feat(subtitles): add local Japanese subtitle generation (#240)

src/core/services/subtitle-generation-download.ts

@@ -0,0 +1,67 @@
+import { createHash } from 'node:crypto';
+import { mkdir, mkdtemp, open, rm } from 'node:fs/promises';
+import path from 'node:path';
+import { publishSubtitleGenerationFile } from './subtitle-generation-files';
+import type { SubtitleGenerationProgress } from '../../shared/subtitle-generation';
+
+export async function downloadSubtitleGenerationArtifact(input: {
+  url: string;
+  size: number;
+  sha256: string;
+  destination: string;
+  label: string;
+  onProgress?: (progress: SubtitleGenerationProgress) => void;
+  signal?: AbortSignal;
+}): Promise<string> {
+  input.signal?.throwIfAborted();
+  await mkdir(path.dirname(input.destination), { recursive: true });
+  const temporaryDirectory = await mkdtemp(
+    path.join(path.dirname(input.destination), '.download-'),
+  );
+  const temporaryPath = path.join(temporaryDirectory, 'model.bin');
+  input.onProgress?.({ stage: 'download', percent: 0, message: `Downloading ${input.label}...` });
+  try {
+    const response = await fetch(input.url, { signal: input.signal });
+    if (!response.ok || !response.body) {
+      throw new Error(`Model download failed: HTTP ${response.status}`);
+    }
+    const file = await open(temporaryPath, 'wx');
+    const reader = response.body.getReader();
+    const digest = createHash('sha256');
+    let received = 0;
+    let previousPercent = -1;
+    try {
+      while (true) {
+        input.signal?.throwIfAborted();
+        const { done, value } = await reader.read();
+        if (done) break;
+        received += value.byteLength;
+        if (received > input.size) throw new Error('Downloaded model exceeds expected size.');
+        digest.update(value);
+        await file.writeFile(value);
+        const percent = Math.min(99, Math.floor((received / input.size) * 100));
+        if (percent !== previousPercent) {
+          previousPercent = percent;
+          input.onProgress?.({
+            stage: 'download',
+            percent,
+            message: `Downloading ${input.label}...`,
+          });
+        }
+      }
+      if (received !== input.size || digest.digest('hex') !== input.sha256) {
+        throw new Error('Downloaded model failed integrity verification. Try downloading again.');
+      }
+      await file.sync();
+    } finally {
+      await reader.cancel().catch(() => undefined);
+      await file.close();
+    }
+    input.signal?.throwIfAborted();
+    await publishSubtitleGenerationFile(temporaryPath, input.destination);
+    input.onProgress?.({ stage: 'download', percent: 100, message: `${input.label} is ready.` });
+    return input.destination;
+  } finally {
+    await rm(temporaryDirectory, { recursive: true, force: true });
+  }
+}