refactor(snaps): prioritize nearby point references

packages/excalidraw/snapping.ts

@@ -39,6 +39,10 @@ import type {
 } from "./types";
 
 const SNAP_DISTANCE = 8;
+// Point snaps can come from both nearby and distant references within snap
+// distance. If their visual distances have a large break, prefer the nearest
+// cluster before comparing offsets.
+const SNAP_REFERENCE_CLUSTER_BREAK_DISTANCE = 200;
 
 // do not comput more gaps per axis than this limit
 // TODO increase or remove once we optimize
@@ -49,6 +53,13 @@ export const getSnapDistance = (zoomValue: number) => {
   return SNAP_DISTANCE / zoomValue;
 };
 
+/**
+ * Keeps the cluster threshold visually stable across zoom levels.
+ */
+const getSnapReferenceClusterBreakDistance = (zoomValue: number) => {
+  return SNAP_REFERENCE_CLUSTER_BREAK_DISTANCE / zoomValue;
+};
+
 type Vector2D = {
   x: number;
   y: number;
@@ -84,6 +95,9 @@ export type PointSnap = {
   points: PointPair;
   pointTypes: [SnapPointType, SnapPointType];
   sourceIds: [SnapPointSourceId, SnapPointSourceId];
+  // Distance along the rendered snapline, used to group visually nearby
+  // references before choosing which point snaps should win.
+  visualDistance: number;
   offset: number;
 };
 
@@ -361,18 +375,37 @@ const getSnapPointSourceId = (elements: ExcalidrawElement[]) => {
     .join(",");
 };
 
+const getSelectedFrameIdsForSnapping = (
+  selectedElements: readonly ExcalidrawElement[],
+) => {
+  const selectedFrameIds = new Set<ExcalidrawElement["id"]>();
+
+  for (const element of selectedElements) {
+    if (element.frameId) {
+      selectedFrameIds.add(element.frameId);
+    }
+  }
+
+  return selectedFrameIds;
+};
+
 const getReferenceElements = (
   elements: readonly NonDeletedExcalidrawElement[],
   selectedElements: NonDeletedExcalidrawElement[],
   appState: AppState,
   elementsMap: ElementsMap,
-) =>
-  getVisibleAndNonSelectedElements(
+) => {
+  const selectedFrameIds = getSelectedFrameIdsForSnapping(selectedElements);
+
+  return getVisibleAndNonSelectedElements(
     elements,
     selectedElements,
     appState,
     elementsMap,
-  );
+  ).filter((element) => {
+    return !element.frameId || selectedFrameIds.has(element.frameId);
+  });
+};
 
 export const getVisibleGaps = (
   elements: readonly NonDeletedExcalidrawElement[],
@@ -662,6 +695,119 @@ const getGapSnaps = (
   }
 };
 
