1 files changed, 262 insertions, 0 deletions
diff --git a/apps/web/src/timeline/components/drop-target.ts b/apps/web/src/timeline/components/drop-target.ts
new file mode 100644
index 00000000..be2495e4
--- /dev/null
+++ b/apps/web/src/timeline/components/drop-target.ts
@@ -0,0 +1,262 @@
+import type { TimelineTrack, TimelineElement } from "@/timeline";
+import type { ComputeDropTargetParams, DropTarget } from "@/timeline";
+import { resolveTrackPlacement } from "@/timeline/placement";
+import { TIMELINE_TRACK_GAP_PX } from "./layout";
+import { getTrackHeight } from "./track-layout";
+import { TICKS_PER_SECOND } from "@/wasm";
+
+function findElementAtPosition({
+	mouseX,
+	tracks,
+	trackIndex,
+	targetElementTypes,
+	pixelsPerSecond,
+	zoomLevel,
+}: {
+	mouseX: number;
+	tracks: TimelineTrack[];
+	trackIndex: number;
+	targetElementTypes: string[];
+	pixelsPerSecond: number;
+	zoomLevel: number;
+}): { elementId: string; trackId: string } | null {
+	const time = Math.round(
+		(mouseX / (pixelsPerSecond * zoomLevel)) * TICKS_PER_SECOND,
+	);
+	const track = tracks[trackIndex];
+	if (!track || !("elements" in track)) return null;
+
+	const hit = track.elements.find(
+		(element: TimelineElement) =>
+			targetElementTypes.includes(element.type) &&
+			element.startTime <= time &&
+			time < element.startTime + element.duration,
+	);
+	if (!hit) return null;
+	return { elementId: hit.id, trackId: track.id };
+}
+
+function getTrackAtY({
+	mouseY,
+	tracks,
+	verticalDragDirection,
+}: {
+	mouseY: number;
+	tracks: TimelineTrack[];
+	verticalDragDirection?: "up" | "down" | null;
+}): { trackIndex: number; relativeY: number } | null {
+	let cumulativeHeight = 0;
+
+	for (let i = 0; i < tracks.length; i++) {
+		const trackHeight = getTrackHeight({ type: tracks[i].type });
+		const trackTop = cumulativeHeight;
+		const trackBottom = trackTop + trackHeight;
+
+		if (mouseY >= trackTop && mouseY < trackBottom) {
+			return {
+				trackIndex: i,
+				relativeY: mouseY - trackTop,
+			};
+		}
+
+		if (i < tracks.length - 1 && verticalDragDirection) {
+			const gapTop = trackBottom;
+			const gapBottom = gapTop + TIMELINE_TRACK_GAP_PX;
+			if (mouseY >= gapTop && mouseY < gapBottom) {
+				const isDraggingUp = verticalDragDirection === "up";
+				return {
+					trackIndex: isDraggingUp ? i : i + 1,
+					relativeY: isDraggingUp ? trackHeight - 1 : 0,
+				};
+			}
+		}
+
+		cumulativeHeight += trackHeight + TIMELINE_TRACK_GAP_PX;
+	}
+
+	return null;
+}
+
+const EMPTY_TARGET_ELEMENT = null;
+
+function fallbackNewTrackDropTarget({
+	xPosition,
+}: {
+	xPosition: number;
+}): DropTarget {
+	return {
+		trackIndex: 0,
+		isNewTrack: true,
+		insertPosition: null,
+		xPosition,
+		targetElement: EMPTY_TARGET_ELEMENT,
+	};
+}
+
+export function computeDropTarget({
+	elementType,
+	mouseX,
+	mouseY,
+	tracks,
+	playheadTime,
+	isExternalDrop,
+	elementDuration,
+	pixelsPerSecond,
+	zoomLevel,
+	verticalDragDirection,
+	startTimeOverride,
+	excludeElementId,
+	targetElementTypes,
+}: ComputeDropTargetParams): DropTarget {
+	const orderedTracks = [...tracks.overlay, tracks.main, ...tracks.audio];
+	const mainTrackIndex = tracks.overlay.length;
+	const xPosition =
+		typeof startTimeOverride === "number"
+			? startTimeOverride
+			: isExternalDrop
+				? playheadTime
+				: Math.round(
+				Math.max(0, mouseX / (pixelsPerSecond * zoomLevel)) * TICKS_PER_SECOND,
+			);
+
+	if (orderedTracks.length === 0) {
