refactor: extract sampling primitives into shared module#418
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Move SplitMix64 and draw_exponential from task_dumped.rs into a new pub(crate) sampling module. Rename draw_exponential_ns → draw_exponential since the math is unit-agnostic (the caller decides whether 'mean' is nanoseconds, bytes, or anything else). This prepares for the memory profiler (commit 2+), which will reuse the same geometric/Poisson sampling primitive to sample on bytes allocated rather than nanoseconds idle. Pure refactor — no behavior change. Existing task-dump tests pass unmodified (only the function name changed).
Fluzko
reviewed
May 18, 2026
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| mod tests { | ||
| use super::SplitMix64; | ||
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| #[test] | ||
| fn splitmix_deterministic_with_fixed_seed() { | ||
| let mut rng = SplitMix64::new(42); | ||
| let a = rng.next_u64(); | ||
| let b = rng.next_u64(); | ||
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| let mut rng2 = SplitMix64::new(42); | ||
| assert_eq!(a, rng2.next_u64()); | ||
| assert_eq!(b, rng2.next_u64()); | ||
| } | ||
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| #[test] | ||
| fn draw_exponential_returns_at_least_1() { | ||
| let mut rng = SplitMix64::new(0); | ||
| for _ in 0..1000 { | ||
| assert!(rng.draw_exponential(1) >= 1); | ||
| } | ||
| } | ||
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| #[test] | ||
| fn draw_exponential_mean_approximates_target() { | ||
| let mut rng = SplitMix64::new(123); | ||
| let mean: u64 = 1024; | ||
| let n = 100_000; | ||
| let sum: f64 = (0..n).map(|_| rng.draw_exponential(mean) as f64).sum(); | ||
| let observed_mean = sum / n as f64; | ||
| // Within ±5% of the configured mean. | ||
| assert!( | ||
| (observed_mean - mean as f64).abs() < mean as f64 * 0.05, | ||
| "observed mean {observed_mean} too far from expected {mean}" | ||
| ); | ||
| } | ||
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| #[test] | ||
| fn draw_exponential_handles_large_mean() { | ||
| let mut rng = SplitMix64::new(999); | ||
| let mean: u64 = 1_000_000_000; | ||
| let mut saw_large = false; | ||
| for _ in 0..1000 { | ||
| let v = rng.draw_exponential(mean); | ||
| assert!(v >= 1); | ||
| if v > 1_000_000 { | ||
| saw_large = true; | ||
| } | ||
| } | ||
| assert!(saw_large, "expected some values much larger than 1"); | ||
| } | ||
| } |
Contributor
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These are duplicated, they still live at task_dumped.rs. They're not exactly the same, but pretty much they are, probably worth consolidating them in one place
Fluzko
reviewed
May 18, 2026
| /// | ||
| /// The unit is whatever the caller treats `mean` as (nanoseconds, | ||
| /// bytes, etc.). | ||
| pub(crate) fn draw_exponential(&mut self, mean: u64) -> i64 { |
Contributor
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Suggested change
| pub(crate) fn draw_exponential(&mut self, mean: u64) -> i64 { | |
| pub(crate) fn draw_exponential(&mut self, mean: u64) -> u64 { |
this is guaranteed to return >1 so I would change it to a narrower and more correct type
Fluzko
approved these changes
May 18, 2026
…ted tests - Change draw_exponential return type from i64 to u64 since the value is guaranteed >= 1 - Remove duplicated sampling tests from task_dumped.rs (already covered by sampling.rs with stricter tolerances)
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Move SplitMix64 and draw_exponential from task_dumped.rs into a new pub(crate) sampling module. Rename draw_exponential_ns → draw_exponential since the math is unit-agnostic (the caller decides whether 'mean' is nanoseconds, bytes, or anything else).
This prepares for the memory profiler (commit 2+), which will reuse the same geometric/Poisson sampling primitive to sample on bytes allocated rather than nanoseconds idle.
Pure refactor — no behavior change. Existing task-dump tests pass unmodified (only the function name changed).