feat: impl jaccardSimilarity for improved cross-post detection

src/events/spam-detection/constants.ts

@@ -3,3 +3,5 @@ import { rules } from "./rules-config.js";
 export const MAX_RULE_TIMEFRAME = Math.max(
   ...rules.filter((rule) => rule.type !== "contentBased").map((rule) => rule.timeframe)
 );
+
+export const MESSAGE_SIMILARITY_THRESHOLD = 0.8;

src/events/spam-detection/detectors.ts

@@ -1,5 +1,6 @@
 import type { Message } from "discord.js";
-import { replaceSpoilerHack, stripCode } from "../../utils/messages.js";
+import { jaccardSimilarity, replaceSpoilerHack, stripCode } from "../../utils/messages.js";
+import { MESSAGE_SIMILARITY_THRESHOLD } from "./constants.js";
 
 export const containsLink = (message: Message): boolean => {
   const withoutCode = stripCode(message.content);
@@ -24,11 +25,18 @@ export const containsSpoilerHack = (message: Message) => {
 };
 
 export const isDuplicate = (message: Message, oldMessage: Message) => {
-  return message.content.toLowerCase().trim() === oldMessage.content.toLowerCase().trim();
+  // cheaper comparison first
+  const a = message.content.toLowerCase().trim();
+  const b = oldMessage.content.toLowerCase().trim();
+  if (a === b) {
+    return true;
+  }
+  // followed by jaccard for catching reordered/slightly altered messages with high similarity
+  return jaccardSimilarity(a, b) > MESSAGE_SIMILARITY_THRESHOLD;
 };
 
 export const isCrossPost = (message: Message, oldMessage: Message) => {
-  return isDuplicate(message, oldMessage) && message.channelId !== oldMessage.channelId;
+  return message.channelId !== oldMessage.channelId && isDuplicate(message, oldMessage);
 };
 
 export const anyMessage = () => true;

src/utils/messages.test.ts

@@ -1,6 +1,6 @@
 import assert from "node:assert";
 import { describe, it } from "node:test";
-import { replaceSpoilerHack, stripCode, stripEmoji } from "./messages.js";
+import { replaceSpoilerHack, stripCode, stripEmoji, jaccardSimilarity } from "./messages.js";
 
 describe("utils/messages -> stripCode", () => {
   it("should remove inline code blocks", () => {
@@ -71,3 +71,36 @@ describe("utils/messages -> replaceSpoilerHack", () => {
     assert.strictEqual(actual, expected);
   });
 });
+
+describe("jaccardSimilarity - crosspost detection", () => {
+  it("catches identical self-promotion spam", () => {
+    const msg1 = "Check out my new portfolio website! Built with React and Tailwind";
+    const msg2 = "Check out my new portfolio website! Built with React and Tailwind";
+    const actual = jaccardSimilarity(msg1, msg2);
+
+    assert.strictEqual(actual, 1);
+  });
+
+  it("catches copy-paste spam with minor punctuation differences", () => {
+    const msg1 = "hey guys check out my new website!";
+    const msg2 = "hey guys, check out my new website";
+    const actual = jaccardSimilarity(msg1, msg2);
+
+    assert.strictEqual(actual, 1);
+  });
+
+  it("catches reordered messages", () => {
+    const msg1 = "I just launched my SaaS app! Check it out and let me know what you think";
+    const msg2 = "Check it out and let me know what you think! I just launched my SaaS app";
+    const actual = jaccardSimilarity(msg1, msg2);
+    assert.strictEqual(actual, 1);
+  });
+
+  it("does not flag similar but different questions", () => {
+    const msg1 = "How do I center a div in CSS?";
+    const msg2 = "How do I align a div to the right in CSS?";
+    const actual = jaccardSimilarity(msg1, msg2); // 0.5833333333333334
+
+    assert.ok(actual > 0.5 && actual < 0.8);
+  });
+});

src/utils/messages.ts

@@ -24,3 +24,23 @@ export const stripEmoji = (content: string): string => content.replace(/:\w+:/g,
 export function replaceSpoilerHack(messageContent: string | null, replacement = "[...]") {
   return (messageContent ?? "").replace(/(\|\|\u200b\|\|)+/g, replacement);
 }
+
+// https://en.wikipedia.org/wiki/Jaccard_index
+export function jaccardSimilarity(text1: string, text2: string): number {
+  const words1 = new Set(normalizeText(text1));
+  const words2 = new Set(normalizeText(text2));
+
+  const intersection = words1.intersection(words2);
+  const union = words1.union(words2);
+
+  return union.size === 0 ? 0 : intersection.size / union.size;
+}
+
+const normalizeText = (text: string) => {
+  return text
+    .toLowerCase()
+    .replace(/[^\w\s]/g, "") // Remove punctuation & symbols
+    .trim()
+    .split(/\s+/)
+    .filter(Boolean);
+};

tsconfig.json

@@ -1,7 +1,7 @@
 {
 	"compilerOptions": {
 		"target": "ES2022",
-		"lib": ["ES2022"],
+		"lib": ["ESNext"],
 		"module": "nodenext",
 		"moduleResolution": "nodenext",
 		"allowSyntheticDefaultImports": true,