fix: correctly truncate seenEvents (#282)

The logic to truncate a set was incorrect and could produce and almost
inifinite loop as it was decreasing the counter rather than increasing
it. We extracted the logic to its own module, where it can be tested.

CHANGELOG.md

@@ -6,6 +6,12 @@ The format is loosely based on [Keep a Changelog](https://keepachangelog.com/en/
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 We follow the format used by [Open Telemetry](https://github.com/open-telemetry/opentelemetry-python/blob/main/CHANGELOG.md).
 
+## Unreleased
+
+### Fixed
+- Fix truncation of `seenEvents`
+  ([#282](https://github.com/Topsort/analytics.js/pull/282))
+
 ## Version 2.3.1 (2024-04-11)
 
 Patch release to fix tags

src/detector.ts

@@ -1,6 +1,7 @@
 import { type Config, Entity, TopsortEvent, reportEvent } from "@topsort/sdk";
 import { version } from "../package.json";
 import { ProcessorResult, Queue } from "./queue";
+import { truncateSet } from "./set";
 import { BidStore } from "./store";
 
 const MAX_EVENTS_SIZE = 2500;
@@ -181,13 +182,7 @@ function logEvent(info: ProductEvent, node: Node) {
     return;
   }
   seenEvents.add(id);
-  if (seenEvents.size > MAX_EVENTS_SIZE) {
-    const iterator = seenEvents.values();
-    for (let i = 0; i < seenEvents.size - MAX_EVENTS_SIZE; --i) {
-      iterator.next();
-    }
-    seenEvents = new Set(iterator);
-  }
+  seenEvents = truncateSet(seenEvents, MAX_EVENTS_SIZE);
   queue.append(info);
 
   // Raise a custom event, so that clients can trigger their own logic.

src/set.test.ts

@@ -0,0 +1,17 @@
+import { describe, expect, test } from "vitest";
+import { truncateSet } from "./set";
+
+describe("truncateSet", () => {
+  test("original set below maxSize", () => {
+    const s = new Set([1, 2, 3]);
+    expect(truncateSet(s, 4)).toBe(s);
+  });
+
+  test("resulting set keeps last inserted items", () => {
+    const s = new Set([1, 2, 3]);
+    s.add(4);
+    expect(truncateSet(s, 3)).toEqual(new Set([2, 3, 4]));
+    expect(truncateSet(s, 2)).toEqual(new Set([3, 4]));
+    expect(truncateSet(s, 1)).toEqual(new Set([4]));
+  });
+});

src/set.ts

@@ -0,0 +1,10 @@
+export function truncateSet<T>(set: Set<T>, maxSize: number): Set<T> {
+  if (set.size <= maxSize) {
+    return set;
+  }
+  const iterator = set.values();
+  for (let i = 0; i < set.size - maxSize; ++i) {
+    iterator.next();
+  }
+  return new Set(iterator);
+}