nEngine/packages/mcp/test/rng-namespace.test.ts

import { mkdtemp, mkdir, rm, writeFile } from "node:fs/promises";
import { tmpdir } from "node:os";
import { join } from "node:path";
import { describe, expect, it } from "vitest";
import { createEngine, seedRandom } from "@nengine/core";
import { createEngineNamespaces, McpGateway } from "../src/index.js";

const TIME_YAML = [
  "id: default",
  "dayLengthTicks: 480",
  "seasons:",
  "  - { name: spring, dayCount: 28 }",
  "start: { day: 1, season: spring }",
  "",
].join("\n");

const FARM_YAML = [
  "id: farm",
  "width: 2",
  "height: 2",
  'tilesetRef: "tiles"',
  "spawn: { x: 0, y: 0 }",
  "layers:",
  "  - name: ground",
  "    data:",
  "      - [a, b]",
  "      - [b, a]",
  "",
].join("\n");

interface Setup {
  engine: ReturnType<typeof createEngine>;
  gateway: McpGateway;
  tmp: string;
  close: () => Promise<void>;
}

async function setup(seed?: number): Promise<Setup> {
  const tmp = await mkdtemp(join(tmpdir(), "farm-rng-ns-"));
  await mkdir(join(tmp, "maps"), { recursive: true });
  await writeFile(join(tmp, "time.yaml"), TIME_YAML);
  await writeFile(join(tmp, "maps", "farm.yaml"), FARM_YAML);

  const engine = createEngine({ contentDir: tmp, seed });
  const gateway = new McpGateway();
  for (const namespace of createEngineNamespaces(engine, { gateway })) {
    gateway.register(namespace);
  }
  await engine.start();
  return {
    engine,
    gateway,
    tmp,
    close: async () => {
      await engine.stop();
      await rm(tmp, { recursive: true, force: true });
    },
  };
}

async function callText(gateway: McpGateway, name: string, args: Record<string, unknown> = {}): Promise<{
  ok: boolean;
  text: string;
}> {
  const result = await gateway.callTool(name, args);
  return { ok: result.isError !== true, text: result.content[0]?.text ?? "" };
}

describe("rng namespace", () => {
  it("rng.get reports {seed, calls} anchored to the engine seed", async () => {
    const { gateway, close } = await setup(42);
    try {
      const got = await callText(gateway, "rng.get");
      expect(got.ok).toBe(true);
      expect(JSON.parse(got.text)).toEqual({ seed: 42, calls: 0 });
    } finally {
      await close();
    }
  });

  it("rng.set resets the RNG to a seed and makes subsequent world randomness deterministic", async () => {
    const { engine, gateway, close } = await setup();
    try {
      engine.rng.next();
      const set = await callText(gateway, "rng.set", { seed: 123 });
      expect(set.ok).toBe(true);
      expect(JSON.parse(set.text)).toEqual({ seed: 123, calls: 0 });

      const expected = seedRandom(123);
      expect(engine.rng.next()).toBe(expected.next());
      expect(engine.rng.nextInt(10)).toBe(expected.nextInt(10));
      expect(JSON.parse((await callText(gateway, "rng.get")).text)).toEqual({ seed: 123, calls: 2 });
    } finally {
      await close();
    }
  });

  it("rng.set rejects non-finite and non-number seeds", async () => {
    const { gateway, close } = await setup();
    try {
      const nan = await callText(gateway, "rng.set", { seed: Number.NaN });
      expect(nan.ok).toBe(false);
      expect(nan.text).toMatch(/seed must be a finite number/);

      const inf = await callText(gateway, "rng.set", { seed: Infinity });
      expect(inf.ok).toBe(false);
      expect(inf.text).toMatch(/seed must be a finite number/);

      const string = await callText(gateway, "rng.set", { seed: "123" });
      expect(string.ok).toBe(false);
      expect(string.text).toMatch(/seed must be a finite number/);

      const missing = await callText(gateway, "rng.set", {});
      expect(missing.ok).toBe(false);
      expect(missing.text).toMatch(/missing required arguments: seed/);
    } finally {
      await close();
    }
  });

  it("rng://state resource reflects the live RNG state", async () => {
    const { engine, gateway, close } = await setup(7);
    try {
      const state = await gateway.readResource("rng://state");
      expect(JSON.parse(state.contents[0]?.text ?? "{}")).toEqual({ seed: 7, calls: 0 });
      engine.rng.next();
      const after = await gateway.readResource("rng://state");
      expect(JSON.parse(after.contents[0]?.text ?? "{}")).toEqual({ seed: 7, calls: 1 });
    } finally {
      await close();
    }
  });

  it("rng namespace has no prompts", async () => {
    const { gateway, close } = await setup();
    try {
      const tools = gateway.listTools().map((tool) => tool.name);
      expect(tools).toContain("rng.get");
      expect(tools).toContain("rng.set");
      const prompts = gateway.listPrompts().map((prompt) => prompt.name);
      expect(prompts.some((name) => name.startsWith("rng."))).toBe(false);
    } finally {
      await close();
    }
  });
});