+/**
+ * Narrows point snaps to the nearest visual cluster, then keeps the best offset
+ * inside that cluster. If all references look continuous, this preserves the
+ * old smallest-offset behavior.
+ */
+const filterPointSnapsToNearestCluster = (
+  snaps: Snaps,
+  clusterBreakDistance: number,
+) => {
+  const pointSnaps = snaps.filter(
+    (snap): snap is PointSnap => snap.type === "point",
+  );
+
+  if (pointSnaps.length < 2) {
+    return;
+  }
+
+  const snapsByOffset = new Map<number, PointSnap[]>();
+
+  for (const snap of pointSnaps) {
+    const offset = round(snap.offset);
+    const offsetSnaps = snapsByOffset.get(offset) ?? [];
+    offsetSnaps.push(snap);
+    snapsByOffset.set(offset, offsetSnaps);
+  }
+
+  const keptPointSnaps = new Set<PointSnap>();
+  const offsetGroups = Array.from(snapsByOffset.entries()).map(
+    ([offset, offsetSnaps]) => ({
+      offset,
+      snaps: offsetSnaps,
+      visualDistance: Math.min(
+        ...offsetSnaps.map((snap) => snap.visualDistance),
+      ),
+    }),
+  );
+  const sortedOffsetGroups = offsetGroups
+    .slice()
+    .sort((a, b) => a.visualDistance - b.visualDistance);
+  let keepOffsetGroupsUntil = sortedOffsetGroups.length;
+
+  // Prefer a nearby reference cluster before comparing offset. If there is no
+  // clear distance break, this keeps all offsets and falls back to the old
+  // smallest-offset behavior.
+  for (let i = 1; i < sortedOffsetGroups.length; i++) {
+    const distanceGap =
+      sortedOffsetGroups[i].visualDistance -
+      sortedOffsetGroups[i - 1].visualDistance;
+
+    if (distanceGap > clusterBreakDistance) {
+      keepOffsetGroupsUntil = i;
+      break;
+    }
+  }
+
+  const nearestOffsetGroups = sortedOffsetGroups.slice(
+    0,
+    keepOffsetGroupsUntil,
+  );
+  const minOffset = Math.min(
+    ...nearestOffsetGroups.map(({ offset }) => Math.abs(offset)),
+  );
+  const selectedOffsets = new Set(
+    nearestOffsetGroups
+      .filter(({ offset }) => Math.abs(offset) === minOffset)
+      .map(({ offset }) => offset),
+  );
+
+  for (const offsetSnaps of snapsByOffset.values()) {
+    if (!selectedOffsets.has(round(offsetSnaps[0].offset))) {
+      continue;
+    }
+
+    if (offsetSnaps.length < 2) {
+      offsetSnaps.forEach((snap) => keptPointSnaps.add(snap));
+      continue;
+    }
+
+    const sortedSnaps = offsetSnaps
+      .slice()
+      .sort((a, b) => a.visualDistance - b.visualDistance);
+
+    let keepUntil = sortedSnaps.length;
+
+    // This runs after the winning snap offset is known. A big visual-distance
+    // gap means references after it belong to a separate, farther cluster.
+    for (let i = 1; i < sortedSnaps.length; i++) {
+      const distanceGap =
+        sortedSnaps[i].visualDistance - sortedSnaps[i - 1].visualDistance;
+
+      if (distanceGap > clusterBreakDistance) {
+        keepUntil = i;
+        break;
+      }
+    }
+
+    for (let i = 0; i < keepUntil; i++) {
+      keptPointSnaps.add(sortedSnaps[i]);
+    }
+  }
+
+  let writeIndex = 0;
+
+  for (const snap of snaps) {
+    if (snap.type !== "point" || keptPointSnaps.has(snap)) {
+      snaps[writeIndex] = snap;
+      writeIndex++;
+    }
+  }
+
+  snaps.length = writeIndex;
+};
+
 export const getReferenceSnapPoints = (
   elements: readonly NonDeletedExcalidrawElement[],
   selectedElements: ExcalidrawElement[],
@@ -689,6 +835,11 @@ export const getReferenceSnapPoints = (
     );
 };
 
+/**
+ * Collects point snaps within snap distance. Unlike gap snaps, point snaps first
+ * go through visual-cluster filtering so a nearby reference can beat a slightly
+ * better offset from a far-away reference.
+ */
 const getPointSnaps = (
   selectedElements: ExcalidrawElement[],
   selectionSnapPoints: SnapPoint[],
@@ -714,39 +865,52 @@ const getPointSnaps = (
         const offsetY = otherSnapPoint.point[1] - thisSnapPoint.point[1];
 
         if (Math.abs(offsetX) <= minOffset.x) {
-          if (Math.abs(offsetX) < minOffset.x) {
-            nearestSnapsX.length = 0;
-          }
-
           nearestSnapsX.push({
             type: "point",
             points: [thisSnapPoint.point, otherSnapPoint.point],
             pointTypes: [thisSnapPoint.type, otherSnapPoint.type],
             sourceIds: [thisSnapPoint.sourceId, otherSnapPoint.sourceId],
+            visualDistance: Math.abs(
+              otherSnapPoint.point[1] - thisSnapPoint.point[1],
+            ),
             offset: offsetX,
           });
-
-          minOffset.x = Math.abs(offsetX);
         }
 
