add n_siblings member, and when splits are created / removed update the number of siblings for the remaining nodes

src/datastruct/split_tree.zig

@@ -507,12 +507,16 @@ pub fn SplitTree(comptime V: type) type {
             // We need to increase the reference count of all the nodes.
             try refNodes(gpa, nodes);
 
-            return .{
+            const result: Self = .{
                 .arena = arena,
                 .nodes = nodes,
                 // Splitting always resets zoom state.
                 .zoomed = null,
             };
+
+            result.updateNodesNumberSiblings();
+
+            return result;
         }
 
         /// Remove a node from the tree.
@@ -556,6 +560,8 @@ pub fn SplitTree(comptime V: type) type {
             // Increase the reference count of all the nodes.
             try refNodes(gpa, nodes);
 
+            result.updateNodesNumberSiblings();
+
             return result;
         }
 
@@ -865,6 +871,14 @@ pub fn SplitTree(comptime V: type) type {
             };
         }
 
+        /// Set the number of siblings for each split in the tree
+        fn updateNodesNumberSiblings(self: *const Self) void {
+            var it = self.iterator();
+            while (it.next()) |entry| {
+                entry.view.setNSiblings(self.nodes.len - 1);
+            }
+        }
+
         /// Spatial representation of the split tree. See spatial.
         pub const Spatial = struct {
             /// The slots of the spatial representation in the same order