From ee7f783fc3f540cdce59ab10d9e4972a96e9e22e Mon Sep 17 00:00:00 2001
From: Bernhard Kragl <bernhard.kragl@gmail.com>
Date: Thu, 16 Jan 2025 11:17:27 +0000
Subject: [PATCH] Add a deterministic constructor for `RandomState`
---
library/std/src/hash/random.rs | 38 ++++++++++++++++++++++++++++++++++
1 file changed, 38 insertions(+)
diff --git a/library/std/src/hash/random.rs b/library/std/src/hash/random.rs
index 236803b24a2ec..04382bd93cca8 100644
--- a/library/std/src/hash/random.rs
+++ b/library/std/src/hash/random.rs
@@ -75,6 +75,44 @@ impl RandomState {
RandomState { k0, k1 }
})
}
+
+ /// Constructs a new `RandomState` that is initialized with fixed keys.
+ ///
+ /// There are no guarantees about the returned value other than it being
+ /// deterministic. The returned value may vary across releases.
+ ///
+ /// This constructor is meant for testing purposes, where it is important
+ /// that a program behaves deterministically in order to reproduce test
+ /// failures. Although it is possible to use other deterministic hashers,
+ /// there are libraries which have hash collections with `RandomState`
+ /// hardcoded in their public interface.
+ ///
+ /// # Examples
+ ///
+ /// Rerunning the following program always produces the same output, which
+ /// isn't the case if `s` is created with `RandomState::new`.
+ ///
+ /// ```
+ /// #![feature(deterministic_random_state)]
+ /// use std::collections::HashSet;
+ /// use std::hash::RandomState;
+ ///
+ /// let s = RandomState::deterministic();
+ /// let mut set = HashSet::with_hasher(s);
+ /// set.insert(0);
+ /// set.insert(1);
+ /// for v in set {
+ /// println!("{v}");
+ /// }
+ /// ```
+ #[inline]
+ #[allow(deprecated)]
+ // rand
+ #[must_use]
+ #[unstable(feature = "deterministic_random_state", issue = "none")]
+ pub fn deterministic() -> RandomState {
+ RandomState { k0: 0, k1: 0 }
+ }
}
#[stable(feature = "hashmap_build_hasher", since = "1.7.0")]