         if (Math.abs(offsetY) <= minOffset.y) {
-          if (Math.abs(offsetY) < minOffset.y) {
-            nearestSnapsY.length = 0;
-          }
-
           nearestSnapsY.push({
             type: "point",
             points: [thisSnapPoint.point, otherSnapPoint.point],
             pointTypes: [thisSnapPoint.type, otherSnapPoint.type],
             sourceIds: [thisSnapPoint.sourceId, otherSnapPoint.sourceId],
+            visualDistance: Math.abs(
+              otherSnapPoint.point[0] - thisSnapPoint.point[0],
+            ),
             offset: offsetY,
           });
-
-          minOffset.y = Math.abs(offsetY);
         }
       }
     }
   }
+
+  const clusterBreakDistance = getSnapReferenceClusterBreakDistance(
+    app.state.zoom.value,
+  );
+
+  filterPointSnapsToNearestCluster(nearestSnapsX, clusterBreakDistance);
+  filterPointSnapsToNearestCluster(nearestSnapsY, clusterBreakDistance);
+
+  if (nearestSnapsX.length > 0) {
+    minOffset.x = Math.min(
+      ...nearestSnapsX.map((snap) => Math.abs(snap.offset)),
+    );
+  }
+
+  if (nearestSnapsY.length > 0) {
+    minOffset.y = Math.min(
+      ...nearestSnapsY.map((snap) => Math.abs(snap.offset)),
+    );
+  }
 };
 
 export const snapDraggedElements = (
@@ -899,6 +1063,10 @@ type PointSnapLineBucket = {
   matches: PointSnapLineMatch[];
 };
 
+/**
+ * Describes which source pair produced a rendered point snapline, so later
+ * filtering can reason about redundancy without mixing unrelated references.
+ */
 const getPointSnapLineMatch = (snap: PointSnap): PointSnapLineMatch => {
   return {
     sourceKey: snap.sourceIds.join("->"),
@@ -908,6 +1076,10 @@ const getPointSnapLineMatch = (snap: PointSnap): PointSnapLineMatch => {
   };
 };
 
+/**
+ * Builds the outer snapline coordinates by source pair. Center snaplines use
+ * this map to decide whether the surrounding outer lines already explain them.
+ */
 const getOuterCoordinatesBySourceKey = (
   snapLines: PointSnapLineCandidate[],
   getCoordinate: (snapLine: PointSnapLine) => number,
@@ -963,6 +1135,10 @@ const isRedundantOuterSnapLine = (
   return coordinate !== minCoordinate && coordinate !== maxCoordinate;
 };
 
+/**
+ * Removes point snaplines that are visually redundant, while preserving any line
+ * that is still needed by at least one source pair represented on that line.
+ */
 const filterRedundantPointSnapLines = (
   snapLines: PointSnapLineCandidate[],
   getCoordinate: (snapLine: PointSnapLine) => number,

