Step 3.6: add fuel cost

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

memory-bank/progress.md

@@ -24,7 +24,7 @@
 - [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.
 - [x] **Step 3.5 — Travel time.** `src/travel.ts`: `travelTimeGameHours(ship, distance)` = cruise time + overhead.
-- [ ] Step 3.6 — Fuel cost
+- [x] **Step 3.6 — Fuel cost.** `src/fuel.ts`: `fuelCost(ship, origin, distance)` = distance × cost per million km × origin launch multiplier.
 - [ ] Step 3.7 — Range and landing eligibility
 - [ ] Step 3.8 — Contract model and payout
 - [ ] Step 3.9 — Contract board generation

packages/sim/src/fuel.ts

@@ -0,0 +1,6 @@
+import type { Location, ShipModel } from "@deadhead/content";
+
+/** Fuel cost in credits: distance × the ship's cost per million km × the origin's launch multiplier. */
+export function fuelCost(ship: ShipModel, origin: Location, distanceMillionKm: number): number {
+  return distanceMillionKm * ship.fuelCostPerMillionKm * origin.launchCostMultiplier;
+}

packages/sim/src/index.ts

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

packages/sim/test/fuel.test.ts

@@ -0,0 +1,25 @@
+import { content } from "@deadhead/content";
+import { describe, expect, it } from "vitest";
+import { fuelCost, routeDistance } from "../src/index";
+
+const courier = content.shipModel("courier")!;
+const earth = content.location("earth")!;
+const moon = content.location("moon")!;
+
+describe("fuel cost", () => {
+  it("costs more Earth→Moon than Moon→Earth for the same ship", () => {
+    const t = Date.parse("2100-01-01T00:00:00Z");
+    const distance = routeDistance(earth, moon, t);
+    expect(fuelCost(courier, earth, distance)).toBeGreaterThan(fuelCost(courier, moon, distance));
+  });
+
+  it("scales linearly with distance", () => {
+    expect(fuelCost(courier, earth, 2)).toBe(2 * fuelCost(courier, earth, 1));
+    expect(fuelCost(courier, earth, 10)).toBe(10 * fuelCost(courier, earth, 1));
+  });
+
+  it("matches a hand calculation", () => {
+    // 0.5M km × 200 credits/M km × Earth multiplier 3.
+    expect(fuelCost(courier, earth, 0.5)).toBe(300);
+  });
+});