1 files changed, 82 insertions, 0 deletions
diff --git a/src/testrand.rs b/src/testrand.rs
new file mode 100644
index 0000000..dbb2148
--- /dev/null
+++ b/src/testrand.rs
@@ -0,0 +1,82 @@
+//! A small, seedable random generator for tests (SplitMix64), and a runner for
+//! property tests over many seeds. No dependency; reproducible: a failing case names
+//! its seed.
+
+use std::time::Duration;
+
+pub(crate) struct Rng(u64);
+
+impl Rng {
+    pub(crate) fn new(seed: u64) -> Self {
+        Self(seed)
+    }
+
+    pub(crate) fn next_u64(&mut self) -> u64 {
+        self.0 = self.0.wrapping_add(0x9e37_79b9_7f4a_7c15);
+        let mut z = self.0;
+        z = (z ^ (z >> 30)).wrapping_mul(0xbf58_476d_1ce4_e5b9);
+        z = (z ^ (z >> 27)).wrapping_mul(0x94d0_49bb_1331_11eb);
+        z ^ (z >> 31)
+    }
+
+    /// Uniform in [0, 1).
+    pub(crate) fn next_f64(&mut self) -> f64 {
+        (self.next_u64() >> 11) as f64 / (1_u64 << 53) as f64
+    }
+
+    pub(crate) fn chance(&mut self, probability: f64) -> bool {
+        probability > 0.0 && self.next_f64() < probability
+    }
+
+    /// Uniform in [0, n); 0 if `n` is 0.
+    pub(crate) fn below(&mut self, n: usize) -> usize {
+        if n == 0 {
+            return 0;
+        }
+        (self.next_u64() % n as u64) as usize
+    }
+
+    /// Uniform in [min, max].
+    pub(crate) fn between(&mut self, min: usize, max: usize) -> usize {
+        min + self.below(max - min + 1)
+    }
+
+    pub(crate) fn pick<'a, T>(&mut self, items: &'a [T]) -> &'a T {
+        &items[self.below(items.len())]
+    }
+
+    pub(crate) fn bytes32(&mut self) -> [u8; 32] {
+        let mut bytes = [0; 32];
+        for chunk in bytes.chunks_mut(8) {
+            chunk.copy_from_slice(&self.next_u64().to_le_bytes());
+        }
+        bytes
+    }
+
+    pub(crate) fn shuffle<T>(&mut self, items: &mut [T]) {
+        for i in (1..items.len()).rev() {
+            items.swap(i, self.below(i + 1));
+        }
+    }
+
+    /// Uniform in the range (`min` if it is empty).
+    pub(crate) fn duration_in(&mut self, (min, max): (Duration, Duration)) -> Duration {
+        if max <= min {
+            return min;
+        }
+        min + (max - min).mul_f64(self.next_f64())
+    }
+}
+
+/// Runs `check` once per seed in `0..cases`, each with its own generator. A failing case
+/// says its seed, to rerun it alone with `Rng::new(seed)`.
+pub(crate) fn for_seeds(cases: u64, check: impl Fn(&mut Rng)) {
+    for seed in 0..cases {
+        let result =
+            std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| check(&mut Rng::new(seed))));
+        if let Err(panic) = result {
+            eprintln!("property failed for seed {seed}");
+            std::panic::resume_unwind(panic);
+        }
+    }
+}