Add methods for parsing Ids from GitHub urls (#388)

Test plan:
Unit tests added. Run `yarn travis`.

The GitHub regex code is inspired by work in #98

src/v3/plugins/github/urlIdParse.js

@@ -0,0 +1,44 @@
+// @flow
+
+// Includes base github url, and the repo owner and repo name with trailing slash
+const baseGithubRegex = /^https:\/\/github.com\/(?:[a-zA-Z0-9_-]+)\/(?:[a-zA-Z0-9_-]+)\//;
+
+export function reviewUrlToId(url: string): string {
+  const suffix = /pull\/\d+#pullrequestreview-(\d+)$/;
+  const regex = new RegExp(baseGithubRegex.source + suffix.source);
+  const result = regex.exec(url);
+  if (result == null) {
+    throw new Error(`Error parsing review url ${url}`);
+  }
+  return result[1];
+}
+
+export function issueCommentUrlToId(url: string): string {
+  const suffix = /issues\/\d+#issuecomment-(\d+)$/;
+  const regex = new RegExp(baseGithubRegex.source + suffix.source);
+  const result = regex.exec(url);
+  if (result == null) {
+    throw new Error(`Error parsing issue comment url ${url}`);
+  }
+  return result[1];
+}
+
+export function pullCommentUrlToId(url: string): string {
+  const suffix = /pull\/\d+#issuecomment-(\d+)$/;
+  const regex = new RegExp(baseGithubRegex.source + suffix.source);
+  const result = regex.exec(url);
+  if (result == null) {
+    throw new Error(`Error parsing pull comment url ${url}`);
+  }
+  return result[1];
+}
+
+export function reviewCommentUrlToId(url: string): string {
+  const suffix = /pull\/\d+#discussion_r(\d+)/;
+  const regex = new RegExp(baseGithubRegex.source + suffix.source);
+  const result = regex.exec(url);
+  if (result == null) {
+    throw new Error(`Error parsing review comment url ${url}`);
+  }
+  return result[1];
+}

src/v3/plugins/github/urlIdParse.test.js

@@ -0,0 +1,77 @@
+// @flow
+
+import {
+  issueCommentUrlToId,
+  pullCommentUrlToId,
+  reviewCommentUrlToId,
+  reviewUrlToId,
+} from "./urlIdParse";
+
+describe("plugins/github/urlToId", () => {
+  const issueComment =
+    "https://github.com/example-owner/example-repo0/issues/350#issuecomment-394939349";
+  const pullComment =
+    "https://github.com/example-owner/example-repo0/pull/363#issuecomment-395836900";
+  const reviewComment =
+    "https://github.com/example-owner/example-repo0/pull/380#discussion_r194816899";
+  const review =
+    "https://github.com/example-owner/example-repo0/pull/383#pullrequestreview-128199239";
+
+  describe("works correctly", () => {
+    it("for issueComment", () => {
+      expect(issueCommentUrlToId(issueComment)).toEqual("394939349");
+    });
+    it("for pullComment", () => {
+      expect(pullCommentUrlToId(pullComment)).toEqual("395836900");
+    });
+    it("for reviewComment", () => {
+      expect(reviewCommentUrlToId(reviewComment)).toEqual("194816899");
+    });
+    it("for review", () => {
+      expect(reviewUrlToId(review)).toEqual("128199239");
+    });
+  });
+  describe("errors on", () => {
+    const issueCommentNamed = {instance: issueComment, name: "issue comment"};
+    const reviewCommentNamed = {
+      instance: reviewComment,
+      name: "review comment",
+    };
+    const pullCommentNamed = {instance: pullComment, name: "pull comment"};
+    const reviewNamed = {instance: review, name: "review"};
+    const withWrongTypes = [
+      {
+        f: issueCommentUrlToId,
+        instances: [pullCommentNamed, reviewCommentNamed, reviewNamed],
+      },
+      {
+        f: pullCommentUrlToId,
+        instances: [issueCommentNamed, reviewCommentNamed, reviewNamed],
+      },
+      {
+        f: reviewCommentUrlToId,
+        instances: [pullCommentNamed, issueCommentNamed, reviewNamed],
+      },
+      {
+        f: reviewUrlToId,
+        instances: [pullCommentNamed, issueCommentNamed, reviewCommentNamed],
+      },
+    ];
+    const nullNamed = {instance: null, name: "null"};
+    const undefinedNamed = {instance: undefined, name: "undefined"};
+    const nonsenseNamed = {instance: "nonsense", name: "nonsense"};
+    const garbage = [nullNamed, undefinedNamed, nonsenseNamed];
+
+    withWrongTypes.forEach(({f, instances}) => {
+      describe(f.name, () => {
+        const examples = [...instances, ...garbage];
+        examples.forEach(({instance, name}) => {
+          it(`recieving ${name}`, () => {
+            // $ExpectFlowError
+            expect(() => f(instance)).toThrow("Error parsing");
+          });
+        });
+      });
+    });
+  });
+});