feat(Tree): implement sumOnProperty()

This commit implements the sumOnProperty() function to sum tree nodes
on a given property.  Parent nodes are given the values of the sums of
their children.

server/lib/Tree.js

@@ -59,20 +59,46 @@ function flatten(tree, depth, pruneChildren = true) {
   }, []);
 }
 
+/**
+ * @function sumOnProperty
+ *
+ * @description
+ * Computes the value of all parent nodes in the tree as the sum of the values
+ * of their children for a given property.
+ */
+function sumOnProperty(node, prop) {
+  if (node.children.length > 0) {
+    // recursively compute the value of node[prop] by summing all child[prop]s
+    node[prop] = node.children.reduce((value, child) =>
+      value + sumOnProperty(child, prop), 0);
+  }
+
+  return node[prop];
+}
 
 class Tree {
-  constructor(data = [], parentKey = 'parent', rootId = 0) {
+  constructor(data = [], options = {
+    parentKey : 'parent',
+    rootId : 0,
+  }) {
     this._data = data;
-    this._parentKey = parentKey;
-    this._rootId = rootId;
+
+    this._parentKey = options.parentKey;
+    this._rootId = options.rootId;
 
     // build the tree with the provided root id and parentKey
-    this._tree = buildTreeFromArray(_.cloneDeep(data), rootId, parentKey);
+    this._tree = buildTreeFromArray(_.cloneDeep(data), this._rootId, this._parentKey);
   }
 
   toArray() {
     return flatten(this._tree);
   }
+
+  sumOnProperty(prop) {
+    this._tree.forEach(node => {
+      sumOnProperty(node, prop);
+    });
+  }
 }
 
 module.exports = Tree;

test/server-unit/Tree.spec.js

@@ -7,8 +7,8 @@ function TreeUnitTests() {
    *            ROOT
    *         /    |    \
    *       id:1  id:4  id:6
-   *       /             \
-   *    id:2            id:3
+   *       /           /   \
+   *    id:2          id:7   id:3
    *     /
    *   id:5
    */
@@ -21,15 +21,26 @@ function TreeUnitTests() {
   }, {
     id : 3,
     parent : 6,
+    valueA : 10,
+    valueB : 2,
   }, {
     id : 4,
     parent : 0,
+    valueA : 30,
+    valueB : 4,
   }, {
     id : 5,
     parent : 2,
+    valueA : 9,
+    valueB : 7,
   }, {
     id : 6,
     parent : 0,
+  }, {
+    id : 7,
+    parent : 6,
+    valueA : 10,
+    valueB : 19,
   }];
 
   it('#constructor() should populate private variables', () => {
@@ -64,11 +75,11 @@ function TreeUnitTests() {
     expect(node4.children).to.have.length(0);
   });
 
-  it('#constructor() node id:6 should have one child', () => {
+  it('#constructor() node id:6 should have two children', () => {
     const tree = new Tree(nodes)._tree;
     const node6 = tree[2];
     expect(node6.id).to.equal(6);
-    expect(node6.children).to.have.length(1);
+    expect(node6.children).to.have.length(2);
   });
 
   it('#toArray() should return an array', () => {
@@ -82,6 +93,27 @@ function TreeUnitTests() {
       expect(node).to.have.property('depth');
     });
   });
+
+  it('#sumOnProperty() should compute the balances of level 1 nodes', () => {
+    const tree = new Tree(nodes);
+    const subTree = tree._tree;
+
+    // first level should not be defined
+    expect(subTree[0].valueA).to.be.undefined;
+    expect(subTree[0].valueB).to.be.undefined;
+
+    // this is a level 1 leaf node, so its values are known.
+    expect(subTree[1].valueA).to.equal(30);
+    expect(subTree[1].valueB).to.equal(4);
+
+    tree.sumOnProperty('valueA');
+    expect(subTree[0].valueA).to.equal(9);
+    expect(subTree[0].valueB).to.be.undefined;
+
+    // this condition sums multiple leaves
+    tree.sumOnProperty('valueB');
+    expect(subTree[2].valueB).to.equal(21); // 2 + 19
+  });
 }
 
 describe('Tree.js', TreeUnitTests);