# 0003. Seed-deterministic worldgen

- **Status:** Accepted
- **Date:** 2026-06-27

## Context

Procedural world generation must be reproducible: given the same seed, the same world is expected on any platform and at any time. Reproducibility enables shared seeds, reliable bug reproduction, and consistent behaviour between a server and any client that regenerates terrain locally.

Reproducibility is fragile. Sources of nondeterminism include OS-seeded random number generators, the randomised iteration order of the standard hasher, and platform-specific arithmetic.

## Decision

Worldgen is **seed-deterministic** and is held to bit-for-bit reproducibility across platforms.

- A fixed RNG algorithm (e.g. `wyrand`, `xoshiro`) is used, seeded only from the world seed. `rand::thread_rng()` and any OS-seeded source are prohibited in worldgen.
- Iteration order that feeds RNG draws or content placement must be deterministic. The default randomised-hash `HashMap` iteration order must not be relied upon; use `BTreeMap`, `IndexMap`, or an explicit sort.

## Consequences

- Worldgen code is constrained in its choice of RNG and collection types, and reviewers must watch for nondeterministic iteration order.
- Identical worlds are guaranteed from identical seeds, on any supported platform.
- This guarantee is scoped to worldgen only; see [ADR-0004](0004-server-authoritative-simulation.md) for why the rest of the simulation is deliberately not held to the same standard.