Pulse

A living-city simulation where about 150 residents keep living whether you watch or not.

Open the live city (opens in a new tab)

Role
Developer on a team project: simulation engine, server and 3D web client (every commit so far)
Dates
Nov 2026 – present
Status
In development · team project
Stack
TypeScript, three.js, Fastify · deterministic engine

Pulse, screen by screen

  • The city. Downtown and Riverside: boulevards, junctions and the canal, with residents on the pavements.

  • The country. Meridian sits in a whole simulated country of five states and their capitals.

  • The region. Its region: coastline, the river to the sea, farms and highways that carry trade.

  • Neighbourhoods. Neighbourhoods read differently: Old Quarter shophouses beside Downtown towers.

  • Weather. Rain changes the day: umbrellas come out, and the simulation tracks who got soaked.

  • World Console. A World Console request in action: everyone heads to Riverside Park.

  • Investigations. A theft that really happened in the simulation, with evidence from the record.

  • Experiment lab. Fork the city, change one thing, run both futures with the same luck.

In short

  • Deterministic by design

    The random-number state lives inside the world, so cloning it forks an identical future; experiments and forecasts compare branches that differ only by the intervention.

  • AI at the edges, never in charge

    A model turns plain English into commands from a 33-action allowlist, validated with Zod and executed atomically with rollback; the offline rules parser covers the same vocabulary.

  • Every event knows its causes

    Major events record their causal parents, so “Why is Maya unhappy?” walks the real chain, and a theft investigation draws its evidence from what the simulation recorded.

The problem

City games usually stop when you look away, and their residents are props: you cannot ask why something happened or trust that a what-if changed only what you changed.

  • A simulation people watch needs a world that is interesting without scripted events.
  • A what-if is only meaningful if the two futures share the same randomness.
  • Language models are useful for interpretation and voice but cannot be trusted to own state.

Results

tests passing, including the brief’s 20 acceptance tests
52
allow-listed World Console actions
33
fastest simulation speed
120×

Measured on 9 November 2026 on a local build.

Full engineering notes

3 sections, about 2 minutes to read

Summary

Pulse is a living-city simulation set in Meridian, a fictional city of about 150 residents. Each person has a job, a home, money, family, feelings, memories and a daily routine, and the city keeps running whether anyone watches. The city sits in a whole simulated country: its other cities trade with Meridian, sending produce, fish, timber and imports by truck and ship along real routes, and residents can move away. You can open any life, change the world through a plain-English World Console, ask why something happened and walk the real chain of causes, solve a theft that actually happened in the simulation, and fork the city to test a what-if. The engine is deterministic: the random state lives inside the world, so a forked future differs only because of the change you made. Each visitor gets a private world saved in their own browser. Language models are optional and sit at the edges: they turn a request into allow-listed commands and phrase text from facts, but never write state, and every feature has a rules-based twin that works offline.

My role

What I owned

  • The deterministic engine: world generation, utility-AI decisions, needs, memory and reflection, social life, economy, weather, ecology, life course, and the event log with causes.
  • The server: authoritative 100 ms loop at 1× to 120×, Fastify and Socket.IO, SQLite saves and checkpoints, and the optional AI gateway with rules fallbacks.
  • The 2.5D three.js city and the React panels: People, Investigations, Experiment lab, Director and Studio film export.

What I did not own

  • Pulse is a team project; the commit history so far is mine.
  • Open-source libraries (three.js, React, Fastify, Socket.IO, Drizzle, Zod) and the optional Google Gemini models.

Stack and release record

  • Platforms

    Web (live prototype)

  • Built with

    TypeScript, React 19, Vite, three.js, Tailwind CSS 4, Zustand, Fastify, Socket.IO, SQLite with Drizzle, Zod, Google Gemini (optional), Vitest, Playwright, Google Cloud Run

  • Testing and delivery

    Vitest covers the brief’s 20 acceptance tests, 7 demo scenarios, a realism report and the HTTP API (59 tests); Playwright runs the city, inspector, console, investigation and Studio export against the production build in installed Chrome.

  • Status in full

    Working prototype, live on Google Cloud Run. Each visitor gets a private world saved in their own browser; the first load after a quiet spell can take a few seconds. The World Console uses Google Gemini on the free tier and falls back to built-in rules when its limits are hit. Source is private.

Next case study CareerOS Notices when you ship, and drafts résumé and LinkedIn lines from evidence it can cite.

All work and archive

Hiring interns for January 2027?

I’m available full-time from 25 January to 23 April 2027, and for graduate roles in Malaysia or overseas from August 2027. Email is the quickest way to reach me.

  • Subang Jaya, Malaysia
  • local time

yousofselim2@gmail.com

Email me