1 files changed, 161 insertions, 0 deletions
diff --git a/library/std/src/hash/random.rs b/library/std/src/hash/random.rs
new file mode 100644
index 00000000000..a1ccbb25369
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+++ b/library/std/src/hash/random.rs
@@ -0,0 +1,161 @@
+//! This module exists to isolate [`RandomState`] and [`DefaultHasher`] outside of the
+//! [`collections`] module without actually publicly exporting them, so that parts of that
+//! implementation can more easily be moved to the [`alloc`] crate.
+//!
+//! Although its items are public and contain stability attributes, they can't actually be accessed
+//! outside this crate.
+//!
+//! [`collections`]: crate::collections
+#[allow(deprecated)]
+use super::{BuildHasher, Hasher, SipHasher13};
+use crate::cell::Cell;
+use crate::fmt;
+use crate::sys;
+
+/// `RandomState` is the default state for [`HashMap`] types.
+///
+/// A particular instance `RandomState` will create the same instances of
+/// [`Hasher`], but the hashers created by two different `RandomState`
+/// instances are unlikely to produce the same result for the same values.
+///
+/// [`HashMap`]: crate::collections::HashMap
+///
+/// # Examples
+///
+/// ```
+/// use std::collections::HashMap;
+/// use std::hash::RandomState;
+///
+/// let s = RandomState::new();
+/// let mut map = HashMap::with_hasher(s);
+/// map.insert(1, 2);
+/// ```
+#[stable(feature = "hashmap_build_hasher", since = "1.7.0")]
+#[derive(Clone)]
+pub struct RandomState {
+    k0: u64,
+    k1: u64,
+}
+
+impl RandomState {
+    /// Constructs a new `RandomState` that is initialized with random keys.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::hash::RandomState;
+    ///
+    /// let s = RandomState::new();
+    /// ```
+    #[inline]
+    #[allow(deprecated)]
+    // rand
+    #[must_use]
+    #[stable(feature = "hashmap_build_hasher", since = "1.7.0")]
+    pub fn new() -> RandomState {
+        // Historically this function did not cache keys from the OS and instead
+        // simply always called `rand::thread_rng().gen()` twice. In #31356 it
+        // was discovered, however, that because we re-seed the thread-local RNG
+        // from the OS periodically that this can cause excessive slowdown when
+        // many hash maps are created on a thread. To solve this performance
+        // trap we cache the first set of randomly generated keys per-thread.
+        //
+        // Later in #36481 it was discovered that exposing a deterministic
+        // iteration order allows a form of DOS attack. To counter that we
+        // increment one of the seeds on every RandomState creation, giving
+        // every corresponding HashMap a different iteration order.
+        thread_local!(static KEYS: Cell<(u64, u64)> = {
+            Cell::new(sys::hashmap_random_keys())
+        });
+
+        KEYS.with(|keys| {
+            let (k0, k1) = keys.get();
+            keys.set((k0.wrapping_add(1), k1));
+            RandomState { k0, k1 }
+        })
+    }
+}
+
+#[stable(feature = "hashmap_build_hasher", since = "1.7.0")]
+impl BuildHasher for RandomState {
+    type Hasher = DefaultHasher;
+    #[inline]
+    #[allow(deprecated)]
+    fn build_hasher(&self) -> DefaultHasher {
+        DefaultHasher(SipHasher13::new_with_keys(self.k0, self.k1))
+    }
+}
+
+/// The default [`Hasher`] used by [`RandomState`].
+///
+/// The internal algorithm is not specified, and so it and its hashes should
+/// not be relied upon over releases.
+#[allow(deprecated)]
+#[derive(Clone, Debug)]
+#[stable(feature = "hashmap_build_hasher", since = "1.7.0")]
+pub struct DefaultHasher(SipHasher13);
+
+impl DefaultHasher {
+    /// Creates a new `DefaultHasher`.
+    ///
+    /// This hasher is not guaranteed to be the same as all other
+    /// `DefaultHasher` instances, but is the same as all other `DefaultHasher`
+    /// instances created through `new` or `default`.
+    #[stable(feature = "hashmap_default_hasher", since = "1.13.0")]
+    #[inline]
+    #[allow(deprecated)]
+    #[rustc_const_unstable(feature = "const_hash", issue = "104061")]
+    #[must_use]
+    pub const fn new() -> DefaultHasher {
+        DefaultHasher(SipHasher13::new_with_keys(0, 0))
+    }
+}
+
+#[stable(feature = "hashmap_default_hasher", since = "1.13.0")]
+impl Default for DefaultHasher {
+    /// Creates a new `DefaultHasher` using [`new`].
+    /// See its documentation for more.
+    ///
+    /// [`new`]: DefaultHasher::new
+    #[inline]
+    fn default() -> DefaultHasher {
+        DefaultHasher::new()
+    }
+}
+
+#[stable(feature = "hashmap_default_hasher", since = "1.13.0")]
+impl Hasher for DefaultHasher {
+    // The underlying `SipHasher13` doesn't override the other
+    // `write_*` methods, so it's ok not to forward them here.
+
+    #[inline]
+    fn write(&mut self, msg: &[u8]) {
+        self.0.write(msg)
+    }
+
+    #[inline]
+    fn write_str(&mut self, s: &str) {
+        self.0.write_str(s);
+    }
+
+    #[inline]
+    fn finish(&self) -> u64 {
+        self.0.finish()
+    }
+}
+
+#[stable(feature = "hashmap_build_hasher", since = "1.7.0")]
+impl Default for RandomState {
+    /// Constructs a new `RandomState`.
+    #[inline]
+    fn default() -> RandomState {
+        RandomState::new()
+    }
+}
+
+#[stable(feature = "std_debug", since = "1.16.0")]
+impl fmt::Debug for RandomState {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("RandomState").finish_non_exhaustive()
+    }
+}