packages/excalidraw/tests/snapping.test.ts

@@ -0,0 +1,509 @@
+import { arrayToMap } from "@excalidraw/common";
+import { pointFrom, type GlobalPoint, type Radians } from "@excalidraw/math";
+
+import type { ExcalidrawElement } from "@excalidraw/element/types";
+
+import { getDefaultAppState } from "../appState";
+import {
+  getElementsCorners,
+  getReferenceSnapPoints,
+  SnapCache,
+  snapDraggedElements,
+} from "../snapping";
+
+import { API } from "./helpers/api";
+
+import type { AppClassProperties, AppState } from "../types";
+
+type ReferenceSnapPoints = NonNullable<
+  ReturnType<typeof SnapCache.getReferenceSnapPoints>
+>;
+
+const NO_MODIFIER_KEYS = {
+  altKey: false,
+  ctrlKey: false,
+  metaKey: false,
+  shiftKey: false,
+};
+
+const createSnappingApp = (appState: Partial<AppState> = {}) =>
+  ({
+    props: {},
+    state: {
+      ...getDefaultAppState(),
+      objectsSnapModeEnabled: true,
+      width: 1000,
+      height: 1000,
+      offsetLeft: 0,
+      offsetTop: 0,
+      ...appState,
+    },
+  } as AppClassProperties);
+
+const getHorizontalPointSnapLineCoordinates = (
+  snapLines: ReturnType<typeof snapDraggedElements>["snapLines"],
+) => {
+  return snapLines
+    .filter((snapLine) => snapLine.type === "points")
+    .filter((snapLine) => {
+      const [firstPoint, lastPoint] = snapLine.points;
+
+      return firstPoint[1] === lastPoint[1];
+    })
+    .map((snapLine) => {
+      return snapLine.points[0][1];
+    })
+    .sort((a, b) => a - b);
+};
+
+const getHorizontalPointSnapLineMaxX = (
+  snapLines: ReturnType<typeof snapDraggedElements>["snapLines"],
+) => {
+  const horizontalSnapLine = snapLines
+    .filter((snapLine) => snapLine.type === "points")
+    .find((snapLine) => {
+      const [firstPoint, lastPoint] = snapLine.points;
+
+      return firstPoint[1] === lastPoint[1];
+    });
+
+  if (!horizontalSnapLine) {
+    return null;
+  }
+
+  return horizontalSnapLine.points[horizontalSnapLine.points.length - 1][0];
+};
+
+const getPointKeys = (points: ReturnType<typeof getElementsCorners>) => {
+  return points.map((point) => point.join(","));
+};
+
+const getReferenceSnapPointKeys = (
+  elements: ExcalidrawElement[],
+  selectedElements: ExcalidrawElement[],
+  app: AppClassProperties,
+) => {
+  return new Set(
+    getReferenceSnapPoints(
+      elements,
+      selectedElements,
+      app.state,
+      arrayToMap(elements),
+    ).map((snapPoint) => snapPoint.point.join(",")),
+  );
+};
+
+const primeReferenceSnapPoints = (
+  elements: ExcalidrawElement[],
+  selectedElements: ExcalidrawElement[],
+) => {
+  const selectedElementIds = new Set(
+    selectedElements.map((element) => element.id),
+  );
+  const elementsMap = arrayToMap(elements);
+
+  SnapCache.setReferenceSnapPoints(
+    elements
+      .filter((element) => !selectedElementIds.has(element.id))
+      .flatMap((element) => {
+        const corners = getElementsCorners([element], elementsMap);
+
+        return corners.map((point, index) => ({
+          point,
+          type: index === corners.length - 1 ? "center" : "outer",
+          sourceId: element.id,
+        }));
+      }) as Parameters<typeof SnapCache.setReferenceSnapPoints>[0],
+  );
+};
+
+describe("snapping", () => {
+  afterEach(() => {
+    SnapCache.destroy();
+  });
+
+  it("does not use frame children as references when snapping outside elements", () => {
+    const frame = API.createElement({
+      type: "frame",
+      id: "frame",
+      x: 0,
+      y: 0,
+      width: 300,
+      height: 300,
+    });
+    const frameChild = API.createElement({
+      type: "rectangle",
+      id: "frameChild",
+      x: 37,
+      y: 53,
+      width: 71,
+      height: 83,
+      frameId: frame.id,
+    });
+    const selected = API.createElement({
+      type: "rectangle",
+      id: "selected",
+      x: 400,
+      y: 50,
+      width: 100,
+      height: 100,
+    });
+    const elements = [frame, frameChild, selected];
+    const app = createSnappingApp({
+      selectedElementIds: { [selected.id]: true },
+    });
+
+    const referenceSnapPointKeys = getReferenceSnapPointKeys(
+      elements,
+      [selected],
+      app,
+    );
+    const frameChildPointKeys = getPointKeys(
+      getElementsCorners([frameChild], arrayToMap(elements)),
+    );
+
+    expect(
