1 files changed, 54 insertions, 0 deletions
diff --git a/apps/web/src/timeline/placement/main-track.ts b/apps/web/src/timeline/placement/main-track.ts
new file mode 100644
index 00000000..ab2ab7bd
--- /dev/null
+++ b/apps/web/src/timeline/placement/main-track.ts
@@ -0,0 +1,54 @@
+import type { SceneTracks, TimelineElement, VideoTrack } from "@/timeline";
+
+export const MAIN_TRACK_NAME = "Main Track";
+
+export function getEarliestMainTrackElement({
+	mainTrack,
+	excludeElementId,
+}: {
+	mainTrack: VideoTrack;
+	excludeElementId?: string;
+}): TimelineElement | 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: SceneTracks;
+	targetTrackId: string;
+	requestedStartTime: number;
+	excludeElementId?: string;
+}): number {
+	if (tracks.main.id !== targetTrackId) {
+		return requestedStartTime;
+	}
+
+	const earliestElement = getEarliestMainTrackElement({
+		mainTrack: tracks.main,
+		excludeElementId,
+	});
+	if (!earliestElement) {
+		return 0;
+	}
+
+	if (requestedStartTime <= earliestElement.startTime) {
+		return 0;
+	}
+
+	return requestedStartTime;
+}