Add an equality function for `Graph` (#61)

Summary:
We need this for testing graph equality: deep-equality is not sufficient
because two graphs can be logically equal even if, say, two nodes are
added in different orders.

This commit adds a dependency on `lodash.isequal` for deep equality.

Test Plan:
New unit tests added. Run `yarn flow && yarn test`.

wchargin-branch: graph-equals

package.json

@@ -31,6 +31,7 @@
     "husky": "^0.14.3",
     "jest": "20.0.4",
     "lint-staged": "^6.1.1",
+    "lodash.isequal": "^4.5.0",
     "object-assign": "4.1.1",
     "postcss-flexbugs-fixes": "3.2.0",
     "postcss-loader": "2.0.8",

src/backend/graph.js

@@ -1,5 +1,7 @@
 // @flow
 
+import deepEqual from "lodash.isequal";
+
 export type Address = {|
   +repositoryName: string,
   +pluginName: string,
@@ -37,6 +39,12 @@ export class Graph {
     this._inEdges = {};
   }
 
+  equals(that: Graph): boolean {
+    return (
+      deepEqual(this._nodes, that._nodes) && deepEqual(this._edges, that._edges)
+    );
+  }
+
   addNode(node: Node<mixed>) {
     if (node == null) {
       throw new Error(`node is ${String(node)}`);

src/backend/graph.test.js

@@ -388,6 +388,46 @@ describe("graph", () => {
         }).toThrow(/no node for address/);
       });
     });
+
+    describe("#equals", () => {
+      it("returns true for identity-equal graphs", () => {
+        const g = advancedMealGraph();
+        expect(g.equals(g)).toBe(true);
+      });
+      it("returns true for deep-equal graphs", () => {
+        expect(advancedMealGraph().equals(advancedMealGraph())).toBe(true);
+      });
+      it("returns false when the LHS has nodes missing in the RHS", () => {
+        expect(advancedMealGraph().equals(simpleMealGraph())).toBe(false);
+      });
+      it("returns false when the RHS has nodes missing in the LHS", () => {
+        expect(simpleMealGraph().equals(advancedMealGraph())).toBe(false);
+      });
+      const extraNode1 = () => ({
+        address: makeAddress("octorok"),
+        payload: {},
+      });
+      const extraNode2 = () => ({
+        address: makeAddress("hinox"),
+        payload: {status: "sleeping"},
+      });
+      it("returns false when the LHS has edges missing in the RHS", () => {
+        const g1 = advancedMealGraph();
+        const g2 = advancedMealGraph().addNode(extraNode1());
+        expect(g1.equals(g2)).toBe(false);
+      });
+      it("returns false when the LHS has edges missing in the RHS", () => {
+        const g1 = advancedMealGraph().addNode(extraNode1());
+        const g2 = advancedMealGraph();
+        expect(g1.equals(g2)).toBe(false);
+      });
+      it("returns true when nodes are added in different orders", () => {
+        const g1 = new Graph().addNode(extraNode1()).addNode(extraNode2());
+        const g2 = new Graph().addNode(extraNode2()).addNode(extraNode1());
+        expect(g1.equals(g2)).toBe(true);
+        expect(g2.equals(g1)).toBe(true);
+      });
+    });
   });
 
   describe("string functions", () => {

yarn.lock

@@ -4163,6 +4163,10 @@ lodash.defaults@^4.2.0:
   version "4.2.0"
   resolved "https://registry.yarnpkg.com/lodash.defaults/-/lodash.defaults-4.2.0.tgz#d09178716ffea4dde9e5fb7b37f6f0802274580c"
 
+lodash.isequal@^4.5.0:
+  version "4.5.0"
+  resolved "https://registry.yarnpkg.com/lodash.isequal/-/lodash.isequal-4.5.0.tgz#415c4478f2bcc30120c22ce10ed3226f7d3e18e0"
+
 lodash.memoize@^4.1.2:
   version "4.1.2"
   resolved "https://registry.yarnpkg.com/lodash.memoize/-/lodash.memoize-4.1.2.tgz#bcc6c49a42a2840ed997f323eada5ecd182e0bfe"