1 files changed, 292 insertions, 0 deletions
diff --git a/apps/web/src/lib/preview/preview-snap.ts b/apps/web/src/lib/preview/preview-snap.ts
new file mode 100644
index 00000000..bd53f0ca
--- /dev/null
+++ b/apps/web/src/lib/preview/preview-snap.ts
@@ -0,0 +1,292 @@
+export interface SnapLine {
+	type: "horizontal" | "vertical";
+	position: number;
+}
+
+const SNAP_THRESHOLD = 10;
+const ROTATION_SNAP_STEP_DEGREES = 90;
+const ROTATION_SNAP_THRESHOLD_DEGREES = 5;
+export const MIN_SCALE = 0.01;
+
+export interface SnapResult {
+	snappedPosition: { x: number; y: number };
+	activeLines: SnapLine[];
+}
+
+export function snapPosition({
+	proposedPosition,
+	canvasSize,
+	elementSize,
+}: {
+	proposedPosition: { x: number; y: number };
+	canvasSize: { width: number; height: number };
+	elementSize: { width: number; height: number };
+}): SnapResult {
+	const centerX = 0;
+	const centerY = 0;
+	const left = -canvasSize.width / 2;
+	const right = canvasSize.width / 2;
+	const top = -canvasSize.height / 2;
+	const bottom = canvasSize.height / 2;
+
+	const halfWidth = elementSize.width / 2;
+	const halfHeight = elementSize.height / 2;
+	const activeLines: SnapLine[] = [];
+
+	type AxisSnapCandidate = {
+		snappedPosition: number;
+		line: SnapLine;
+		distance: number;
+	};
+
+	function getClosestAxisSnap({
+		candidates,
+	}: {
+		candidates: AxisSnapCandidate[];
+	}): AxisSnapCandidate | null {
+		const snapCandidatesWithinThreshold = candidates.filter(
+			(candidate) => candidate.distance <= SNAP_THRESHOLD,
+		);
+		if (snapCandidatesWithinThreshold.length === 0) {
+			return null;
+		}
+		return snapCandidatesWithinThreshold.reduce((closest, current) =>
+			current.distance < closest.distance ? current : closest,
+		);
+	}
+
+	const verticalTargets = [left, centerX, right];
+	const horizontalTargets = [top, centerY, bottom];
+
+	const xCandidates: AxisSnapCandidate[] = [];
+	for (const targetX of verticalTargets) {
+		xCandidates.push({
+			snappedPosition: targetX,
+			line: { type: "vertical", position: targetX },
+			distance: Math.abs(proposedPosition.x - targetX),
+		});
+		xCandidates.push({
+			snappedPosition: targetX + halfWidth,
+			line: { type: "vertical", position: targetX },
+			distance: Math.abs(proposedPosition.x - halfWidth - targetX),
+		});
+		xCandidates.push({
+			snappedPosition: targetX - halfWidth,
+			line: { type: "vertical", position: targetX },
+			distance: Math.abs(proposedPosition.x + halfWidth - targetX),
+		});
+	}
+	const yCandidates: AxisSnapCandidate[] = [];
+	for (const targetY of horizontalTargets) {
+		yCandidates.push({
+			snappedPosition: targetY,
+			line: { type: "horizontal", position: targetY },
+			distance: Math.abs(proposedPosition.y - targetY),
+		});
+		yCandidates.push({
+			snappedPosition: targetY + halfHeight,
+			line: { type: "horizontal", position: targetY },
+			distance: Math.abs(proposedPosition.y - halfHeight - targetY),
+		});
+		yCandidates.push({
+			snappedPosition: targetY - halfHeight,
+			line: { type: "horizontal", position: targetY },
+			distance: Math.abs(proposedPosition.y + halfHeight - targetY),
+		});
+	}
+
+	const closestX = getClosestAxisSnap({ candidates: xCandidates });
+	const closestY = getClosestAxisSnap({ candidates: yCandidates });
+
+	const x = closestX?.snappedPosition ?? proposedPosition.x;
+	const y = closestY?.snappedPosition ?? proposedPosition.y;
+	if (closestX) {
+		activeLines.push(closestX.line);
+	}
+	if (closestY) {
+		activeLines.push(closestY.line);
+	}
+
+	return {
+		snappedPosition: { x, y },
+		activeLines,
+	};
+}
+
+export interface ScaleSnapResult {
+	snappedScale: number;
+	activeLines: SnapLine[];
+}
+
+export function snapScale({
+	proposedScale,
+	position,
+	baseWidth,
+	baseHeight,
+	canvasSize,
+}: {
+	proposedScale: number;
+	position: { x: number; y: number };
+	baseWidth: number;
+	baseHeight: number;
+	canvasSize: { width: number; height: number };
+}): ScaleSnapResult {
+	const centerX = 0;
+	const centerY = 0;
+	const left = -canvasSize.width / 2;
+	const right = canvasSize.width / 2;
+	const top = -canvasSize.height / 2;
+	const bottom = canvasSize.height / 2;
+
