8 files changed, 398 insertions, 0 deletions
diff --git a/apps/web/src/retime/__tests__/resolve.test.ts b/apps/web/src/retime/__tests__/resolve.test.ts
new file mode 100644
index 00000000..bb7b9a6d
--- /dev/null
+++ b/apps/web/src/retime/__tests__/resolve.test.ts
@@ -0,0 +1,61 @@
+import { describe, expect, test } from "bun:test";
+import {
+	getClipTimeAtSourceTime,
+	getEffectiveRateAt,
+	getSourceTimeAtClipTime,
+	getTimelineDurationForSourceSpan,
+} from "@/retime";
+import type { RetimeConfig } from "@/timeline";
+
+const twoX: RetimeConfig = { rate: 2 };
+const halfX: RetimeConfig = { rate: 0.5 };
+
+describe("retime resolve", () => {
+	test("maps clip time to source time at 2x speed", () => {
+		expect(getSourceTimeAtClipTime({ clipTime: 5, retime: twoX })).toBe(10);
+	});
+
+	test("maps clip time to source time at 0.5x speed", () => {
+		expect(getSourceTimeAtClipTime({ clipTime: 4, retime: halfX })).toBe(2);
+	});
+
+	test("returns clip time unchanged when no retime", () => {
+		expect(getSourceTimeAtClipTime({ clipTime: 7 })).toBe(7);
+	});
+
+	test("inverts source time back to clip time at 2x speed", () => {
+		expect(getClipTimeAtSourceTime({ sourceTime: 10, retime: twoX })).toBe(5);
+	});
+
+	test("returns effective rate", () => {
+		expect(getEffectiveRateAt({ retime: twoX })).toBe(2);
+		expect(getEffectiveRateAt({})).toBe(1);
+	});
+
+	test("derives timeline duration for a visible source span", () => {
+		expect(
+			getTimelineDurationForSourceSpan({ sourceSpan: 10, retime: twoX }),
+		).toBe(5);
+		expect(
+			getTimelineDurationForSourceSpan({ sourceSpan: 10, retime: halfX }),
+		).toBe(20);
+	});
+
+	test("clamps invalid rates to 1", () => {
+		expect(getSourceTimeAtClipTime({ clipTime: 5, retime: { rate: 0 } })).toBe(
+			5,
+		);
+		expect(
+			getSourceTimeAtClipTime({ clipTime: 5, retime: { rate: -1 } }),
+		).toBe(5);
+	});
+
+	test("caps retime rates above 5x", () => {
+		expect(getSourceTimeAtClipTime({ clipTime: 5, retime: { rate: 100 } })).toBe(
+			25,
+		);
+		expect(
+			getTimelineDurationForSourceSpan({ sourceSpan: 10, retime: { rate: 100 } }),
+		).toBe(2);
+	});
+});
diff --git a/apps/web/src/retime/__tests__/split.test.ts b/apps/web/src/retime/__tests__/split.test.ts
new file mode 100644
index 00000000..3c7bef63
--- /dev/null
+++ b/apps/web/src/retime/__tests__/split.test.ts
@@ -0,0 +1,31 @@
+import { describe, expect, test } from "bun:test";
+import {
+	getSourceSpanAtClipTime,
+	splitRetimeAtClipTime,
+} from "@/retime";
+import type { RetimeConfig } from "@/timeline";
+
+describe("retime split", () => {
+	test("measures source span at a clip time", () => {
+		const retime: RetimeConfig = { rate: 2 };
+		expect(getSourceSpanAtClipTime({ clipTime: 5, retime })).toBe(10);
+	});
+
+	test("returns zero for non-positive clip time", () => {
+		expect(getSourceSpanAtClipTime({ clipTime: 0 })).toBe(0);
+		expect(getSourceSpanAtClipTime({ clipTime: -1 })).toBe(0);
+	});
+
+	test("passes the same retime to both halves when splitting", () => {
+		const retime: RetimeConfig = { rate: 1.5 };
+		const result = splitRetimeAtClipTime({ retime, splitClipTime: 3 });
+		expect(result.left).toBe(retime);
+		expect(result.right).toBe(retime);
+	});
+
+	test("returns undefined on both sides when no retime", () => {
+		const result = splitRetimeAtClipTime({ splitClipTime: 3 });
+		expect(result.left).toBeUndefined();
+		expect(result.right).toBeUndefined();
+	});
+});
diff --git a/apps/web/src/retime/audio-stretch.ts b/apps/web/src/retime/audio-stretch.ts
new file mode 100644
index 00000000..7df67f36
--- /dev/null
+++ b/apps/web/src/retime/audio-stretch.ts
@@ -0,0 +1,182 @@
+import { PitchShifter } from "soundtouchjs";
+import { clampRetimeRate, shouldMaintainPitch } from "@/retime/rate";
+import type { RetimeConfig } from "@/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,
