perf: avoid BigInt alloc in calculateTrackHash binary layout

src/chart/track-hasher.ts

@@ -49,60 +49,71 @@ export function calculateTrackHash(parsedChart: ParsedChart, instrument: Instrum
 	const uint8Array = new Uint8Array(buffer)
 	const view = new DataView(buffer, 0)
 
+	// For a chart, every `tick` and `length` is a non-negative 32-bit integer —
+	// max per-tick values are well below 2^31 for any real song. So we can write
+	// each 8-byte little-endian int64 slot as [low 32 bits, high 32 bits = 0],
+	// skipping the `BigInt(x)` allocation that `setBigInt64` forces. A full
+	// hash on a dense track fires tens of thousands of these — the BigInt
+	// allocations dominate GC in the hasher path.
+	function writeInt64LE(offset: number, v: number): void {
+		view.setUint32(offset, v >>> 0, true)
+		view.setUint32(offset + 4, 0, true)
+	}
+
 	view.setUint32(0, 0x43484e46, false) // Big endian for format header, little endian for everything else
 	view.setUint32(4, 20240320, true)
 	view.setUint32(8, parsedChart.resolution, true)
 	let i = 12
 	view.setUint32(i, tempoData.length, true)
 	i += 4
 	for (const tempo of tempoData) {
-		view.setBigInt64(i, BigInt(tempo.tick), true)
+		writeInt64LE(i, tempo.tick)
 		view.setFloat64(i + 8, tempo.beatsPerMinute, true)
 		i += 16
 	}
 	view.setUint32(i, timeSignatureData.length, true)
 	i += 4
 	for (const timeSignature of timeSignatureData) {
-		view.setBigInt64(i, BigInt(timeSignature.tick), true)
+		writeInt64LE(i, timeSignature.tick)
 		view.setUint32(i + 8, timeSignature.numerator, true)
 		view.setUint32(i + 12, timeSignature.denominator, true)
 		i += 16
 	}
 	view.setUint32(i, starPowerData.length, true)
 	i += 4
 	for (const starPower of starPowerData) {
-		view.setBigInt64(i, BigInt(starPower.tick), true)
-		view.setBigInt64(i + 8, BigInt(starPower.length), true)
+		writeInt64LE(i, starPower.tick)
+		writeInt64LE(i + 8, starPower.length)
 		i += 16
 	}
 	view.setUint32(i, soloSectionData.length, true)
 	i += 4
 	for (const soloSection of soloSectionData) {
-		view.setBigInt64(i, BigInt(soloSection.tick), true)
-		view.setBigInt64(i + 8, BigInt(soloSection.length), true)
+		writeInt64LE(i, soloSection.tick)
+		writeInt64LE(i + 8, soloSection.length)
 		i += 16
 	}
 	view.setUint32(i, flexLanesData.length, true)
 	i += 4
 	for (const flexLane of flexLanesData) {
-		view.setBigInt64(i, BigInt(flexLane.tick), true)
-		view.setBigInt64(i + 8, BigInt(flexLane.length), true)
+		writeInt64LE(i, flexLane.tick)
+		writeInt64LE(i + 8, flexLane.length)
 		view.setUint8(i + 16, flexLane.isDouble ? 1 : 0)
 		i += 17
 	}
 	view.setInt32(i, drumFreestyleSectionData.length, true)
 	i += 4
 	for (const drumFreestyleSection of drumFreestyleSectionData) {
-		view.setBigInt64(i, BigInt(drumFreestyleSection.tick), true)
-		view.setBigInt64(i + 8, BigInt(drumFreestyleSection.length), true)
+		writeInt64LE(i, drumFreestyleSection.tick)
+		writeInt64LE(i + 8, drumFreestyleSection.length)
 		view.setUint8(i + 16, drumFreestyleSection.isCoda ? 1 : 0)
 		i += 17
 	}
 	view.setInt32(i, notesData.length, true)
 	i += 4
 	for (const note of notesData) {
-		view.setBigInt64(i, BigInt(note.tick), true)
-		view.setBigInt64(i + 8, BigInt(note.length), true)
+		writeInt64LE(i, note.tick)
+		writeInt64LE(i + 8, note.length)
 		view.setUint32(i + 16, note.type, true)
 		view.setUint32(i + 20, note.flags, true)
 		i += 24