Add a copy method to `analysis/weights`

It's very simple: a method that creates a copy of a `Weights`.
While writing this, I realized I should probably refactor the weights
module so that it exports a class rather than a bunch of methods
operating on a data structure. It would just be a cleaner API. But I'm
leaving that for another day.

Test plan: Unit tests added.

src/analysis/weights.js

@@ -48,6 +48,14 @@ export function defaultWeights(): Weights {
   };
 }
 
+export function copy(w: Weights): Weights {
+  return {
+    nodeTypeWeights: new Map(w.nodeTypeWeights),
+    edgeTypeWeights: new Map(w.edgeTypeWeights),
+    nodeManualWeights: new Map(w.nodeManualWeights),
+  };
+}
+
 export type WeightsJSON = Compatible<{|
   +nodeTypeWeights: {[NodeAddressT]: NodeWeight},
   +edgeTypeWeights: {[EdgeAddressT]: EdgeWeight},

src/analysis/weights.test.js

@@ -2,9 +2,20 @@
 
 import stringify from "json-stable-stringify";
 import {NodeAddress, EdgeAddress} from "../core/graph";
-import {toJSON, fromJSON, defaultWeights} from "./weights";
+import {toJSON, fromJSON, defaultWeights, copy} from "./weights";
 
 describe("analysis/weights", () => {
+  it("copy makes a copy", () => {
+    const w = defaultWeights();
+    const w1 = copy(w);
+    w1.nodeTypeWeights.set(NodeAddress.empty, 33);
+    w1.edgeTypeWeights.set(EdgeAddress.empty, {forwards: 34, backwards: 39});
+    w1.nodeManualWeights.set(NodeAddress.empty, 35);
+    expect(w1).not.toEqual(w);
+    expect(w1.nodeTypeWeights).not.toEqual(w.nodeTypeWeights);
+    expect(w1.edgeTypeWeights).not.toEqual(w.edgeTypeWeights);
+    expect(w1.nodeManualWeights).not.toEqual(w.nodeManualWeights);
+  });
   describe("toJSON/fromJSON", () => {
     it("works for the default weights", () => {
       const weights = defaultWeights();