+	const leftEdge = position.x - (baseWidth * proposedScale) / 2;
+	const rightEdge = position.x + (baseWidth * proposedScale) / 2;
+	const topEdge = position.y - (baseHeight * proposedScale) / 2;
+	const bottomEdge = position.y + (baseHeight * proposedScale) / 2;
+
+	interface SnapCandidate {
+		scale: number;
+		distance: number;
+		lines: SnapLine[];
+	}
+
+	const candidates: SnapCandidate[] = [];
+
+	const verticalTargets = [
+		{ position: left, line: { type: "vertical" as const, position: left } },
+		{
+			position: centerX,
+			line: { type: "vertical" as const, position: centerX },
+		},
+		{ position: right, line: { type: "vertical" as const, position: right } },
+	];
+
+	for (const target of verticalTargets) {
+		const distanceLeft = Math.abs(leftEdge - target.position);
+		if (distanceLeft <= SNAP_THRESHOLD) {
+			const scale = (2 * (position.x - target.position)) / baseWidth;
+			if (scale > MIN_SCALE) {
+				candidates.push({
+					scale,
+					distance: distanceLeft,
+					lines: [target.line],
+				});
+			}
+		}
+		const distanceRight = Math.abs(rightEdge - target.position);
+		if (distanceRight <= SNAP_THRESHOLD) {
+			const scale = (2 * (target.position - position.x)) / baseWidth;
+			if (scale > MIN_SCALE) {
+				candidates.push({
+					scale,
+					distance: distanceRight,
+					lines: [target.line],
+				});
+			}
+		}
+	}
+
+	const horizontalTargets = [
+		{ position: top, line: { type: "horizontal" as const, position: top } },
+		{
+			position: centerY,
+			line: { type: "horizontal" as const, position: centerY },
+		},
+		{
+			position: bottom,
+			line: { type: "horizontal" as const, position: bottom },
+		},
+	];
+
+	for (const target of horizontalTargets) {
+		const distanceTop = Math.abs(topEdge - target.position);
+		if (distanceTop <= SNAP_THRESHOLD) {
+			const scale = (2 * (position.y - target.position)) / baseHeight;
+			if (scale > MIN_SCALE) {
+				candidates.push({
+					scale,
+					distance: distanceTop,
+					lines: [target.line],
+				});
+			}
+		}
+		const distanceBottom = Math.abs(bottomEdge - target.position);
+		if (distanceBottom <= SNAP_THRESHOLD) {
+			const scale = (2 * (target.position - position.y)) / baseHeight;
+			if (scale > MIN_SCALE) {
+				candidates.push({
+					scale,
+					distance: distanceBottom,
+					lines: [target.line],
+				});
+			}
+		}
+	}
+
+	if (candidates.length === 0) {
+		return { snappedScale: proposedScale, activeLines: [] };
+	}
+
+	const best = candidates.reduce((acc, candidate) =>
+		candidate.distance < acc.distance ? candidate : acc,
+	);
+
+	const snappedLeft = position.x - (baseWidth * best.scale) / 2;
+	const snappedRight = position.x + (baseWidth * best.scale) / 2;
+	const snappedTop = position.y - (baseHeight * best.scale) / 2;
+	const snappedBottom = position.y + (baseHeight * best.scale) / 2;
+
+	const activeLines: SnapLine[] = [];
+	const seenKeys = new Set<string>();
+
+	function addLine({ line }: { line: SnapLine }) {
+		const key = `${line.type}-${line.position}`;
+		if (!seenKeys.has(key)) {
+			seenKeys.add(key);
+			activeLines.push(line);
+		}
+	}
+
+	for (const target of verticalTargets) {
+		if (
+			Math.abs(snappedLeft - target.position) <= 1 ||
+			Math.abs(snappedRight - target.position) <= 1
+		) {
+			addLine({ line: target.line });
+		}
+	}
+	for (const target of horizontalTargets) {
+		if (
+			Math.abs(snappedTop - target.position) <= 1 ||
+			Math.abs(snappedBottom - target.position) <= 1
+		) {
+			addLine({ line: target.line });
+		}
+	}
+
+	return {
+		snappedScale: best.scale,
+		activeLines,
+	};
+}
+
+export interface RotationSnapResult {
+	snappedRotation: number;
+	isSnapped: boolean;
+}
+
+export function snapRotation({
+	proposedRotation,
+}: {
+	proposedRotation: number;
+}): RotationSnapResult {
+	const nearestRotationSnap =
+		Math.round(proposedRotation / ROTATION_SNAP_STEP_DEGREES) *
+		ROTATION_SNAP_STEP_DEGREES;
+	const distanceToNearestSnap = Math.abs(
+		proposedRotation - nearestRotationSnap,
+	);
+	if (distanceToNearestSnap <= ROTATION_SNAP_THRESHOLD_DEGREES) {
+		return { snappedRotation: nearestRotationSnap, isSnapped: true };
+	}
+	return { snappedRotation: proposedRotation, isSnapped: false };
+}