1 files changed, 182 insertions, 0 deletions
diff --git a/apps/web/src/lib/retime/audio-stretch.ts b/apps/web/src/lib/retime/audio-stretch.ts
new file mode 100644
index 00000000..51ef38d0
--- /dev/null
+++ b/apps/web/src/lib/retime/audio-stretch.ts
@@ -0,0 +1,182 @@
+import { PitchShifter } from "soundtouchjs";
+import { clampRetimeRate, shouldMaintainPitch } from "@/constants/retime-constants";
+import type { RetimeConfig } from "@/lib/timeline";
+import { getSourceTimeAtClipTime } from "./resolve";
+
+const RATE_EPSILON = 1e-6;
+
+function sampleLinear({
+	channelData,
+	position,
+}: {
+	channelData: Float32Array;
+	position: number;
+}): number {
+	if (position <= 0) {
+		return channelData[0] ?? 0;
+	}
+	const lower = Math.floor(position);
+	const upper = Math.min(channelData.length - 1, lower + 1);
+	if (lower >= channelData.length) {
+		return 0;
+	}
+	const fraction = position - lower;
+	return channelData[lower] * (1 - fraction) + channelData[upper] * fraction;
+}
+
+function buildResampledBuffer({
+	audioContext,
+	sourceBuffer,
+	trimStart,
+	clipDuration,
+	targetSampleRate,
+	retime,
+}: {
+	audioContext: BaseAudioContext;
+	sourceBuffer: AudioBuffer;
+	trimStart: number;
+	clipDuration: number;
+	targetSampleRate: number;
+	retime?: RetimeConfig;
+}): AudioBuffer {
+	const outputLength = Math.max(1, Math.ceil(clipDuration * targetSampleRate));
+	const numChannels = Math.max(1, Math.min(2, sourceBuffer.numberOfChannels));
+	const outputBuffer = audioContext.createBuffer(
+		numChannels,
+		outputLength,
+		targetSampleRate,
+	);
+
+	for (let channel = 0; channel < numChannels; channel++) {
+		const sourceData = sourceBuffer.getChannelData(
+			Math.min(channel, sourceBuffer.numberOfChannels - 1),
+		);
+		const outputData = outputBuffer.getChannelData(channel);
+
+		for (let i = 0; i < outputLength; i++) {
+			const clipTime = i / targetSampleRate;
+			const sourceTime =
+				trimStart + getSourceTimeAtClipTime({ clipTime, retime });
+			outputData[i] = sampleLinear({
+				channelData: sourceData,
+				position: sourceTime * sourceBuffer.sampleRate,
+			});
+		}
+	}
+
+	return outputBuffer;
+}
+
+async function buildPitchPreservedBuffer({
+	sourceBuffer,
+	trimStart,
+	clipDuration,
+	rate,
+	targetSampleRate,
+}: {
+	sourceBuffer: AudioBuffer;
+	trimStart: number;
+	clipDuration: number;
+	rate: number;
+	targetSampleRate: number;
+}): Promise<AudioBuffer> {
+	const nativeSampleRate = sourceBuffer.sampleRate;
+	const sourceDuration = clipDuration * rate;
+	const startSample = Math.max(0, Math.floor(trimStart * nativeSampleRate));
+	const numSourceSamples = Math.max(
+		1,
+		Math.ceil(sourceDuration * nativeSampleRate),
+	);
+	const available = Math.max(0, sourceBuffer.length - startSample);
+	const actualSamples = Math.max(1, Math.min(numSourceSamples, available));
+	const numChannels = Math.max(1, Math.min(2, sourceBuffer.numberOfChannels));
+
+	// Resample to targetSampleRate first — soundtouchjs reads raw channel data
+	// and does not respect the source buffer's native sample rate.
+	const resampledLength = Math.max(
+		1,
+		Math.ceil(sourceDuration * targetSampleRate),
+	);
+	const resampleCtx = new OfflineAudioContext(
+		numChannels,
+		resampledLength,
+		targetSampleRate,
+	);
+	const nativeBuffer = resampleCtx.createBuffer(
+		numChannels,
+		actualSamples,
+		nativeSampleRate,
+	);
+
+	for (let ch = 0; ch < numChannels; ch++) {
+		const src = sourceBuffer.getChannelData(
+			Math.min(ch, sourceBuffer.numberOfChannels - 1),
+		);
+		nativeBuffer.copyToChannel(
+			src.subarray(startSample, startSample + actualSamples),
+			ch,
+		);
+	}
+
+	const resampleSourceNode = resampleCtx.createBufferSource();
+	resampleSourceNode.buffer = nativeBuffer;
+	resampleSourceNode.connect(resampleCtx.destination);
+	resampleSourceNode.start(0);
+	const resampledBuffer = await resampleCtx.startRendering();
+
+	const outputSamples = Math.max(
+		1,
+		Math.ceil(clipDuration * targetSampleRate),
+	);
+	const stretchCtx = new OfflineAudioContext(
+		numChannels,
+		outputSamples,
+		targetSampleRate,
+	);
+	const shifter = new PitchShifter(stretchCtx, resampledBuffer, 4096);
+	shifter.tempo = rate;
+	shifter.pitch = 1;
+	shifter.connect(stretchCtx.destination);
+	return stretchCtx.startRendering();
+}
+
+export async function renderRetimedBuffer({
+	audioContext,
+	sourceBuffer,
+	trimStart,
+	clipDuration,
+	retime,
+	maintainPitch = false,
+}: {
+	audioContext: BaseAudioContext;
+	sourceBuffer: AudioBuffer;
+	trimStart: number;
+	clipDuration: number;
+	retime?: RetimeConfig;
+	maintainPitch?: boolean;
+}): Promise<AudioBuffer> {
+	const targetSampleRate = audioContext.sampleRate;
+	const rate = clampRetimeRate({ rate: retime?.rate ?? 1 });
+	const usePitchPreservation =
+		shouldMaintainPitch({ rate, maintainPitch }) &&
+		Math.abs(rate - 1) > RATE_EPSILON;
+
+	if (usePitchPreservation) {
+		return buildPitchPreservedBuffer({
+			sourceBuffer,
+			trimStart,
+			clipDuration,
+			rate,
+			targetSampleRate,
+		});
+	}
+
+	return buildResampledBuffer({
+		audioContext,
+		sourceBuffer,
+		trimStart,
+		clipDuration,
+		targetSampleRate,
+		retime,
+	});
+}