refactor discourse createGraph (#1409)

This is a minor refactor to re-organize the createGraph function in the
Discourse plugin to use a class under the hood. Using a hidden class
makes sense because there is a fair bit of shared state that's needed
while creating the graph.

The proximate cause for this refactor is tha adding reference edges will
bloat the `addPost` section of the function, which was already a little
too complex. Simply shoving in more complexity would make it unweidy. So
I opted for this minor refactor. It's internal-only (no public APIs are
changed).

Test plan: `yarn test` passes. As noted, refactor is internal-only.

This is progress towards [Discourse reference and mention detection][1].

[1]: https://discourse.sourcecred.io/t/discourse-reference-mention-detection/270

src/plugins/discourse/createGraph.js

@@ -128,27 +128,59 @@ export function likesEdge(serverUrl: string, like: LikeAction): Edge {
 }
 
 export function createGraph(serverUrl: string, data: DiscourseData): Graph {
+  const gc = new _GraphCreator(serverUrl, data);
+  return gc.graph;
+}
+
+class _GraphCreator {
+  graph: Graph;
+  serverUrl: string;
+  data: DiscourseData;
+  topicIdToTitle: Map<TopicId, string>;
+
+  constructor(serverUrl: string, data: DiscourseData) {
     if (serverUrl.endsWith("/")) {
       throw new Error(`by convention, serverUrl should not end with /`);
     }
-  const g = new Graph();
-  const topicIdToTitle: Map<TopicId, string> = new Map();
+    this.serverUrl = serverUrl;
+    this.data = data;
+    this.graph = new Graph();
+    this.topicIdToTitle = new Map();
 
     for (const username of data.users()) {
-    g.addNode(userNode(serverUrl, username));
+      this.graph.addNode(userNode(serverUrl, username));
     }
 
     for (const topic of data.topics()) {
-    topicIdToTitle.set(topic.id, topic.title);
-    g.addNode(topicNode(serverUrl, topic));
-    g.addEdge(authorsTopicEdge(serverUrl, topic));
+      this.topicIdToTitle.set(topic.id, topic.title);
+      this.graph.addNode(topicNode(serverUrl, topic));
+      this.graph.addEdge(authorsTopicEdge(serverUrl, topic));
     }
 
     for (const post of data.posts()) {
-    const topicTitle = topicIdToTitle.get(post.topicId) || "[unknown topic]";
-    g.addNode(postNode(serverUrl, post, topicTitle));
-    g.addEdge(authorsPostEdge(serverUrl, post));
-    g.addEdge(topicContainsPostEdge(serverUrl, post));
+      this.addPost(post);
+    }
+
+    for (const like of data.likes()) {
+      this.graph.addEdge(likesEdge(serverUrl, like));
+    }
+  }
+
+  addPost(post: Post) {
+    const topicTitle =
+      this.topicIdToTitle.get(post.topicId) || "[unknown topic]";
+    this.graph.addNode(postNode(this.serverUrl, post, topicTitle));
+    this.graph.addEdge(authorsPostEdge(this.serverUrl, post));
+    this.graph.addEdge(topicContainsPostEdge(this.serverUrl, post));
+    this.maybeAddPostRepliesEdge(post);
+  }
+
+  /**
+   * Any post that is not the first post in the thread is a reply to some post.
+   * This method adds those reply edges. It is a bit hairy to work around unintuitive
+   * choices in the Discourse API.
+   */
+  maybeAddPostRepliesEdge(post: Post) {
     let replyToPostIndex = post.replyToPostIndex;
     if (replyToPostIndex == null && post.indexWithinTopic > 1) {
       // For posts that are a reply to the first posts (or, depending on how you look at it,
@@ -158,16 +190,13 @@ export function createGraph(serverUrl: string, data: DiscourseData): Graph {
       replyToPostIndex = 1;
     }
     if (replyToPostIndex != null) {
-      const basePostId = data.findPostInTopic(post.topicId, replyToPostIndex);
+      const basePostId = this.data.findPostInTopic(
+        post.topicId,
+        replyToPostIndex
+      );
       if (basePostId != null) {
-        g.addEdge(postRepliesEdge(serverUrl, post, basePostId));
+        this.graph.addEdge(postRepliesEdge(this.serverUrl, post, basePostId));
       }
     }
   }
-
-  for (const like of data.likes()) {
-    g.addEdge(likesEdge(serverUrl, like));
-  }
-
-  return g;
 }