From 1e3c27b7aa0b3727c4f637ed9bcad0521f878c47 Mon Sep 17 00:00:00 2001
From: Eli White <github@eli-white.com>
Date: Sun, 19 Apr 2026 23:31:20 -0700
Subject: [PATCH] perf: swap writer's spread-with-deltaTime for Object.assign
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tsup transpiles `{...ev, deltaTime: 0}` to __spreadProps(__spreadValues(...))
which iterates property descriptors — ~7% of total writer profile time was
spent inside __spreadProps. Object.assign({}, ev, { deltaTime: 0 }) is a
native builtin on V8 and 2-3× faster for small objects.

Applied at the three writer sites that clone-with-delta-reset a source
event: buildUnrecognizedTrack (top-level unrecognized MIDI tracks) and
the per-track unrecognizedMidiEvents passes in buildDrumTrack and
buildFretTrack.

Measured on writer autoresearch bench (2000 charts, 8 workers):
  previous: 5.710 ms mean, 4.883 ms p50, 15.040 ms p95, 5.132s wall
  now:      4.566 ms mean, 4.129 ms p50, 10.424 ms p95, 4.926s wall
  delta:    -20.0% mean, -30.7% p95

Second run (variance check): 4.609 ms mean — stable.

0 hash mismatches, 442/442 tests still green.

Cumulative: baseline 92 ms -> 4.57 ms, 20× faster.
---
 src/chart/midi-writer.ts | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/src/chart/midi-writer.ts b/src/chart/midi-writer.ts
index 96b214a..e51be26 100644
--- a/src/chart/midi-writer.ts
+++ b/src/chart/midi-writer.ts
@@ -257,7 +257,10 @@ function buildUnrecognizedTrack(events: MidiEvent[]): MidiEvent[] {
 	const out: MidiEvent[] = []
 	for (const e of events) {
 		const absTick = e.deltaTime
-		out.push({ ...e, deltaTime: absTick - prevTick })
+		// Object.assign is noticeably faster than {...e, …} spread on V8 — tsup
+		// transpiles the spread to __spreadProps(__spreadValues(...)) which
+		// iterates property descriptors.
+		out.push(Object.assign({}, e, { deltaTime: absTick - prevTick }) as MidiEvent)
 		prevTick = absTick
 	}
 	return out
@@ -539,7 +542,7 @@ function buildDrumTrack(
 				events.push({
 					tick: ev.deltaTime,
 					seq: unrecSeqBase + unrecSeq++,
-					event: { ...ev, deltaTime: 0 } as MidiEvent,
+					event: Object.assign({}, ev, { deltaTime: 0 }) as MidiEvent,
 				})
 			}
 		}
@@ -1002,7 +1005,7 @@ function buildFretTrack(
 				events.push({
 					tick: ev.deltaTime,
 					seq: unrecSeqBase + unrecSeq++,
-					event: { ...ev, deltaTime: 0 } as MidiEvent,
+					event: Object.assign({}, ev, { deltaTime: 0 }) as MidiEvent,
 				})
 			}
 		}