Add tree utility and tests

api/src/tree.utility.test.ts

@@ -0,0 +1,35 @@
+import { describe, it } from 'node:test';
+import assert from 'node:assert/strict';
+import { buildCategoryTree, CategoryNode } from './tree.utility.ts';
+
+describe('Tree Utility - buildCategoryTree()', () => {
+  it('should transform a flat list of ltree rows into a nested JSON structure', () => {
+    const flatNodes: CategoryNode[] = [
+      { id: 1, name: 'Root', path: 'Top' },
+      { id: 3, name: 'Astronomy', path: 'Top.Science.Astronomy' },
+      { id: 2, name: 'Science', path: 'Top.Science' },
+      { id: 4, name: 'Gardening', path: 'Top.Hobbies.Gardening' },
+      { id: 5, name: 'Hobbies', path: 'Top.Hobbies' }
+    ];
+
+    const tree = buildCategoryTree(flatNodes);
+
+    // Verify root nodes count
+    assert.equal(tree.length, 1);
+    assert.equal(tree[0].name, 'Root');
+
+    // Verify second level ("Science" and "Hobbies")
+    const rootChildren = tree[0].children;
+    assert.equal(rootChildren.length, 2);
+    
+    const scienceNode = rootChildren.find(c => c.name === 'Science');
+    const hobbiesNode = rootChildren.find(c => c.name === 'Hobbies');
+    
+    assert.ok(scienceNode);
+    assert.ok(hobbiesNode);
+
+    // Verify third level deeply nested child ("Astronomy")
+    assert.equal(scienceNode.children.length, 1);
+    assert.equal(scienceNode.children[0].name, 'Astronomy');
+  });
+});

api/src/tree.utility.ts

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+export interface CategoryNode {
+  id: number;
+  name: string;
+  path: string;
+}
+
+export interface TreeResponseNode {
+  id: number;
+  name: string;
+  path: string;
+  children: TreeResponseNode[];
+}
+
+export function buildCategoryTree(nodes: CategoryNode[]): TreeResponseNode[] {
+  const tree: TreeResponseNode[] = [];
+  const nodeMap = new Map<string, TreeResponseNode>();
+
+  // Sort nodes by path depth (nlevel) implicitly by splitting path strings
+  // This guarantees parents are always processed before their children
+  const sortedNodes = [...nodes].sort((a, b) => a.path.split('.').length - b.path.split('.').length);
+
+  // Build the map and establish relationships
+  for (const node of sortedNodes) {
+    const treeNode: TreeResponseNode = { ...node, children: [] };
+    nodeMap.set(node.path, treeNode);
+
+    const pathParts = node.path.split('.');
+    
+    if (pathParts.length === 1) {
+      // Root level node (e.g., "Top")
+      tree.push(treeNode);
+    } else {
+      // Find parent path by dropping the last element (e.g., "Top.Science" -> "Top")
+      const parentPath = pathParts.slice(0, -1).join('.');
+      const parentNode = nodeMap.get(parentPath);
+
+      if (parentNode) {
+        parentNode.children.push(treeNode);
+      } else {
+        // Fallback: If parent isn't in the dataset, treat it as an orphan root
+        tree.push(treeNode);
+      }
+    }
+  }
+
+  return tree;
+}