+		const placementResult = resolveTrackPlacement({
+			tracks,
+			elementType,
+			timeSpans: [{ startTime: xPosition, duration: elementDuration, excludeElementId }],
+			strategy: {
+				type: "preferIndex",
+				trackIndex: 0,
+				hoverDirection: "below",
+				createNewTrackOnly: true,
+			},
+		});
+		const emptyTimelineResult =
+			placementResult?.kind === "newTrack" ? placementResult : null;
+		if (!emptyTimelineResult) {
+			return fallbackNewTrackDropTarget({ xPosition });
+		}
+
+		return {
+			trackIndex: emptyTimelineResult.insertIndex,
+			isNewTrack: true,
+			insertPosition: emptyTimelineResult.insertPosition,
+			xPosition,
+			targetElement: EMPTY_TARGET_ELEMENT,
+		};
+	}
+
+	const trackAtMouse = getTrackAtY({
+		mouseY,
+		tracks: orderedTracks,
+		verticalDragDirection,
+	});
+
+	if (!trackAtMouse) {
+		const isAboveAllTracks = mouseY < 0;
+
+		const placementResult = resolveTrackPlacement({
+			tracks,
+			elementType,
+			timeSpans: [{ startTime: xPosition, duration: elementDuration, excludeElementId }],
+			strategy: {
+				type: "preferIndex",
+				trackIndex: isAboveAllTracks ? 0 : orderedTracks.length - 1,
+				hoverDirection: isAboveAllTracks ? "above" : "below",
+				createNewTrackOnly: true,
+			},
+		});
+		const outOfBoundsResult =
+			placementResult?.kind === "newTrack" ? placementResult : null;
+		if (!outOfBoundsResult) {
+			return fallbackNewTrackDropTarget({ xPosition });
+		}
+
+		return {
+			trackIndex: outOfBoundsResult.insertIndex,
+			isNewTrack: true,
+			insertPosition: outOfBoundsResult.insertPosition,
+			xPosition,
+			targetElement: EMPTY_TARGET_ELEMENT,
+		};
+	}
+
+	const { trackIndex, relativeY } = trackAtMouse;
+	const track = orderedTracks[trackIndex];
+
+	if (targetElementTypes && targetElementTypes.length > 0) {
+		const targetElement = findElementAtPosition({
+			mouseX,
+			tracks: orderedTracks,
+			trackIndex,
+			targetElementTypes,
+			pixelsPerSecond,
+			zoomLevel,
+		});
+		if (targetElement) {
+			return {
+				trackIndex,
+				isNewTrack: false,
+				insertPosition: null,
+				xPosition,
+				targetElement,
+			};
+		}
+	}
+
+	const trackHeight = getTrackHeight({ type: track.type });
+	const placementResult = resolveTrackPlacement({
+		tracks,
+		elementType,
+		timeSpans: [{ startTime: xPosition, duration: elementDuration, excludeElementId }],
+		strategy: {
+			type: "preferIndex",
+			trackIndex,
+			hoverDirection: relativeY < trackHeight / 2 ? "above" : "below",
+			verticalDragDirection,
+		},
+	});
+	if (!placementResult) {
+		return fallbackNewTrackDropTarget({ xPosition });
+	}
+
+	if (placementResult.kind === "existingTrack") {
+		const adjustedXPosition = placementResult.adjustedStartTime ?? xPosition;
+
+		return {
+			trackIndex: placementResult.trackIndex,
+			isNewTrack: false,
+			insertPosition: null,
+			xPosition: adjustedXPosition,
+			targetElement: EMPTY_TARGET_ELEMENT,
+		};
+	}
+
+	return {
+		trackIndex: placementResult.insertIndex,
+		isNewTrack: true,
+		insertPosition: placementResult.insertPosition,
+		xPosition,
+		targetElement: EMPTY_TARGET_ELEMENT,
+	};
+}
+
+export function getDropLineY({
+	dropTarget,
+	tracks,
+}: {
+	dropTarget: DropTarget;
+	tracks: TimelineTrack[];
+}): number {
+	const safeTrackIndex = Math.min(
+		Math.max(dropTarget.trackIndex, 0),
+		tracks.length,
+	);
+	let y = 0;
+
+	for (let i = 0; i < safeTrackIndex; i++) {
+		y += getTrackHeight({ type: tracks[i].type }) + TIMELINE_TRACK_GAP_PX;
+	}
+
+	return y;
+}