refactor(youtube): extract subtitle generation pipeline

launcher/youtube/srt.ts

@@ -0,0 +1,40 @@
+export interface SrtCue {
+  index: number;
+  start: string;
+  end: string;
+  text: string;
+}
+
+const TIMING_LINE_PATTERN =
+  /^(?<start>\d{2}:\d{2}:\d{2},\d{3}) --> (?<end>\d{2}:\d{2}:\d{2},\d{3})$/;
+
+export function parseSrt(content: string): SrtCue[] {
+  const normalized = content.replace(/\r\n/g, '\n').trim();
+  if (!normalized) return [];
+
+  return normalized
+    .split(/\n{2,}/)
+    .map((block) => {
+      const lines = block.split('\n');
+      const index = Number.parseInt(lines[0] || '', 10);
+      const timingLine = lines[1] || '';
+      const timingMatch = TIMING_LINE_PATTERN.exec(timingLine);
+      if (!Number.isInteger(index) || !timingMatch?.groups) {
+        throw new Error(`Invalid SRT cue block: ${block}`);
+      }
+      return {
+        index,
+        start: timingMatch.groups.start!,
+        end: timingMatch.groups.end!,
+        text: lines.slice(2).join('\n').trim(),
+      } satisfies SrtCue;
+    })
+    .filter((cue) => cue.text.length > 0);
+}
+
+export function stringifySrt(cues: SrtCue[]): string {
+  return cues
+    .map((cue, idx) => `${idx + 1}\n${cue.start} --> ${cue.end}\n${cue.text.trim()}\n`)
+    .join('\n')
+    .trimEnd();
+}