1 files changed, 88 insertions, 0 deletions
diff --git a/apps/web/src/timeline/placement/insert-index.ts b/apps/web/src/timeline/placement/insert-index.ts
new file mode 100644
index 00000000..5e2786dc
--- /dev/null
+++ b/apps/web/src/timeline/placement/insert-index.ts
@@ -0,0 +1,88 @@
+import type { SceneTracks, TrackType } from "@/timeline";
+
+export function getDefaultInsertIndexForTrack({
+	tracks,
+	trackType,
+}: {
+	tracks: SceneTracks;
+	trackType: TrackType;
+}): number {
+	if (trackType === "audio") {
+		return tracks.overlay.length + 1 + tracks.audio.length;
+	}
+
+	if (trackType === "effect") {
+		return 0;
+	}
+
+	return tracks.overlay.length;
+}
+
+export function getHighestInsertIndexForTrack({
+	tracks,
+	trackType,
+}: {
+	tracks: SceneTracks;
+	trackType: TrackType;
+}): number {
+	if (trackType === "audio") {
+		return tracks.overlay.length + 1;
+	}
+
+	return 0;
+}
+
+export function resolvePreferredNewTrackPlacement({
+	tracks,
+	trackType,
+	preferredIndex,
+	direction,
+}: {
+	tracks: SceneTracks;
+	trackType: TrackType;
+	preferredIndex: number;
+	direction: "above" | "below";
+}): { insertIndex: number; insertPosition: "above" | "below" | null } {
+	const trackCount = tracks.overlay.length + 1 + tracks.audio.length;
+	if (trackCount === 0) {
+		return {
+			insertIndex: 0,
+			insertPosition: trackType === "audio" ? "below" : null,
+		};
+	}
+
+	const safePreferredIndex = Math.min(
+		Math.max(preferredIndex, 0),
+		trackCount - 1,
+	);
+	const mainTrackIndex = tracks.overlay.length;
+
+	if (trackType === "audio") {
+		if (safePreferredIndex <= mainTrackIndex) {
+			return {
+				insertIndex: mainTrackIndex + 1,
+				insertPosition: "below",
+			};
+		}
+
+		return {
+			insertIndex:
+				direction === "above" ? safePreferredIndex : safePreferredIndex + 1,
+			insertPosition: direction,
+		};
+	}
+
+	const insertIndex =
+		direction === "above" ? safePreferredIndex : safePreferredIndex + 1;
+	if (mainTrackIndex >= 0 && insertIndex > mainTrackIndex) {
+		return {
+			insertIndex: mainTrackIndex,
+			insertPosition: "above",
+		};
+	}
+
+	return {
+		insertIndex,
+		insertPosition: direction,
+	};
+}