UniRef: add GithubReferenceDetector (#1543)

As a RelationalView is not designed for multiple repositories, we
should implement our own merging of mappings obtained from
RelationalViews.

fromRelationalViews is a factory function which does this for us.
And by accepting an array of RelationalViews it's more apparent
it should be used this way.

src/plugins/github/referenceDetector.js

@@ -0,0 +1,37 @@
+// @flow
+
+import dedent from "../../util/dedent";
+import {type NodeAddressT, NodeAddress} from "../../core/graph";
+import {MappedReferenceDetector, type URL} from "../../core/references";
+import {RelationalView} from "./relationalView";
+
+/**
+ * Builds a GithubReferenceDetector using multiple RelationalView.
+ * As RelationalView should only be used for one repository at a time, you will
+ * commonly want to compose several of them into one GithubReferenceDetector.
+ *
+ * Note: duplicates are normally expected. However for any URL, the corresponding
+ * NodeAddressT should be the same, or we'll throw an error.
+ */
+export function fromRelationalViews(
+  views: $ReadOnlyArray<RelationalView>
+): GithubReferenceDetector {
+  const map: Map<URL, NodeAddressT> = new Map();
+  for (const view of views) {
+    for (const [url, addr] of view.urlReferenceMap().entries()) {
+      const existing = map.get(url);
+      if (existing && existing != addr) {
+        throw new Error(dedent`\
+          An entry for ${url} already existed, but with a different NodeAddressT.
+          This is probably a bug with SourceCred. Please report it on GitHub.
+          Old: ${NodeAddress.toString(existing)}
+          New: ${NodeAddress.toString(addr)}
+        `);
+      }
+      map.set(url, addr);
+    }
+  }
+  return new GithubReferenceDetector(map);
+}
+
+export const GithubReferenceDetector = MappedReferenceDetector;

src/plugins/github/referenceDetector.test.js

@@ -0,0 +1,75 @@
+// @flow
+
+import {exampleRelationalView} from "./example/example";
+import {RelationalView} from "./relationalView";
+import {
+  GithubReferenceDetector,
+  fromRelationalViews,
+} from "./referenceDetector";
+import {MappedReferenceDetector} from "../../core/references";
+import {NodeAddress} from "../../core/graph";
+import dedent from "../../util/dedent";
+
+describe("plugins/github/referenceDetector", () => {
+  describe("GithubReferenceDetector", () => {
+    it("should be a MappedReferenceDetector", () => {
+      // Given
+      const map = new Map();
+
+      // When
+      const refs = new GithubReferenceDetector(map);
+
+      // Then
+      expect(refs).toBeInstanceOf(MappedReferenceDetector);
+    });
+  });
+
+  describe("fromRelationalViews", () => {
+    it("should use urlReferenceMap to create the instance", () => {
+      // Given
+      const rv = new RelationalView();
+      const urlReferenceMap = jest.spyOn(rv, "urlReferenceMap");
+
+      // When
+      const refs = fromRelationalViews([rv, rv]);
+
+      // Then
+      expect(refs).toBeInstanceOf(MappedReferenceDetector);
+      expect(urlReferenceMap).toBeCalledTimes(2);
+    });
+
+    it("should deduplicate silently given the same entries", () => {
+      // Given
+      const rv = exampleRelationalView();
+
+      // When
+      const refs = fromRelationalViews([rv, rv]);
+
+      // Then
+      expect(refs.map).toEqual(rv.urlReferenceMap());
+    });
+
+    it("should throw when encountering duplicate keys with different values", () => {
+      // Given
+      const url = "http://foo.bar";
+      const nodeA = NodeAddress.fromParts(["test", "A"]);
+      const nodeB = NodeAddress.fromParts(["test", "B"]);
+      const rv1 = new RelationalView();
+      const rv2 = new RelationalView();
+      const rv1Spy = jest.spyOn(rv1, "urlReferenceMap");
+      const rv2Spy = jest.spyOn(rv2, "urlReferenceMap");
+      rv1Spy.mockReturnValue(new Map([[url, nodeA]]));
+      rv2Spy.mockReturnValue(new Map([[url, nodeB]]));
+
+      // When
+      const fn = () => fromRelationalViews([rv1, rv2]);
+
+      // Then
+      expect(fn).toThrow(dedent`\
+        An entry for http://foo.bar already existed, but with a different NodeAddressT.
+        This is probably a bug with SourceCred. Please report it on GitHub.
+        Old: NodeAddress["test","A"]
+        New: NodeAddress["test","B"]`);
+    });
+  });
+});