Copy `github/findReferences` from v1 to v3 (#410)

The code will be refactored so that references are expressed in terms of
the GitHub node address code; the implementation is copied first so that
the review will be cleaner.

Test plan:
`yarn travis` passes.

src/v3/plugins/github/findReferences.js

@@ -0,0 +1,58 @@
+// @flow
+
+function findAllMatches(re: RegExp, s: string): any[] {
+  // modified from: https://stackoverflow.com/a/6323598
+  let m;
+  const matches = [];
+  do {
+    m = re.exec(s);
+    if (m) {
+      matches.push(m);
+    }
+  } while (m);
+  return matches;
+}
+
+export function findReferences(body: string): string[] {
+  // Note to maintainer: If it becomes necessary to encode references in a
+  // richer format, consider implementing the type signature described in
+  // https://github.com/sourcecred/sourcecred/pull/130#pullrequestreview-113849998
+  return [
+    ...findNumericReferences(body),
+    ...findGithubUrlReferences(body),
+    ...findUsernameReferences(body),
+  ];
+}
+
+function findNumericReferences(body: string): string[] {
+  return findAllMatches(/(?:\W|^)(#\d+)(?:\W|$)/g, body).map((x) => x[1]);
+}
+
+function findUsernameReferences(body: string): string[] {
+  return findAllMatches(/(?:\W|^)(@[a-zA-Z0-9-]+)(?:\W|$)/g, body).map(
+    (x) => x[1]
+  );
+}
+
+function findGithubUrlReferences(body: string): string[] {
+  const githubNamePart = /(?:[a-zA-Z0-9_-]+)/.source;
+  const urlRegex = new RegExp(
+    "" +
+      /(?:\W|^)/.source +
+      "(" +
+      /http(?:s)?:\/\/github.com\//.source +
+      githubNamePart +
+      "(?:" +
+      /\//.source +
+      githubNamePart +
+      /\/(?:issues|pull)\//.source +
+      /(?:\d+)/.source +
+      /(?:#(?:issue|issuecomment|pullrequestreview|discussion_r)-?(?:\d+))?/
+        .source +
+      ")?" +
+      ")" +
+      /(?:[^\w/]|$)/.source,
+    "gm"
+  );
+  return findAllMatches(urlRegex, body).map((match) => match[1]);
+}

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

@@ -0,0 +1,131 @@
+// @flow
+
+import {findReferences} from "./findReferences.js";
+
+describe("findReferences", () => {
+  it("finds no references when not present", () => {
+    expect(findReferences("foo bar bod boink")).toHaveLength(0);
+    expect(findReferences("")).toHaveLength(0);
+  });
+
+  it("finds trivial numeric references", () => {
+    expect(findReferences("#1, #2, and #3")).toEqual(["#1", "#2", "#3"]);
+  });
+
+  it("finds numeric references in a multiline string", () => {
+    const example = `
+    This is a multiline string.
+    It refers to #1. Oh, and to #2 too.
+    (#42 might be included too - who knows?)`;
+    expect(findReferences(example)).toEqual(["#1", "#2", "#42"]);
+  });
+
+  it("does not find bad references", () => {
+    expect(findReferences("foo#123 #124bar")).toHaveLength(0);
+  });
+
+  it("does not yet find concise cross-repo links", () => {
+    // The link below is valid, when we add cross-repo support we
+    // should fix this test case
+    expect(findReferences("sourcecred/sourcecred#12")).toHaveLength(0);
+  });
+
+  it("finds a trivial url reference", () => {
+    expect(
+      findReferences("https://github.com/sourcecred/sourcecred/issues/86")
+    ).toHaveLength(1);
+  });
+
+  it("finds url references", () => {
+    const example = `
+    A directly linked issue:
+https://github.com/sourcecred/example-github/issues/1
+
+    A directly linked issue with fragment:
+https://github.com/sourcecred/example-github/issues/1#issue-300934818
+
+    A directly linked pull request:
+https://github.com/sourcecred/example-github/pull/3
+
+    A directly linked pull request with fragment:
+https://github.com/sourcecred/example-github/pull/3#issue-171887741
+
+    A directly linked issue comment:
+https://github.com/sourcecred/example-github/issues/6#issuecomment-373768442
+
+    A directly linked pull request review:
+https://github.com/sourcecred/example-github/pull/5#pullrequestreview-100313899
+
+    A directly linked pull request review comment:
+https://github.com/sourcecred/example-github/pull/5#discussion_r171460198
+
+    A directly linked pull request comment:
+https://github.com/sourcecred/example-github/pull/3#issuecomment-369162222
+    `;
+
+    const expected = [
+      "https://github.com/sourcecred/example-github/issues/1",
+      "https://github.com/sourcecred/example-github/issues/1#issue-300934818",
+      "https://github.com/sourcecred/example-github/pull/3",
+      "https://github.com/sourcecred/example-github/pull/3#issue-171887741",
+      "https://github.com/sourcecred/example-github/issues/6#issuecomment-373768442",
+      "https://github.com/sourcecred/example-github/pull/5#pullrequestreview-100313899",
+      "https://github.com/sourcecred/example-github/pull/5#discussion_r171460198",
+      "https://github.com/sourcecred/example-github/pull/3#issuecomment-369162222",
+    ];
+
+    expect(findReferences(example)).toEqual(expected);
+  });
+
+  it("doesn't find urls mangled with word characters", () => {
+    expect(
+      findReferences("foohttps://github.com/sourcecred/sourcecred/pull/94")
+    ).toHaveLength(0);
+
+    expect(
+      findReferences("https://github.com/sourcecred/sourcecred/pull/94foo")
+    ).toHaveLength(0);
+
+    expect(
+      findReferences("(https://github.com/sourcecred/sourcecred/pull/94)")
+    ).toHaveLength(1);
+  });
+
+  it("allows but excludes leading and trailing punctuation", () => {
+    const base = "https://github.com/sourcecred/sourcecred/pull/94";
+    expect(findReferences(`!${base}`)).toEqual([base]);
+    expect(findReferences(`${base}!`)).toEqual([base]);
+    expect(findReferences(`!${base}!`)).toEqual([base]);
+  });
+
+  it("finds username references", () => {
+    expect(findReferences("hello to @wchargin from @decentralion!")).toEqual([
+      "@wchargin",
+      "@decentralion",
+    ]);
+  });
+
+  it("finds usernames with hypens and numbers", () => {
+    expect(findReferences("@paddy-hack and @0x00 are valid usernames")).toEqual(
+      ["@paddy-hack", "@0x00"]
+    );
+  });
+
+  it("finds username references by exact url", () => {
+    expect(findReferences("greetings https://github.com/wchargin")).toEqual([
+      "https://github.com/wchargin",
+    ]);
+  });
+
+  it("finds a mix of reference types", () => {
+    expect(
+      findReferences(
+        "@wchargin commented on #125, eg https://github.com/sourcecred/sourcecred/pull/125#pullrequestreview-113402856"
+      )
+    ).toEqual([
+      "#125",
+      "https://github.com/sourcecred/sourcecred/pull/125#pullrequestreview-113402856",
+      "@wchargin",
+    ]);
+  });
+});