Subcosm

We kept coming back to one question: what did our community become overnight? Subreddits generate an enormous amount of activity every single day, and then it scrolls away forever. We wanted to turn that fleeting activity into something persistent, legible, and beautiful — a single shared artifact that a community grows together, the way an agate, a tree ring, or a coral grows: layer by layer, each layer a record of the conditions that made it.

That gave us Subcosm — a universe grown from your community. r/place energy, but biographical instead of spatial: not where everyone clicked, but who your community was, day after day.

What it does

Each subreddit grows one shared cosmos from its own daily activity. Depth is time: a glowing genesis core (install day) sits at the center, and each day adds a concentric shell of stars synthesized from that day's posts, comments, contributors, and conflict. The outermost shell is the live frontier that fills during the day and freezes forever at the daily tick — so every morning the community wakes up to what their universe became overnight.

• Activity is the texture, not a parameter: posts → stars, big threads → bright clusters, contributors → density, conflict → red turbulence. A quiet day is sparse; a drama day burns red; an AMA day blooms with a few huge clusters.
• Travel through time: a depth scrubber and zoom let you fly inward through every shell back to the genesis core — the community's whole history in one image.
• Shape today: players nudge the live frontier (spread / symmetry / hue). Nudges bias the mean of a parameter — they never dictate the result, so it stays a shared organism, not a single user's drawing.
• One engine, many worlds: behaviour (Genome) and look (StyleTemplate) are data, not code, so the same engine renders provably different cosmoses per community.

The daily game (the hook). Every day the community gets a shared goal — a "genome quest" like grow a symmetric spiral, tame the conflict-red, or ignite a rare twin-star. Players spend a limited budget of nudges to steer the live frontier toward it (and call how the day will go); overnight the frontier freezes and the reveal resolves the goal — a success leaves a permanent reward star in that ring, visible forever. Steering only biases the odds, never dictates the outcome, so it stays a shared organism, not one player's drawing. That loop — set a goal → shape it together → reveal overnight — is the retention hook, and it's literally built from the community's own activity.

How we built it

The heart is a pure, deterministic rendering engine with a strict typed seam:

\( \text{synthesize}(\text{DayVector},\, \text{Genome}) \rightarrow \text{Scene} \quad\text{then}\quad \text{paint}(\text{Scene},\, \text{StyleTemplate}) \rightarrow \text{pixels} \)

• Functional core, imperative shell. Synthesis is style-agnostic; paint never touches raw data; the camera never mutates the scene. Reddit/Devvit code is isolated in its own layer and attaches at a single seam.
• Determinism by construction. Every shell is reproducible from its day vector and a seed, with a seeded mulberry32 PRNG and a fixed value-consumption order. The per-day seed is \( s = \text{hash}(\text{subId},\, \text{day},\, \text{genomeVersion}) \), and shells are laid out at radius $$ r_i = R_{\max}\cdot 0.85^{\,i} $$ so the same inputs always produce a byte-identical scene on every device.
• Zod as the single source of truth. All four contracts (DayVector, Scene, Genome, StyleTemplate) are Zod schemas; TypeScript types are inferred from them, and data is validated only at system boundaries.
• Simulator first. We built a data simulator that generates realistic DayVector[] (growth, busy/quiet days, a drama spike, an AMA day, a cold-start day-1) so we could tune the data→visuals mapping with zero platform risk before wiring Reddit. The simulator's schema is the contract the real collector fills.
• Mobile-first rendering. Canvas 2D with bake-on-freeze caching (only the live frontier animates), pre-cached gradients, and a capped device-pixel-ratio to hold 60 fps in the Reddit post viewport.

[TODO: replace with the real build narrative + final numbers once implemented — e.g. measured fps on device, ring count, draw-call budget.]

Challenges we faced

• Determinism is fragile. A single Math.random() anywhere in the engine silently breaks reproducibility across clients — we ban it in the engine with ESLint and verify with a snapshot test.
• Reddit has no vote trigger. There's no score-delta event, so the "conflict" signal can't be streamed — it's a composite derived from comment events plus a tick-time score snapshot.
• The scheduler is UTC-only. Per-community day boundaries are computed in an hourly sweeper using IANA time zones, with deterministic hash(subId) % 60 minute jitter.
• Mobile Canvas 2D has performance cliffs. Per-star gradients and uncapped DPR will tank the frame rate; the cache and DPR cap are part of the first implementation, not a later optimization.
• Avoiding "AI slop." The hardest aesthetic goal: look genuinely self-authored, not a generic neon fractal. We anchor the visual grammar to natural references (agate, tree rings, coral).

What we learned

• A typed seam between what the data means and how it looks is what makes a real template engine — adding a new style or genome becomes one data file, zero engine changes.
• Parse at the boundary, trust types inside. Validating once at the edges (and never in the render loop) keeps the hot path allocation-free.
• A lot about the Devvit platform's real surface — triggers, UTC scheduling, Redis without key-scan, realtime channel naming — which shaped the architecture before we wrote platform code.

What's next

• Deeper game layers: a daily prediction + streaks, and a collection of rare overnight mutations.
• More flagship styles (Comic, Pixel) — each is pure data on the same engine.
• Rare overnight mutations, the full signal→parameter genome matrix, and an fbm/WebGL shader layer (our Phaser target).
• Optional, fair free-to-play extras via Reddit Gold — cosmetics, collection, supporter badges. Never pay-to-win: the shared community outcome is never for sale.
• A connected multiverse: subreddits as galaxies that owners can opt-in link into shared "quadrants," with graphical travel between them.

Built With

• canvas-2d
• devvit
• html5
• mulberry32
• phaser.js
• realtime
• reddit-developer-platform
• redis
• typescript
• vite
• vitest
• zod