feat(prng): order-independent derived streams and distribution helpers - #38
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`fork(label)` consumes a draw from the parent, so a child stream depends on how many values were drawn before it. That is right for per-entity streams created in a fixed order, and it is left exactly as it was. Procedural generation needs the opposite: adding a draw to one step (placing a new kind of landmark, say) must not reshuffle every other step. - `derive(label)`: a child that is a pure function of this stream's seed identity and the label. It never advances the parent and ignores prior draws, and nested labels compose. The identity is carried in exportState/restoreState; states exported before this change (no `origin` field) still restore. - Helpers used constantly by procedural code: `range(min, max)`, `pick(items)`, `weighted(items, weights)`, `gaussian(mean, stdDev)`, `shuffle(items)` and `fn()` for libraries that take a random function, such as noise generators. - `gaussian` sums twelve uniforms rather than using Box-Muller, whose Math.log and Math.cos are not bit-identical across JavaScript engines; a world generated with it could differ between two browsers in a lockstep session. Bounded to six standard deviations. - Diagnostics RND_0501 to RND_0504 for empty picks and invalid weights. - The base PCG32 sequence and `fork` are byte-identical to before. Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
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fork(label)consumes a draw from the parent, so a child stream depends on how many values were drawn before it. That's right for per-entity streams created in a fixed order, andforkis unchanged. Procedural generation needs the opposite: adding a draw to one generation step must not reshuffle every other step.derive(label): a child that is a pure function of the stream's seed identity and the label. It never advances the parent, and nested labels compose. The identity is carried inexportState/restoreState, and older states without it still restore.range,pick,weighted,gaussian,shuffle, andfn()for libraries that take a random function (noise generators).gaussiansums twelve uniforms (Irwin–Hall) instead of Box–Muller.Math.log/Math.cosare not bit-identical across JS engines, which would break lockstep determinism. The result is bounded to ±6σ.forkare byte-identical to before.Verification: typecheck, build, 242/242 tests.
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