artifacts/api-server/src/lib/__tests__/shadow-active-pairings.unit.test.ts

/**
 * Unit tests for `summariseActivePairings` — the pure helper that
 * groups shadow sample rows by `activeVariantId` to power the
 * "compared against active" badge in the admin Evolution UI.
 *
 * Pure-function only: lives in `shadow-core.ts` so the test stays
 * free of DB / Express / events imports.
 *
 * Run with:
 *   node --test artifacts/api-server/src/lib/__tests__/shadow-active-pairings.unit.test.ts
 */
import { test } from "node:test";
import assert from "node:assert/strict";

import { summariseActivePairings } from "../evolution/shadow-core.ts";

test("summariseActivePairings: empty input → empty output", () => {
  assert.deepEqual(summariseActivePairings([]), []);
});

test("summariseActivePairings: single pairing → one row with null timestamps when createdAt absent", () => {
  const out = summariseActivePairings([
    { activeVariantId: "v_active_a" },
    { activeVariantId: "v_active_a" },
    { activeVariantId: "v_active_a" },
  ]);
  assert.deepEqual(out, [
    {
      activeVariantId: "v_active_a",
      sampleCount: 3,
      firstSampleAt: null,
      lastSampleAt: null,
    },
  ]);
});

test("summariseActivePairings: multiple pairings sorted by descending count", () => {
  const out = summariseActivePairings([
    { activeVariantId: "v_active_b" },
    { activeVariantId: "v_active_a" },
    { activeVariantId: "v_active_a" },
    { activeVariantId: "v_active_a" },
    { activeVariantId: "v_active_b" },
  ]);
  assert.deepEqual(out, [
    {
      activeVariantId: "v_active_a",
      sampleCount: 3,
      firstSampleAt: null,
      lastSampleAt: null,
    },
    {
      activeVariantId: "v_active_b",
      sampleCount: 2,
      firstSampleAt: null,
      lastSampleAt: null,
    },
  ]);
});

test("summariseActivePairings: ties broken by lexicographic variant id", () => {
  // Deterministic ordering matters because the admin UI keys list
  // entries by activeVariantId — flapping the order between renders
  // would defeat React's reconciliation and cause the warning badge to
  // jitter when two actives share an equal sample count.
  const out = summariseActivePairings([
    { activeVariantId: "v_zzz" },
    { activeVariantId: "v_aaa" },
    { activeVariantId: "v_zzz" },
    { activeVariantId: "v_aaa" },
  ]);
  assert.equal(out.length, 2);
  assert.equal(out[0]!.activeVariantId, "v_aaa");
  assert.equal(out[1]!.activeVariantId, "v_zzz");
});

test("summariseActivePairings: caller cannot mutate the input array", () => {
  // Defensive: the helper accepts a ReadonlyArray, so passing a frozen
  // array must not throw.
  const rows = Object.freeze([
    { activeVariantId: "v_active_a" },
    { activeVariantId: "v_active_b" },
  ]);
  const out = summariseActivePairings(rows);
  assert.equal(out.length, 2);
});

test("summariseActivePairings: tracks first/last sample timestamps per pairing", () => {
  // The admin overview tooltips the chip with this date range, so we
  // need the earliest sample for the pairing and the latest one — even
  // when rows arrive out of order (the route reads them desc by
  // createdAt, but the helper must not assume any input ordering).
  const out = summariseActivePairings([
    { activeVariantId: "v_a", createdAt: new Date("2026-01-03T00:00:00Z") },
    { activeVariantId: "v_a", createdAt: new Date("2026-01-01T00:00:00Z") },
    { activeVariantId: "v_a", createdAt: new Date("2026-01-02T00:00:00Z") },
    { activeVariantId: "v_b", createdAt: new Date("2026-01-04T00:00:00Z") },
  ]);
  assert.equal(out[0]!.activeVariantId, "v_a");
  assert.equal(
    out[0]!.firstSampleAt!.toISOString(),
    "2026-01-01T00:00:00.000Z",
  );
  assert.equal(
    out[0]!.lastSampleAt!.toISOString(),
    "2026-01-03T00:00:00.000Z",
  );
  assert.equal(out[1]!.activeVariantId, "v_b");
  assert.equal(
    out[1]!.firstSampleAt!.toISOString(),
    "2026-01-04T00:00:00.000Z",
  );
  assert.equal(
    out[1]!.lastSampleAt!.toISOString(),
    "2026-01-04T00:00:00.000Z",
  );
});

test("summariseActivePairings: rows missing createdAt do not corrupt the timestamps", () => {
  // Defensive: a half-populated row (missing createdAt) must not blow
  // away timestamps already collected from sibling rows.
  const out = summariseActivePairings([
    { activeVariantId: "v_a", createdAt: new Date("2026-01-02T00:00:00Z") },
    { activeVariantId: "v_a" },
    { activeVariantId: "v_a", createdAt: null },
  ]);
  assert.equal(out[0]!.sampleCount, 3);
  assert.equal(
    out[0]!.firstSampleAt!.toISOString(),
    "2026-01-02T00:00:00.000Z",
  );
  assert.equal(
    out[0]!.lastSampleAt!.toISOString(),
    "2026-01-02T00:00:00.000Z",
  );
});