feat: news ledger with lane-delayed price snapshots

src/sim/news.js

@@ -0,0 +1,35 @@
+import { GOODS, NEWS_DAYS_PER_LANE, SNAPSHOT_KEEP } from './constants.js';
+import { advancePlanet, priceOf } from './economy.js';
+
+// Daily price snapshots per planet. Knowledge from `hops` lanes away
+// is the snapshot from hops * NEWS_DAYS_PER_LANE days ago.
+export class NewsLedger {
+  constructor() {
+    this.history = new Map(); // planetId -> [{time, prices}]
+  }
+
+  record(galaxy, t) {
+    for (const planet of galaxy.planets) {
+      advancePlanet(planet, t);
+      const prices = Object.fromEntries(GOODS.map((g) => [g, priceOf(g, planet.stock[g])]));
+      let arr = this.history.get(planet.id);
+      if (!arr) this.history.set(planet.id, (arr = []));
+      arr.push({ time: t, prices });
+      if (arr.length > SNAPSHOT_KEEP) arr.shift();
+    }
+  }
+
+  // Latest snapshot of planetId that has had time to travel `hops` lanes by time t.
+  // Clamped to t=0: the initial state is universally known.
+  knownPrices(planetId, hops, t) {
+    const arr = this.history.get(planetId);
+    if (!arr || arr.length === 0) return null;
+    const asOf = Math.max(0, t - hops * NEWS_DAYS_PER_LANE);
+    let best = null;
+    for (const snap of arr) {
+      if (snap.time <= asOf) best = snap;
+      else break;
+    }
+    return best ?? arr[0];
+  }
+}

tests/news.test.js

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+import { describe, it, expect } from 'vitest';
+import { NewsLedger } from '../src/sim/news.js';
+import { generateGalaxy } from '../src/sim/galaxy.js';
+import { priceOf } from '../src/sim/economy.js';
+
+describe('NewsLedger', () => {
+  it('records snapshots and serves them with lane delay', () => {
+    const galaxy = generateGalaxy('news-test', { systemCount: 8 });
+    const news = new NewsLedger();
+    news.record(galaxy, 0);
+    news.record(galaxy, 1);
+    news.record(galaxy, 2);
+
+    const pid = galaxy.planets[0].id;
+    // 0 hops away at t=2: freshest snapshot (t=2)
+    expect(news.knownPrices(pid, 0, 2).time).toBe(2);
+    // 2 hops away at t=2: only the t=0 snapshot has reached us
+    expect(news.knownPrices(pid, 2, 2).time).toBe(0);
+    // far away early on: clamps to the universally-known t=0 snapshot
+    expect(news.knownPrices(pid, 5, 1).time).toBe(0);
+  });
+
+  it('snapshot prices match the live price curve at record time', () => {
+    const galaxy = generateGalaxy('news-test', { systemCount: 8 });
+    const news = new NewsLedger();
+    news.record(galaxy, 0);
+    const p = galaxy.planets[0];
+    const snap = news.knownPrices(p.id, 0, 0);
+    expect(snap.prices.ore).toBeCloseTo(priceOf('ore', p.stock.ore), 5);
+  });
+
+  it('trims history beyond SNAPSHOT_KEEP entries', () => {
+    const galaxy = generateGalaxy('news-test', { systemCount: 8 });
+    const news = new NewsLedger();
+    for (let t = 0; t <= 50; t++) news.record(galaxy, t);
+    // a 45-day-old snapshot is gone; we get the oldest retained one instead
+    const snap = news.knownPrices(galaxy.planets[0].id, 45, 50);
+    expect(snap.time).toBeGreaterThanOrEqual(50 - 30);
+  });
+});