From 0d3bfa11cfdbafe529a50b02196ac22e009f51c5 Mon Sep 17 00:00:00 2001
From: Jeff Quast <contact@jeffquast.com>
Date: Mon, 28 Sep 2026 18:45:55 +0000
Subject: [PATCH] Seed the RNG in simInit and load a city once

initWillStuff() called randomlySeedRandom(), and every path to it runs
doSimInit(), whose map scans draw random numbers and write the results
into map tiles, so initWillStuff() overwrote a seed the caller had set
with seedRandom() before that seed could take effect. Seed in simInit(),
where a fresh simulation gets a random world, and leave reproducible runs
to callers.

Call seedRandom() before loadCity(). loadFileData() runs doSimInit(),
whose map scans draw from the same stream, so a seed set afterwards finds
the loaded city on the clock-seeded stream. Measured both ways: seeding
after loadCity gives a different map on every run, seeding before gives
the same map every time.

loadCity() also called doSimInit() after loadFileData() had already run
it, so a loaded city came back one initialization step past the file.

A save and reload changes about 240 map tiles, and the change accumulates
over repeated round trips, because loadFileData() runs doSimInit() and
that call is kept here.
---
 .../src/lib/cityLoadDeterminism.test.ts       | 36 +++++++++++++++++++
 packages/micropolis-engine/src/fileio.cpp     |  2 --
 packages/micropolis-engine/src/initialize.cpp |  1 -
 packages/micropolis-engine/src/micropolis.cpp |  1 +
 4 files changed, 37 insertions(+), 3 deletions(-)
 create mode 100644 apps/micropolis/src/lib/cityLoadDeterminism.test.ts

diff --git a/apps/micropolis/src/lib/cityLoadDeterminism.test.ts b/apps/micropolis/src/lib/cityLoadDeterminism.test.ts
new file mode 100644
index 00000000..71f9c56c
--- /dev/null
+++ b/apps/micropolis/src/lib/cityLoadDeterminism.test.ts
@@ -0,0 +1,36 @@
+import { describe, it, expect } from 'vitest';
+import { loadMicropolisMainModule } from './wasm/node';
+import { createNoopJsCallback } from './wasm/callbacks';
+import { createMapMopViews } from './wasm/views';
+import type { MainModule, Micropolis } from '../types/micropolisengine.d.js';
+
+const CITY = '/cities/kobe.cty';
+
+const loadWithSeed = (engine: MainModule, seed: number): Micropolis => {
+	const micropolis = new engine.Micropolis();
+	micropolis.setCallback(createNoopJsCallback(engine), {});
+	micropolis.init();
+	micropolis.seedRandom(seed);
+	expect(micropolis.loadCity(CITY)).toBe(true);
+	return micropolis;
+};
+
+const mapHash = (engine: MainModule, micropolis: Micropolis): number => {
+	let hash = 0;
+	for (const tile of createMapMopViews(engine, micropolis)!.mapData) hash = (Math.imul(hash, 31) + tile) | 0;
+	return hash >>> 0;
+};
+
+describe('city load determinism', () => {
+	it('same seed gives the same map, a different seed gives another', async () => {
+		const engine = await loadMicropolisMainModule();
+		const first = loadWithSeed(engine, 42);
+		const second = loadWithSeed(engine, 42);
+		const other = loadWithSeed(engine, 7);
+
+		expect(mapHash(engine, second)).toBe(mapHash(engine, first));
+		expect(mapHash(engine, other)).not.toBe(mapHash(engine, first));
+
+		for (const micropolis of [first, second, other]) micropolis.delete();
+	});
+});
diff --git a/packages/micropolis-engine/src/fileio.cpp b/packages/micropolis-engine/src/fileio.cpp
index 30e91a04..94cea582 100644
--- a/packages/micropolis-engine/src/fileio.cpp
+++ b/packages/micropolis-engine/src/fileio.cpp
@@ -592,8 +592,6 @@ bool Micropolis::loadCity(const std::string &filename)
         std::string newCityName = cityFileName.substr(pos, last - pos);
         setCityName(newCityName);
 
-        doSimInit();
-
         didLoadCity(filename);
 
         return true;
diff --git a/packages/micropolis-engine/src/initialize.cpp b/packages/micropolis-engine/src/initialize.cpp
index 9ff81d07..e7b50071 100644
--- a/packages/micropolis-engine/src/initialize.cpp
+++ b/packages/micropolis-engine/src/initialize.cpp
@@ -84,7 +84,6 @@
 /** Reset many game state variables */
 void Micropolis::initWillStuff()
 {
-    randomlySeedRandom();
     initGraphMax();
     destroyAllSprites();
 
diff --git a/packages/micropolis-engine/src/micropolis.cpp b/packages/micropolis-engine/src/micropolis.cpp
index 5230c74e..ebf955b3 100644
--- a/packages/micropolis-engine/src/micropolis.cpp
+++ b/packages/micropolis-engine/src/micropolis.cpp
@@ -738,6 +738,7 @@ void Micropolis::simInit()
     resetMapState();
     resetEditorState();
     clearMap();
+    randomlySeedRandom();
     initWillStuff();
     setFunds(5000);
     setGameLevelFunds(LEVEL_EASY);