SubMiner/src/core/services/subsync-extract.ts

import * as fs from 'fs';
import * as os from 'os';
import * as path from 'path';
import {
  codecToExtension,
  fileExists,
  MpvTrack,
  resolveLocalMediaPath,
  runCommand,
  summarizeCommandFailure,
} from '../../subsync/utils';
import { downloadToFile, isRemoteMediaPath } from '../../jimaku/utils';
import { createLogger } from '../../logger';
import {
  ResolvedMpvHttpHeaders,
  toFfmpegInputHttpArgs,
  toRequestHeaders,
} from './mpv-http-headers';

const logger = createLogger('subsync');

export interface FileExtractionResult {
  path: string;
  temporary: boolean;
}

export interface SubtitleExtractionInput {
  /**
   * Resolved lazily: an external track never shells out to ffmpeg, so a stream
   * whose subtitles arrive by URL must not fail just because ffmpeg is absent.
   */
  resolveFfmpegPath: () => string;
  videoPath: string;
  track: MpvTrack;
  /** mpv's request context, so stream-hosted tracks stay reachable. */
  httpHeaders: ResolvedMpvHttpHeaders | null;
}

function extensionForUrl(url: string, fallback: string): string {
  try {
    const parsed = new URL(url);
    const ext = path.extname(parsed.pathname).replace(/^\./, '').toLowerCase();
    return /^[a-z0-9]{1,5}$/.test(ext) ? ext : fallback;
  } catch {
    return fallback;
  }
}

/**
 * Drop a scratch directory whose extraction never completed.
 *
 * Removal is recursive here — unlike `cleanupTemporaryFile`, nothing in a failed
 * directory is worth keeping, and a half-written download would otherwise leave
 * the directory behind for the life of the machine.
 */
function discardTemporaryDirectory(tempDir: string): void {
  try {
    fs.rmSync(tempDir, { recursive: true, force: true });
  } catch {}
}

/**
 * Pull a subtitle track mpv loaded from a URL down to disk.
 *
 * Extension-backed and Jellyfin streams add their subtitles by URL, and neither
 * alass nor ffmpeg can read one — the whole subsync path used to reject these
 * with "Subtitle file not found: https://…".
 */
async function downloadRemoteSubtitleTrack(
  url: string,
  httpHeaders: ResolvedMpvHttpHeaders | null,
): Promise<FileExtractionResult> {
  const tempDir = fs.mkdtempSync(path.join(os.tmpdir(), 'subminer-subsync-'));
  try {
    const outputPath = path.join(tempDir, `remote_track.${extensionForUrl(url, 'srt')}`);

    const result = await downloadToFile(url, outputPath, toRequestHeaders(httpHeaders));
    if (!result.ok) {
      throw new Error(
        `Failed to download subtitle track: ${result.error?.error ?? 'unknown error'}`,
      );
    }
    if (!fileExists(outputPath)) {
      throw new Error(`Downloaded subtitle track is missing: ${url}`);
    }

    logger.info(`Downloaded remote subtitle track to ${outputPath}`);
    return { path: outputPath, temporary: true };
  } catch (error) {
    discardTemporaryDirectory(tempDir);
    throw error;
  }
}

async function extractInternalTrack(input: SubtitleExtractionInput): Promise<FileExtractionResult> {
  const ffIndex = input.track['ff-index'];
  const extension = codecToExtension(input.track.codec);
  if (typeof ffIndex !== 'number' || !Number.isInteger(ffIndex) || ffIndex < 0) {
    throw new Error('Internal subtitle track has no valid ff-index');
  }
  if (!extension) {
    throw new Error(`Unsupported subtitle codec: ${input.track.codec ?? 'unknown'}`);
  }
  const ffmpegMuxer = extension === 'vtt' ? 'webvtt' : extension;

  const tempDir = fs.mkdtempSync(path.join(os.tmpdir(), 'subminer-subsync-'));
  try {
    const outputPath = path.join(tempDir, `track_${ffIndex}.${extension}`);
    // Header args configure the HTTP demuxer, so they belong before `-i`.
    const httpArgs = isRemoteMediaPath(input.videoPath)
      ? toFfmpegInputHttpArgs(input.httpHeaders)
      : [];

    const extraction = await runCommand(input.resolveFfmpegPath(), [
      '-hide_banner',
      '-nostdin',
      '-y',
      '-loglevel',
      'error',
      ...httpArgs,
      '-an',
      '-vn',
      '-i',
      input.videoPath,
      '-map',
      `0:${ffIndex}`,
      '-f',
      ffmpegMuxer,
      outputPath,
    ]);

    if (!extraction.ok || !fileExists(outputPath)) {
      throw new Error(
        `Failed to extract internal subtitle track with ffmpeg: ${summarizeCommandFailure(
          'ffmpeg',
          extraction,
        )}`,
      );
    }

    return { path: outputPath, temporary: true };
  } catch (error) {
    discardTemporaryDirectory(tempDir);
    throw error;
  }
}

export async function extractSubtitleTrackToFile(
  input: SubtitleExtractionInput,
): Promise<FileExtractionResult> {
  if (input.track.external) {
    const externalFilename = input.track['external-filename'];
    if (typeof externalFilename !== 'string' || externalFilename.length === 0) {
      throw new Error('External subtitle track has no file path');
    }
    // A dropped subtitle arrives as a `file://` URL, which is local, not remote.
    const externalPath = resolveLocalMediaPath(externalFilename);
    if (isRemoteMediaPath(externalPath)) {
      return downloadRemoteSubtitleTrack(externalPath, input.httpHeaders);
    }
    if (!fileExists(externalPath)) {
      throw new Error(`Subtitle file not found: ${externalPath}`);
    }
    return { path: externalPath, temporary: false };
  }

  return extractInternalTrack(input);
}

/**
 * Drop a temporary extraction once alass/ffsubsync is done with it.
 *
 * `preservePath` is the retimed subtitle mpv is about to load. With
 * `replace: false` it lands in this same temp directory, so the removal has to
 * skip it — and the directory removal must stay non-recursive, since that is
 * what keeps the retimed file alive for mpv.
 */
export function cleanupTemporaryFile(
  extraction: FileExtractionResult,
  preservePath?: string,
): void {
  if (!extraction.temporary) return;
  if (preservePath && path.resolve(preservePath) === path.resolve(extraction.path)) return;

  try {
    if (fileExists(extraction.path)) {
      fs.unlinkSync(extraction.path);
    }
  } catch {}
  try {
    const dir = path.dirname(extraction.path);
    if (fs.existsSync(dir)) {
      fs.rmdirSync(dir);
    }
  } catch {}
}