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pub trait Entropy: Send + Sync + 'static { fn next_u64(&self) -> u64;}
#[derive(Clone, Copy, Debug, Default)]pub struct OsEntropy;
impl Entropy for OsEntropy { fn next_u64(&self) -> u64 { let mut buf = [0u8; 8]; getrandom::fill(&mut buf).expect("os entropy source unavailable"); u64::from_le_bytes(buf) }}
#[derive(Debug)]pub struct SeededEntropy { state: AtomicU64,}
impl SeededEntropy { const GOLDEN: u64 = 0x9E37_79B9_7F4A_7C15;
pub fn new(seed: u64) -> Self { Self { state: AtomicU64::new(seed), } }}
impl Entropy for SeededEntropy { fn next_u64(&self) -> u64 { let prev = self.state.fetch_add(Self::GOLDEN, Ordering::Relaxed); let z = prev.wrapping_add(Self::GOLDEN); let z = (z ^ (z >> 30)).wrapping_mul(0xBF58_476D_1CE4_E5B9); let z = (z ^ (z >> 27)).wrapping_mul(0x94D0_49BB_1331_11EB); z ^ (z >> 31) }}
#[cfg(test)]mod tests { use super::*; use std::sync::Arc; use std::sync::atomic::{AtomicU64, Ordering};
struct CountingEntropy(AtomicU64);
impl Entropy for CountingEntropy { fn next_u64(&self) -> u64 { self.0.fetch_add(1, Ordering::SeqCst) } }
#[test] fn external_impl_works_behind_dyn_arc() { let e: Arc<dyn Entropy> = Arc::new(CountingEntropy(AtomicU64::new(100))); assert_eq!(e.next_u64(), 100); assert_eq!(e.next_u64(), 101); }
#[test] fn seeded_entropy_two_constructions_with_same_seed_yield_same_stream() { let a = SeededEntropy::new(0xCAFE_F00D); let b = SeededEntropy::new(0xCAFE_F00D); for _ in 0..32 { assert_eq!( a.next_u64(), b.next_u64(), "splitmix stream must reproduce exactly across constructions with same seed", ); } }
#[test] fn seeded_entropy_different_seeds_diverge() { let a = SeededEntropy::new(1); let b = SeededEntropy::new(2); let mut all_match = true; for _ in 0..32 { if a.next_u64() != b.next_u64() { all_match = false; break; } } assert!( !all_match, "two seeded entropies with distinct seeds must diverge inside 32 draws", ); }
#[test] fn seeded_entropy_does_not_repeat_inside_short_window() { let e = SeededEntropy::new(0); let mut seen = std::collections::HashSet::new(); for _ in 0..1024 { assert!( seen.insert(e.next_u64()), "splitmix collided inside 1024 draws" ); } }
#[test] fn os_entropy_does_not_collide_on_back_to_back_calls() { let e = OsEntropy; let a = e.next_u64(); let b = e.next_u64(); assert_ne!( a, b, "back-to-back os entropy collided, getrandom likely not actually wired up", ); }}