feat(users/Profpatsch/lyric/ext): add bpm quantization

It’s a bit crappy and really depends on the input field opening
quickly again (which it often doesn’t really do…), but it was the
easiest way I figured how to do it haha.

Aligning to eigth notes is pretty much the easiest way to sync
everything up after tapping in the timestamps (for most songs).

Change-Id: Ibbb072f62b6ee17d983e81b6c1554bc3516fa636
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12551
Reviewed-by: Profpatsch <mail@profpatsch.de>
Tested-by: BuildkiteCI

users/Profpatsch/lyric/extension/src/quantize-lrc.ts

@@ -0,0 +1,82 @@
+export function bpmToEighthNoteDuration(bpm: number): number {
+  // Convert BPM to eighth-note duration in milliseconds
+  const quarterNoteDuration = (60 / bpm) * 1000; // in ms
+  const eighthNoteDuration = quarterNoteDuration / 2;
+  return eighthNoteDuration;
+}
+
+function parseTimestamp(timestamp: string): number {
+  // Parse [mm:ss.ms] format into milliseconds
+  const [min, sec] = timestamp.split(':');
+
+  const minutes = parseInt(min, 10);
+  const seconds = parseFloat(sec);
+
+  return minutes * 60 * 1000 + seconds * 1000;
+}
+
+function formatTimestamp(ms: number): string {
+  // Format milliseconds back into [mm:ss.ms]
+  const minutes = Math.floor(ms / 60000);
+  ms %= 60000;
+  const seconds = (ms / 1000).toFixed(2);
+
+  return `${String(minutes).padStart(2, '0')}:${seconds}`;
+}
+
+export function adjustTimestampToEighthNote(
+  timestampMs: number,
+  eighthNoteDuration: number,
+): number {
+  // Find the closest multiple of the eighth-note duration
+  return Math.round(timestampMs / eighthNoteDuration) * eighthNoteDuration;
+}
+
+function adjustTimestamps(bpm: number, timestamps: string[]): string[] {
+  const eighthNoteDuration = bpmToEighthNoteDuration(bpm);
+
+  return timestamps.map(timestamp => {
+    const timestampMs = parseTimestamp(timestamp);
+    const adjustedMs = adjustTimestampToEighthNote(timestampMs, eighthNoteDuration);
+    return formatTimestamp(adjustedMs);
+  });
+}
+
+// Parse a .lrc file into an array of objects with timestamp and text
+// Then adjust the timestamps to the closest eighth note
+// Finally, format the adjusted timestamps back into [mm:ss.ms] format and put them back into the lrc object
+//
+// Example .lrc file:
+// [01:15.66] And the reviewers bewail
+// [01:18.18] There'll be no encore
+// [01:21.65] 'Cause you're not begging for more
+// [01:25.00]
+// [01:34.64] She may seem self-righteous and holier-than-thou
+// [01:39.77] She may sound like she has all the answers
+// [01:45.20] But beyond she may feel just a bit anyhow
+function parseLrc(lrc: string): { timestamp: string; text: string }[] {
+  return lrc
+    .trimEnd()
+    .split('\n')
+    .map(line => {
+      const match = line.match(/\[(\d+:\d+\.\d+)\](.*)/);
+      const [, timestamp, text] = match!;
+      return { timestamp, text };
+    });
+}
+
+function formatLrc(lrc: { timestamp: string; text: string }[]): string {
+  return lrc.map(({ timestamp, text }) => `[${timestamp}] ${text}`).join('\n');
+}
+
+function adjustLrc(lrc: string, bpm: number): string {
+  const lrcArray = parseLrc(lrc);
+  const timestamps = lrcArray.map(({ timestamp }) => timestamp);
+  const adjustedTimestamps = adjustTimestamps(bpm, timestamps);
+
+  lrcArray.forEach((line, i) => {
+    line.timestamp = adjustedTimestamps[i];
+  });
+
+  return formatLrc(lrcArray);
+}