+	});
+}
diff --git a/apps/web/src/retime/index.ts b/apps/web/src/retime/index.ts
new file mode 100644
index 00000000..2ab1d10f
--- /dev/null
+++ b/apps/web/src/retime/index.ts
@@ -0,0 +1,5 @@
+export * from "./rate";
+export * from "./audio-stretch";
+export * from "./presets";
+export * from "./resolve";
+export * from "./split";
diff --git a/apps/web/src/retime/presets.ts b/apps/web/src/retime/presets.ts
new file mode 100644
index 00000000..816c8675
--- /dev/null
+++ b/apps/web/src/retime/presets.ts
@@ -0,0 +1,12 @@
+import type { RetimeConfig } from "@/timeline";
+import { clampRetimeRate } from "@/retime/rate";
+
+export function buildConstantRetime({
+	rate,
+	maintainPitch = false,
+}: {
+	rate: number;
+	maintainPitch?: boolean;
+}): RetimeConfig {
+	return { rate: clampRetimeRate({ rate }), maintainPitch };
+}
diff --git a/apps/web/src/retime/rate.ts b/apps/web/src/retime/rate.ts
new file mode 100644
index 00000000..75ea530b
--- /dev/null
+++ b/apps/web/src/retime/rate.ts
@@ -0,0 +1,25 @@
+export const DEFAULT_RETIME_RATE = 1;
+export const MIN_RETIME_RATE = 0.01;
+export const MAX_RETIME_RATE = 5;
+
+export function clampRetimeRate({ rate }: { rate: number }): number {
+	if (!Number.isFinite(rate) || rate <= 0) {
+		return DEFAULT_RETIME_RATE;
+	}
+
+	return Math.min(Math.max(rate, MIN_RETIME_RATE), MAX_RETIME_RATE);
+}
+
+export function canMaintainPitch({ rate }: { rate: number }): boolean {
+	return Number.isFinite(rate) && rate > 0;
+}
+
+export function shouldMaintainPitch({
+	rate,
+	maintainPitch,
+}: {
+	rate: number;
+	maintainPitch?: boolean;
+}): boolean {
+	return maintainPitch === true && canMaintainPitch({ rate });
+}
diff --git a/apps/web/src/retime/resolve.ts b/apps/web/src/retime/resolve.ts
new file mode 100644
index 00000000..0dea9dd8
--- /dev/null
+++ b/apps/web/src/retime/resolve.ts
@@ -0,0 +1,48 @@
+import type { RetimeConfig } from "@/timeline";
+import { clampRetimeRate } from "@/retime/rate";
+
+function getSafeRate({ rate }: { rate: number }): number {
+	return clampRetimeRate({ rate });
+}
+
+export function getSourceTimeAtClipTime({
+	clipTime,
+	retime,
+}: {
+	clipTime: number;
+	retime?: RetimeConfig;
+}): number {
+	return clipTime * getSafeRate({ rate: retime?.rate ?? 1 });
+}
+
+export function getClipTimeAtSourceTime({
+	sourceTime,
+	retime,
+}: {
+	sourceTime: number;
+	retime?: RetimeConfig;
+}): number {
+	return sourceTime / getSafeRate({ rate: retime?.rate ?? 1 });
+}
+
+export function getEffectiveRateAt({
+	retime,
+}: {
+	clipTime?: number;
+	retime?: RetimeConfig;
+}): number {
+	return getSafeRate({ rate: retime?.rate ?? 1 });
+}
+
+export function getTimelineDurationForSourceSpan({
+	sourceSpan,
+	retime,
+}: {
+	sourceSpan: number;
+	retime?: RetimeConfig;
+}): number {
+	if (sourceSpan <= 0) {
+		return 0;
+	}
+	return sourceSpan / getSafeRate({ rate: retime?.rate ?? 1 });
+}
diff --git a/apps/web/src/retime/split.ts b/apps/web/src/retime/split.ts
new file mode 100644
index 00000000..1b7360cc
--- /dev/null
+++ b/apps/web/src/retime/split.ts
@@ -0,0 +1,34 @@
+import type { RetimeConfig } from "@/timeline";
+import { getSourceTimeAtClipTime } from "./resolve";
+
+export function getSourceSpanAtClipTime({
+	clipTime,
+	retime,
+}: {
+	clipTime: number;
+	retime?: RetimeConfig;
+}): number {
+	return Math.max(0, getSourceTimeAtClipTime({ clipTime, retime }));
+}
+
+export function splitRetimeAtClipTime({
+	retime,
+}: {
+	retime?: RetimeConfig;
+	splitClipTime: number;
+}): {
+	left: RetimeConfig | undefined;
+	right: RetimeConfig | undefined;
+} {
+	return { left: retime, right: retime };
+}
+
+export function adjustRetimeForTrimChange({
+	retime,
+}: {
+	retime?: RetimeConfig;
+	clipTrimTime: number;
+	side: "start" | "end";
+}): RetimeConfig | undefined {
+	return retime;
+}