Step 3.5: add travel time

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

DECISIONS.md

@@ -98,3 +98,7 @@ Implementation decisions not spelled out in `memory-bank/implementation-plan.md`
 ## Step 3.4
 
 - **Route distance is the straight line between the two positions at one game time.** Orbital transfer paths are not modelled.
+
+## Step 3.5
+
+- **Travel time takes the distance as a number**, not two locations and a time. The caller measures the distance at departure, so the function cannot accidentally use a mid-flight distance.

memory-bank/progress.md

@@ -23,7 +23,7 @@
 - [x] **Step 3.2 — Seeded RNG.** `src/rng.ts`: `createRng(seed)` (mulberry32) with `next`, `int`, `pick`; `deriveSeed(parent, label)` for child seeds.
 - [x] **Step 3.3 — Location positions.** `src/positions.ts`: heliocentric ecliptic positions (million km) from `astronomy-engine` for Earth, Mars and Jupiter; Moon = Earth + fixed offset; Ceres on a circular orbit.
 - [x] **Step 3.4 — Route distance.** `src/distance.ts`: `routeDistance(from, to, gameTime)`, straight-line million km.
-- [ ] Step 3.5 — Travel time
+- [x] **Step 3.5 — Travel time.** `src/travel.ts`: `travelTimeGameHours(ship, distance)` = cruise time + overhead.
 - [ ] Step 3.6 — Fuel cost
 - [ ] Step 3.7 — Range and landing eligibility
 - [ ] Step 3.8 — Contract model and payout

packages/sim/src/index.ts

@@ -2,3 +2,4 @@ export * from "./clock";
 export * from "./rng";
 export * from "./positions";
 export * from "./distance";
+export * from "./travel";

packages/sim/src/travel.ts

@@ -0,0 +1,10 @@
+import type { ShipModel } from "@deadhead/content";
+
+/**
+ * Trip duration in game hours: cruise time plus the ship's fixed takeoff/landing
+ * overhead. Pass the distance at departure; it is not recalculated mid-flight.
+ */
+export function travelTimeGameHours(ship: ShipModel, distanceMillionKm: number): number {
+  const cruiseHours = (distanceMillionKm * 1e6) / ship.cruiseSpeedKmPerGameHour;
+  return cruiseHours + ship.overheadGameHours;
+}

packages/sim/test/travel.test.ts

@@ -0,0 +1,28 @@
+import { content } from "@deadhead/content";
+import { describe, expect, it } from "vitest";
+import { routeDistance, travelTimeGameHours } from "../src/index";
+
+const courier = content.shipModel("courier")!;
+const freighter = content.shipModel("freighter")!;
+
+describe("travel time", () => {
+  it("matches a hand calculation", () => {
+    // 150M km at 2M km/h = 75 h cruise, plus 4 h overhead.
+    expect(travelTimeGameHours(freighter, 150)).toBe(79);
+  });
+
+  it("gets a faster ship there sooner on the same route", () => {
+    for (const distance of [0.384, 50, 400]) {
+      expect(travelTimeGameHours(courier, distance)).toBeLessThan(
+        travelTimeGameHours(freighter, distance),
+      );
+    }
+  });
+
+  it("is mostly overhead for a Courier on Earth→Moon", () => {
+    const t = Date.parse("2100-01-01T00:00:00Z");
+    const distance = routeDistance(content.location("earth")!, content.location("moon")!, t);
+    const total = travelTimeGameHours(courier, distance);
+    expect(courier.overheadGameHours).toBeGreaterThan(total / 2);
+  });
+});