bug: separate strokes if erasers do not match

Memola/Canvas/Elements/Core/ElementGroup.swift

@@ -8,23 +8,47 @@
 import Foundation
 
 final class ElementGroup {
+    #if DEBUG
+    let id = UUID()
+    #endif
     var elements: [Element] = []
     var type: ElementGroupType
+    var eraserStrokes: Set<EraserStroke> = []
 
     init(_ element: Element) {
         elements = [element]
         type = element.elementGroupType
+        collect(element)
     }
 
     var isEmpty: Bool { elements.isEmpty }
 
     func add(_ element: Element) {
         elements.append(element)
+        collect(element)
+    }
+
+    private func collect(_ element: Element) {
+        if let stroke = element.stroke(as: PenStroke.self) {
+            eraserStrokes.formUnion(stroke.eraserStrokes)
+        }
+        #if DEBUG
+        NSLog("[Memola] - \(id) eraser count - \(eraserStrokes.count)")
+        #endif
     }
 
     func isSameElement(_ element: Element) -> Bool {
         guard let last = elements.last else { return false }
-        return element ^= last
+        guard element ^= last else {
+            return false
+        }
+        if let stroke = element.stroke(as: PenStroke.self) {
+            if eraserStrokes.count <= stroke.eraserStrokes.count {
+                return true
+            }
+            return eraserStrokes.intersection(stroke.eraserStrokes).count == eraserStrokes.count
+        }
+        return true
     }
 
     func getPenStyle() -> PenStyle? {