refactor: centralize track placement into its own module

1 files changed, 96 insertions, 0 deletions

diff --git a/apps/web/src/lib/timeline/placement/main-track.ts b/apps/web/src/lib/timeline/placement/main-track.ts
new file mode 100644
index 00000000..1783352c
--- /dev/null
+++ b/apps/web/src/lib/timeline/placement/main-track.ts
@@ -0,0 +1,96 @@
+import type { TimelineElement, TimelineTrack, VideoTrack } from "@/lib/timeline";
+import { generateUUID } from "@/utils/id";
+
+const MAIN_TRACK_NAME = "Main Track";
+
+export function isMainTrack(track: TimelineTrack): track is VideoTrack {
+	return track.type === "video" && track.isMain === true;
+}
+
+export function getMainTrack({
+	tracks,
+}: {
+	tracks: TimelineTrack[];
+}): VideoTrack | null {
+	return tracks.find((track) => isMainTrack(track)) ?? null;
+}
+
+export function ensureMainTrack({
+	tracks,
+}: {
+	tracks: TimelineTrack[];
+}): TimelineTrack[] {
+	if (tracks.some((track) => isMainTrack(track))) {
+		return tracks;
+	}
+
+	return [
+		{
+			id: generateUUID(),
+			name: MAIN_TRACK_NAME,
+			type: "video",
+			elements: [],
+			muted: false,
+			isMain: true,
+			hidden: false,
+		},
+		...tracks,
+	];
+}
+
+export function getEarliestMainTrackElement({
+	tracks,
+	excludeElementId,
+}: {
+	tracks: TimelineTrack[];
+	excludeElementId?: string;
+}): TimelineElement | null {
+	const mainTrack = getMainTrack({ tracks });
+	if (!mainTrack) {
+		return null;
+	}
+
+	const elements = mainTrack.elements.filter((element) => {
+		return !excludeElementId || element.id !== excludeElementId;
+	});
+	if (elements.length === 0) {
+		return null;
+	}
+
+	return elements.reduce((earliestElement, element) => {
+		return element.startTime < earliestElement.startTime
+			? element
+			: earliestElement;
+	});
+}
+
+export function enforceMainTrackStart({
+	tracks,
+	targetTrackId,
+	requestedStartTime,
+	excludeElementId,
+}: {
+	tracks: TimelineTrack[];
+	targetTrackId: string;
+	requestedStartTime: number;
+	excludeElementId?: string;
+}): number {
+	const mainTrack = getMainTrack({ tracks });
+	if (!mainTrack || mainTrack.id !== targetTrackId) {
+		return requestedStartTime;
+	}
+
+	const earliestElement = getEarliestMainTrackElement({
+		tracks,
+		excludeElementId,
+	});
+	if (!earliestElement) {
+		return 0;
+	}
+
+	if (requestedStartTime <= earliestElement.startTime) {
+		return 0;
+	}
+
+	return requestedStartTime;
+}