Restructure code

mixins/tree-flatten.js

@@ -0,0 +1,38 @@
+export const TreeFlattenMixin = (superClass) => class TreeFlattenMixin extends superClass {
+  _flattenNodes() {
+    const results = [];
+    const roots = Array.isArray(this.roots) ? this.roots : [];
+
+    const walk = (nodes, depth, path) => {
+      if (!Array.isArray(nodes)) return;
+      nodes.forEach((node, index) => {
+        const slug = node?.slug ?? `${index}`;
+        const nextPath = [...path, slug];
+        const states = Array.isArray(node?.states) ? node.states : [];
+        const hasChildren = this._isExpandableNode(node);
+        const expanded = states.includes('expanded');
+
+        results.push({
+          node,
+          depth,
+          path: nextPath,
+          hasChildren,
+          expanded,
+          key: nextPath.join('/'),
+        });
+
+        if (hasChildren && expanded && Array.isArray(node?.children) && node.children.length > 0) {
+          walk(node.children, depth + 1, nextPath);
+        }
+      });
+    };
+
+    walk(roots, 0, []);
+    return results;
+  }
+
+  _findFlatIndexByKey(key) {
+    const items = Array.isArray(this._flatItems) ? this._flatItems : [];
+    return items.findIndex((it) => it?.key === key);
+  }
+};