf(name);
  if (!kind) throw new FilesError("forbidden", `不支持预览 .${ext || "无扩展名"} 文件`);
  const real = await recheck(r.abs, r.rootReal);

  if (kind === "image") {
    const { buf, size } = await readHead(real, IMAGE_HEAD);
    const dim = imageSize(buf);
    return {
      type: "image",
      size,
      mime: IMAGE_MIME[ext] ?? "application/octet-stream",
      width: dim?.width ?? null,
      height: dim?.height ?? null,
    };
  }

  if (kind === "torrent") {
    const { buf, size } = await readHead(real, TORRENT_MAX);
    if (size > TORRENT_MAX) throw new FilesError("too_large", `.torrent 文件 ${size} 字节，超过预览上限 4 MiB`);
    try {
      const meta = parseTorrent(buf);
      const root = bdecode(buf) as { [k: string]: BencodeValue };
      const info = root.info as { [k: string]: BencodeValue } | undefined;
      const created = typeof root["creation date"] === "number" ? root["creation date"] : null;
      return {
        type: "torrent",
        size,
        name: meta.name,
        hash: meta.hash,
        infohashV1: meta.infohashV1,
        infohashV2: meta.infohashV2,
        private: meta.private,
        totalSize: meta.totalSize,
        fileCount: meta.files.length,
        files: meta.files.slice(0, TORRENT_FILES).map((f) => ({ path: f.path, length: Math.max(0, f.length) })),
        trackers: trackersOf(root),
        comment: str(root["comment.utf-8"]) ?? str(root.comment),
        createdBy: str(root["created by"]),
        createdAt: created && created > 0 ? new Date(created * 1000).toISOString() : null,
        pieceLength: typeof info?.["piece length"] === "number" ? info["piece length"] : null,
      };
    } catch (e) {
      if (e instanceof BencodeError) throw new FilesError("bad_request", "不是有效的 .torrent 文件");
      throw e;
    }
  }

  const { buf, size } = await readHead(real, TEXT_PREVIEW_MAX);
  const truncated = size > buf.length;

  if (kind === "subtitle") {
    if (ext === "sup") {
      const all = parseSup(buf);
      return {
        type: "subtitle",
        size,
        format: "sup",
        encoding: null,
        cues: all.slice(0, CUE_LIMIT),
        parsed: all.length,
        truncated,
        title: null,
      };
    }
    const { text, encoding } = decodeText(buf, truncated);
    const ass = ext === "ass" || ext === "ssa";
    const parsed = ass ? parseAss(text) : { cues: parseSrtLike(text), title: null };
    const cues = [...parsed.cues].sort((a, b) => a.start - b.start);
    return {
      type: "subtitle",
      size,
      format: ext as "srt" | "vtt" | "ass" | "ssa",
      encoding,
      cues: cues.slice(0, CUE_LIMIT),
      parsed: cues.length,
      truncated,
      title: parsed.title,
    };
  }

  const { text, encoding } = decodeText(buf, truncated);
  return { type: "text", size, format: ext, encoding, text, truncated };
}