Initiatives: create InitiativeId concept (#1646)

Previously we were relying on the `Initiative.tracker` to define the
address of an Initiative. Based on feedback at #1643 we want to remove
trackers. So we'll need a replacement ID.

This will enforce a new InitiativeId convention. As well as how to derive
a NodeAddressT from it. In a follow-up we'll remove the tracker concept.

src/plugins/initiatives/initiative.js

@@ -1,9 +1,26 @@
 // @flow
 
-import {type NodeAddressT} from "../../core/graph";
+import {type NodeAddressT, NodeAddress} from "../../core/graph";
+import {initiativeNodeType} from "./declaration";
 
 export type URL = string;
 
+// Composite ID, used as input for NodeAddressT.
+export opaque type InitiativeId = string[];
+
+// Enforce that each ID has at least a sub-type and 1..n components.
+export function createId(
+  subType: string,
+  requiredId: string,
+  ...rest: string[]
+): InitiativeId {
+  return [subType, requiredId, ...rest];
+}
+
+export function addressFromId(id: InitiativeId): NodeAddressT {
+  return NodeAddress.append(initiativeNodeType.prefix, ...id);
+}
+
 /**
  * An intermediate representation of an Initiative.
  *

src/plugins/initiatives/initiative.test.js

@@ -0,0 +1,49 @@
+// @flow
+
+import {NodeAddress} from "../../core/graph";
+import {createId, addressFromId} from "./initiative";
+
+describe("plugins/initiatives/initiative", () => {
+  describe("createId", () => {
+    it("should require a subtype as first argument", () => {
+      // $ExpectFlowError
+      createId();
+    });
+
+    it("should require at least one ID component as second argument", () => {
+      // $ExpectFlowError
+      createId("SUBTYPE");
+    });
+
+    it("should work given at least a subtype and one ID component", () => {
+      const id = createId("SUBTYPE", "123");
+      expect(id).toEqual(["SUBTYPE", "123"]);
+    });
+
+    it("should work with an arbitrary number of ID components", () => {
+      const id = createId("SUBTYPE", "123", "456", "789");
+      expect(id).toEqual(["SUBTYPE", "123", "456", "789"]);
+    });
+  });
+
+  describe("addressFromId", () => {
+    it("should add the correct prefix to an InitiativeId", () => {
+      // Given
+      const id = createId("EXAMPLE", "123");
+
+      // When
+      const address = addressFromId(id);
+
+      // Then
+      expect(address).toEqual(
+        NodeAddress.fromParts([
+          "sourcecred",
+          "initiatives",
+          "initiative",
+          "EXAMPLE",
+          "123",
+        ])
+      );
+    });
+  });
+});