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Determinism tools for the core's types: canonical encoding, hash, version for saves #22
Description
Problem
A game that replays or saves has to build its own determinism tools around the core: a canonical form of its state, a hash to compare two runs, and a version check so a save is not read by another version of the rules.
Found by the arena (#16): arena/src/sim/save.ts writes its own canonical JSON and FNV-1a digest, and stamps a save with the content hash and contractVersion() so a save from another contract is refused.
Evidence from comparable libraries
- rapier.js is deterministic only for the same version, initial state and insertion order, and suggests comparing a hash of snapshots (https://rapier.rs/docs/user_guides/javascript/determinism); it offers
takeSnapshot/restoreSnapshot(https://rapier.rs/docs/user_guides/javascript/serialization). - GGRS requires the game to handle save, load and advance requests and lets it attach a checksum to each saved state to detect desyncs (https://docs.rs/ggrs/latest/ggrs/).
- Factorio compares a CRC of the whole state every tick to find the first tick that differs (https://www.factorio.com/blog/post/fff-47).
Idea
For the core's own types (progress, rolled items, random states, a character sheet if #17 lands): a canonical encoding and a stable hash, and a version a save can carry.
Open — decided when this is picked up
- Which types; whether the hash is part of the contract (stable across releases) or only within one version.
- Relation to the existing native/WebAssembly determinism check.
Found by the arena: #16.
Activity