feat: improve label stacking to reuse lanes and prevent overlaps

Previously, stack indices were assigned by simple incrementing counter, which caused unnecessary gaps in the label stacks. Now, the algorithm checks for available lanes and reuses them when possible.

Author: hamed-musallam

src/component/utility/stackOverlappingLabels.ts

@@ -38,28 +38,46 @@ export function stackOverlappingLabelsArray<T extends Record<string, any>>(
     });
   });
 }
-
+/**
+ * Assigns a stack index (vertical lane) to each item so that horizontally
+ * overlapping labels are pushed to different lanes, while non-overlapping
+ * labels reuse the lowest available lane.
+ * */
 export function stackOverlappingLabelsMap<T extends Record<string, any>>(
   items: T[],
   options: StackOverlappingLabelsOptions<T> & { idKey: keyof T },
 ): StackOverlappingLabelsMapReturnType {
-  const { idKey } = options;
+  const { idKey, startPositionKey, labelWidthKey, padding = 0 } = options;
   const groups = stackOverlappingLabels(items, options);
 
   const stackMap: StackOverlappingLabelsMapReturnType = {};
+
   for (const group of groups) {
-    let i = 0;
+    const laneEdges: number[] = [];
+
     for (const item of group) {
       const key = item[idKey];
       if (key === undefined || key === null) {
         throw new Error(
           `Invalid or missing idKey value for item: ${JSON.stringify(item)}`,
         );
       }
-      stackMap[key as string | number] = i;
-      if (item.assignment) i++;
+
+      const startPosition = item[startPositionKey];
+      const itemEnd = startPosition + item[labelWidthKey] + padding;
+
+      // Find the first lane whose right edge does not overlap this item is start.
+      // If no such lane exists, open a new one.
+      let assignedLane = laneEdges.findIndex((edge) => edge <= startPosition);
+      if (assignedLane === -1) {
+        assignedLane = laneEdges.length;
+      }
+
+      laneEdges[assignedLane] = itemEnd;
+      stackMap[key] = assignedLane;
     }
   }
+
   return stackMap;
 }
 
@@ -84,7 +102,7 @@ function stackOverlappingLabels<T extends Record<string, any>>(
       }
       currentGroup = [item];
     }
-    lastInPixel = startPosition + labelWidth + padding;
+    lastInPixel = Math.max(lastInPixel, startPosition + labelWidth + padding);
   }
 
   if (currentGroup.length > 0) {