+      frameChildPointKeys.some((pointKey) =>
+        referenceSnapPointKeys.has(pointKey),
+      ),
+    ).toBe(false);
+  });
+
+  it("uses frame siblings as references when snapping elements in the same frame", () => {
+    const frame = API.createElement({
+      type: "frame",
+      id: "frame",
+      x: 0,
+      y: 0,
+      width: 300,
+      height: 300,
+    });
+    const sibling = API.createElement({
+      type: "rectangle",
+      id: "sibling",
+      x: 37,
+      y: 53,
+      width: 71,
+      height: 83,
+      frameId: frame.id,
+    });
+    const selected = API.createElement({
+      type: "rectangle",
+      id: "selected",
+      x: 150,
+      y: 50,
+      width: 100,
+      height: 100,
+      frameId: frame.id,
+    });
+    const elements = [frame, sibling, selected];
+    const app = createSnappingApp({
+      selectedElementIds: { [selected.id]: true },
+    });
+
+    const referenceSnapPointKeys = getReferenceSnapPointKeys(
+      elements,
+      [selected],
+      app,
+    );
+    const siblingPointKeys = getPointKeys(
+      getElementsCorners([sibling], arrayToMap(elements)),
+    );
+
+    expect(
+      siblingPointKeys.some((pointKey) => referenceSnapPointKeys.has(pointKey)),
+    ).toBe(true);
+  });
+
+  it("does not use frame children as references when snapping the frame itself", () => {
+    const frame = API.createElement({
+      type: "frame",
+      id: "frame",
+      x: 0,
+      y: 0,
+      width: 300,
+      height: 300,
+    });
+    const frameChild = API.createElement({
+      type: "rectangle",
+      id: "frameChild",
+      x: 37,
+      y: 53,
+      width: 71,
+      height: 83,
+      frameId: frame.id,
+    });
+    const elements = [frame, frameChild];
+    const app = createSnappingApp({
+      selectedElementIds: { [frame.id]: true },
+    });
+
+    const referenceSnapPointKeys = getReferenceSnapPointKeys(
+      elements,
+      [frame],
+      app,
+    );
+    const frameChildPointKeys = getPointKeys(
+      getElementsCorners([frameChild], arrayToMap(elements)),
+    );
+
+    expect(
+      frameChildPointKeys.some((pointKey) =>
+        referenceSnapPointKeys.has(pointKey),
+      ),
+    ).toBe(false);
+  });
+
+  it("filters center and inner outer point snaplines for the same reference", () => {
+    const angle = 0.68 as Radians;
+    const reference = API.createElement({
+      type: "rectangle",
+      id: "reference",
+      x: 0,
+      y: 0,
+      width: 140,
+      height: 140,
+      angle,
+    });
+    const selected = API.createElement({
+      type: "rectangle",
+      id: "selected",
+      x: 200,
+      y: 0,
+      width: 140,
+      height: 140,
+      angle,
+    });
+    const elements = [reference, selected];
+    const app = createSnappingApp({
+      selectedElementIds: { [selected.id]: true },
+    });
+
+    primeReferenceSnapPoints(elements, [selected]);
+
+    const { snapLines } = snapDraggedElements(
+      elements,
+      { x: 0, y: 0 },
+      app,
+      NO_MODIFIER_KEYS,
+      arrayToMap(elements),
+    );
+
+    expect(getHorizontalPointSnapLineCoordinates(snapLines)).toHaveLength(2);
+  });
+
+  it("keeps a snapline that is redundant for one reference but needed for another", () => {
+    const angle = 0.68 as Radians;
+    const selected = API.createElement({
+      type: "rectangle",
+      id: "selected",
+      x: 200,
+      y: 0,
+      width: 140,
+      height: 140,
+      angle,
+    });
+    const elements = [selected];
+    const elementsMap = arrayToMap(elements);
+    const selectedSnapPoints = getElementsCorners([selected], elementsMap);
+    const outerSnapPoints = selectedSnapPoints.slice(0, -1);
+    const centerSnapPoint = selectedSnapPoints[selectedSnapPoints.length - 1];
+    const innerOuterSnapPoint = [...outerSnapPoints].sort(
+      (a, b) => a[1] - b[1],
+    )[1];
+    const app = createSnappingApp({
+      selectedElementIds: { [selected.id]: true },
+    });
+
+    const referenceSnapPoints: ReferenceSnapPoints = [
+      ...outerSnapPoints.map((point) => ({
+        point: pointFrom<GlobalPoint>(point[0] - 200, point[1]),
+        type: "outer" as const,
+        sourceId: "referenceA",
+      })),
+      {
+        point: pointFrom<GlobalPoint>(
+          centerSnapPoint[0] - 200,
+          centerSnapPoint[1],
+        ),
+        type: "center" as const,
