feat: add shared unstable listing classifier

packages/core/test/unstable-listing-mode.test.ts

@@ -0,0 +1,84 @@
+import { describe, expect, test } from "bun:test";
+import type { ListingDetails } from "../src/types/common";
+import { classifyUnstableListings } from "../src/utils/unstable";
+
+interface TestListing extends ListingDetails {
+  id: string;
+}
+
+function makeListing(id: string, cents: number): TestListing {
+  return {
+    id,
+    url: `https://example.com/${id}`,
+    title: id,
+    listingPrice: {
+      amountFormatted: `$${(cents / 100).toFixed(2)}`,
+      cents,
+      currency: "CAD",
+    },
+    listingType: "test",
+    listingStatus: "active",
+  };
+}
+
+describe("classifyUnstableListings", () => {
+  test("moves listings below 80% of median into unstableResults", () => {
+    const listings = [
+      makeListing("stable-1", 100_00),
+      makeListing("stable-2", 110_00),
+      makeListing("unstable", 70_00),
+    ];
+
+    const buckets = classifyUnstableListings(listings);
+
+    expect(buckets.results.map((listing) => listing.id)).toEqual(["stable-1", "stable-2"]);
+    expect(buckets.unstableResults.map((listing) => listing.id)).toEqual(["unstable"]);
+  });
+
+  test("uses the midpoint median for even-sized priced inputs", () => {
+    const listings = [
+      makeListing("low", 79_00),
+      makeListing("mid-low", 100_00),
+      makeListing("mid-high", 120_00),
+      makeListing("high", 140_00),
+    ];
+
+    const buckets = classifyUnstableListings(listings);
+
+    expect(buckets.results.map((listing) => listing.id)).toEqual(["mid-low", "mid-high", "high"]);
+    expect(buckets.unstableResults.map((listing) => listing.id)).toEqual(["low"]);
+  });
+
+  test("keeps non-positive prices in results and excludes them from the median input", () => {
+    const listings = [
+      makeListing("zero", 0),
+      makeListing("negative", -500),
+      makeListing("stable-1", 100_00),
+      makeListing("stable-2", 120_00),
+      makeListing("unstable", 70_00),
+    ];
+
+    const buckets = classifyUnstableListings(listings);
+
+    expect(buckets.results.map((listing) => listing.id)).toEqual([
+      "zero",
+      "negative",
+      "stable-1",
+      "stable-2",
+    ]);
+    expect(buckets.unstableResults.map((listing) => listing.id)).toEqual(["unstable"]);
+  });
+
+  test("returns all listings in results when fewer than two valid prices are present", () => {
+    const listings = [
+      makeListing("zero", 0),
+      makeListing("negative", -100),
+      makeListing("only-valid", 150_00),
+    ];
+
+    const buckets = classifyUnstableListings(listings);
+
+    expect(buckets.results.map((listing) => listing.id)).toEqual(["zero", "negative", "only-valid"]);
+    expect(buckets.unstableResults).toEqual([]);
+  });
+});