Fix LinkStiffness calculation

Author: li3zhen1

Examples/ForceDirectedGraphExample/ForceDirectedGraphExample/MyRing.swift

@@ -26,27 +26,27 @@ struct MyRing: View {
                 NodeMark(id: 3 * i + 0)
                     .symbolSize(radius: 6.0)
                     .foregroundStyle(.green)
-                    .stroke(3*i+0 == draggingNodeID ? .black : .clear, Self.storkeStyle)
+                    .stroke(3*i+0 == draggingNodeID ? .secondary : .clear, Self.storkeStyle)
 
                 NodeMark(id: 3 * i + 1)
                     .symbol(.pentagon)
                     .symbolSize(radius:10)
                     .foregroundStyle(.blue)
-                    .stroke(3*i+1 == draggingNodeID ? .black : .clear, Self.storkeStyle)
+                    .stroke(3*i+1 == draggingNodeID ? .secondary : .clear, Self.storkeStyle)
 
                 NodeMark(id: 3 * i + 2)
                     .symbol(.circle)
                     .symbolSize(radius:6.0)
                     .foregroundStyle(.yellow)
-                    .stroke(3*i+2 == draggingNodeID ? .black : .clear, Self.storkeStyle)
+                    .stroke(3*i+2 == draggingNodeID ? .secondary : .clear, Self.storkeStyle)
 
                 LinkMark(from: 3 * i + 0, to: 3 * i + 1)
                 LinkMark(from: 3 * i + 1, to: 3 * i + 2)
                 LinkMark(from: 3 * i + 0, to: 3 * ((i + 1) % 20) + 0)
                 LinkMark(from: 3 * i + 1, to: 3 * ((i + 1) % 20) + 1)
                 LinkMark(from: 3 * i + 2, to: 3 * ((i + 1) % 20) + 2)
             }
-            .stroke(.black, Self.storkeStyle)
+            .stroke(.secondary, Self.storkeStyle)
 
         } force: {
             .manyBody(strength: -15)

Sources/ForceSimulation/Forces/LinkForce.swift

@@ -22,7 +22,7 @@ extension Kinetics {
                                 lookup.count[l.source, default: 0],
                                 lookup.count[l.target, default: 0]
                             )
-                        )+1)
+                        ))
                 }
             }
         }