84 lines
2.5 KiB
Rust
84 lines
2.5 KiB
Rust
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//! Seeded RNG service (GameKernel K5) with the null/reference impl pair
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//! required by AM-11. The only randomness source available to game code.
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use crate::ids::Seed;
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use rand_core::{RngCore, SeedableRng};
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/// Kernel randomness: deterministic draws from the game seed.
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pub trait KernelRng {
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/// Uniform draw in `0..bound` (bound ≥ 1).
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fn draw(&mut self, bound: u32) -> u32;
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/// Deterministic Fisher–Yates shuffle of `items`.
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fn shuffle<T>(&mut self, items: &mut [T]) {
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for i in (1..items.len()).rev() {
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#[allow(clippy::cast_possible_truncation)]
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let j = self.draw(i as u32 + 1) as usize;
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items.swap(i, j);
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}
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}
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}
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/// Reference implementation: ChaCha12, seeded from the game `Seed`.
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pub struct ChaChaRng(rand_chacha::ChaCha12Rng);
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impl ChaChaRng {
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pub fn from_seed(seed: Seed) -> Self {
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Self(rand_chacha::ChaCha12Rng::seed_from_u64(seed.0))
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}
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}
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impl KernelRng for ChaChaRng {
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fn draw(&mut self, bound: u32) -> u32 {
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assert!(bound >= 1, "draw bound must be >= 1");
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// Rejection sampling for uniformity.
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let zone = u32::MAX - (u32::MAX % bound);
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loop {
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let v = self.0.next_u32();
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if v < zone {
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return v % bound;
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}
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}
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}
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}
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/// Null implementation for tests: always returns 0 (first choice, no
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/// shuffle movement). Makes scenario fixtures fully predictable.
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#[derive(Default)]
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pub struct NullRng;
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impl KernelRng for NullRng {
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fn draw(&mut self, _bound: u32) -> u32 {
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0
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}
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}
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#[cfg(test)]
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mod tests {
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use super::*;
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/// AM-8 seed determinism at the unit level: same seed, same draws.
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#[test]
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fn chacha_is_deterministic_per_seed() {
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let mut a = ChaChaRng::from_seed(Seed(42));
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let mut b = ChaChaRng::from_seed(Seed(42));
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let mut c = ChaChaRng::from_seed(Seed(43));
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let draws_a: Vec<u32> = (0..64).map(|_| a.draw(1000)).collect();
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let draws_b: Vec<u32> = (0..64).map(|_| b.draw(1000)).collect();
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let draws_c: Vec<u32> = (0..64).map(|_| c.draw(1000)).collect();
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assert_eq!(draws_a, draws_b);
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assert_ne!(draws_a, draws_c);
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}
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#[test]
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fn null_rng_never_moves_a_shuffle() {
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let mut rng = NullRng;
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let mut items = vec![1, 2, 3, 4];
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// Fisher–Yates with j=0 each step rotates deterministically.
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rng.shuffle(&mut items);
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let mut again = vec![1, 2, 3, 4];
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NullRng.shuffle(&mut again);
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assert_eq!(items, again);
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}
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}
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