+        sourceId: "referenceA",
+      },
+      {
+        point: pointFrom<GlobalPoint>(
+          innerOuterSnapPoint[0] - 300,
+          innerOuterSnapPoint[1],
+        ),
+        type: "outer" as const,
+        sourceId: "referenceB",
+      },
+    ];
+
+    SnapCache.setReferenceSnapPoints(referenceSnapPoints);
+
+    const { snapLines } = snapDraggedElements(
+      elements,
+      { x: 0, y: 0 },
+      app,
+      NO_MODIFIER_KEYS,
+      elementsMap,
+    );
+
+    expect(getHorizontalPointSnapLineCoordinates(snapLines)).toHaveLength(3);
+  });
+
+  it("keeps a center snapline when no outer snaplines imply it", () => {
+    const reference = API.createElement({
+      type: "rectangle",
+      id: "reference",
+      x: 0,
+      y: 0,
+      width: 100,
+      height: 100,
+    });
+    const selected = API.createElement({
+      type: "rectangle",
+      id: "selected",
+      x: 200,
+      y: 25,
+      width: 50,
+      height: 50,
+    });
+    const elements = [reference, selected];
+    const app = createSnappingApp({
+      selectedElementIds: { [selected.id]: true },
+    });
+
+    primeReferenceSnapPoints(elements, [selected]);
+
+    const { snapLines } = snapDraggedElements(
+      elements,
+      { x: 0, y: 0 },
+      app,
+      NO_MODIFIER_KEYS,
+      arrayToMap(elements),
+    );
+
+    expect(getHorizontalPointSnapLineCoordinates(snapLines)).toEqual([50]);
+  });
+
+  it("prefers the nearest visual cluster for same-offset point snaps", () => {
+    const selected = API.createElement({
+      type: "rectangle",
+      id: "selected",
+      x: 0,
+      y: 0,
+      width: 100,
+      height: 100,
+    });
+    const elements = [selected];
+    const app = createSnappingApp({
+      selectedElementIds: { [selected.id]: true },
+    });
+
+    SnapCache.setReferenceSnapPoints([
+      {
+        point: pointFrom<GlobalPoint>(220, 50),
+        type: "center",
+        sourceId: "near",
+      },
+      {
+        point: pointFrom<GlobalPoint>(900, 50),
+        type: "center",
+        sourceId: "far",
+      },
+    ]);
+
+    const { snapLines } = snapDraggedElements(
+      elements,
+      { x: 0, y: 0 },
+      app,
+      NO_MODIFIER_KEYS,
+      arrayToMap(elements),
+    );
+
+    expect(getHorizontalPointSnapLineMaxX(snapLines)).toBe(220);
+  });
+
+  it("prefers a nearby point snap over a slightly better far offset", () => {
+    const selected = API.createElement({
+      type: "rectangle",
+      id: "selected",
+      x: 0,
+      y: 0,
+      width: 100,
+      height: 100,
+    });
+    const elements = [selected];
+    const app = createSnappingApp({
+      selectedElementIds: { [selected.id]: true },
+    });
+
+    SnapCache.setReferenceSnapPoints([
+      {
+        point: pointFrom<GlobalPoint>(220, 54),
+        type: "center",
+        sourceId: "near",
+      },
+      {
+        point: pointFrom<GlobalPoint>(900, 50),
+        type: "center",
+        sourceId: "far",
+      },
+    ]);
+
+    const { snapOffset, snapLines } = snapDraggedElements(
+      elements,
+      { x: 0, y: 0 },
+      app,
+      NO_MODIFIER_KEYS,
+      arrayToMap(elements),
+    );
+
+    expect(snapOffset.y).toBe(4);
+    expect(getHorizontalPointSnapLineMaxX(snapLines)).toBe(220);
+  });
+
+  it("keeps same-offset point snaps when references form a continuous cluster", () => {
+    const selected = API.createElement({
+      type: "rectangle",
+      id: "selected",
+      x: 0,
+      y: 0,
+      width: 100,
+      height: 100,
+    });
+    const elements = [selected];
+    const app = createSnappingApp({
+      selectedElementIds: { [selected.id]: true },
+    });
+
+    SnapCache.setReferenceSnapPoints([
+      {
+        point: pointFrom<GlobalPoint>(200, 50),
+        type: "center",
+        sourceId: "referenceA",
+      },
+      {
+        point: pointFrom<GlobalPoint>(350, 50),
+        type: "center",
+        sourceId: "referenceB",
+      },
+      {
+        point: pointFrom<GlobalPoint>(500, 50),
+        type: "center",
+        sourceId: "referenceC",
+      },
+    ]);
+
+    const { snapLines } = snapDraggedElements(
+      elements,
+      { x: 0, y: 0 },
+      app,
+      NO_MODIFIER_KEYS,
+      arrayToMap(elements),
+    );
+
+    expect(getHorizontalPointSnapLineMaxX(snapLines)).toBe(500);
+  });
+});