Introduce HashLock type

This factors out the memoized hash part of `LazyHash` for manual use

crates/typst-utils/src/hash.rs

@@ -30,9 +30,10 @@ use siphasher::sip128::{Hasher128, SipHasher13};
 /// # Usage
 /// If the value is expected to be cloned, it is best used inside of an `Arc`
 /// or `Rc` to best re-use the hash once it has been computed.
+#[derive(Clone)]
 pub struct LazyHash<T: ?Sized> {
     /// The hash for the value.
-    hash: AtomicU128,
+    hash: HashLock,
     /// The underlying value.
     value: T,
 }
@@ -48,16 +49,7 @@ impl<T> LazyHash<T> {
     /// Wraps an item without pre-computed hash.
     #[inline]
     pub fn new(value: T) -> Self {
-        Self { hash: AtomicU128::new(0), value }
+        Self { hash: HashLock::new(), value }
-    }
-
-    /// Wrap an item with a pre-computed hash.
-    ///
-    /// **Important:** The hash must be correct for the value. This cannot be
-    /// enforced at compile time, so use with caution.
-    #[inline]
-    pub fn reuse<U: ?Sized>(value: T, existing: &LazyHash<U>) -> Self {
-        LazyHash { hash: AtomicU128::new(existing.load_hash()), value }
     }
 
     /// Returns the wrapped value.
@@ -67,33 +59,11 @@ impl<T> LazyHash<T> {
     }
 }
 
-impl<T: ?Sized> LazyHash<T> {
-    /// Get the hash, returns zero if not computed yet.
-    #[inline]
-    fn load_hash(&self) -> u128 {
-        // We only need atomicity and no synchronization of other operations, so
-        // `Relaxed` is fine.
-        self.hash.load(Ordering::Relaxed)
-    }
-}
-
 impl<T: Hash + ?Sized + 'static> LazyHash<T> {
     /// Get the hash or compute it if not set yet.
     #[inline]
     fn load_or_compute_hash(&self) -> u128 {
-        let mut hash = self.load_hash();
+        self.hash.get_or_insert_with(|| hash_item(&self.value))
-        if hash == 0 {
-            hash = hash_item(&self.value);
-            self.hash.store(hash, Ordering::Relaxed);
-        }
-        hash
-    }
-
-    /// Reset the hash to zero.
-    #[inline]
-    fn reset_hash(&mut self) {
-        // Because we have a mutable reference, we can skip the atomic.
-        *self.hash.get_mut() = 0;
     }
 }
 
@@ -140,23 +110,14 @@ impl<T: ?Sized> Deref for LazyHash<T> {
     }
 }
 
-impl<T: Hash + ?Sized + 'static> DerefMut for LazyHash<T> {
+impl<T: ?Sized + 'static> DerefMut for LazyHash<T> {
     #[inline]
     fn deref_mut(&mut self) -> &mut Self::Target {
-        self.reset_hash();
+        self.hash.reset();
         &mut self.value
     }
 }
 
-impl<T: Hash + Clone + 'static> Clone for LazyHash<T> {
-    fn clone(&self) -> Self {
-        Self {
-            hash: AtomicU128::new(self.load_hash()),
-            value: self.value.clone(),
-        }
-    }
-}
-
 impl<T: Debug> Debug for LazyHash<T> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         self.value.fmt(f)
@@ -233,3 +194,51 @@ impl<T: Debug> Debug for ManuallyHash<T> {
         self.value.fmt(f)
     }
 }
+
+/// Storage for lazy hash computation.
+pub struct HashLock(AtomicU128);
+
+impl HashLock {
+    /// Create a new unset hash cell.
+    pub const fn new() -> Self {
+        Self(AtomicU128::new(0))
+    }
+
+    /// Get the hash or compute it if not set yet.
+    #[inline]
+    pub fn get_or_insert_with(&self, f: impl FnOnce() -> u128) -> u128 {
+        let mut hash = self.get();
+        if hash == 0 {
+            hash = f();
+            self.0.store(hash, Ordering::Relaxed);
+        }
+        hash
+    }
+
+    /// Reset the hash to unset.
+    #[inline]
+    pub fn reset(&mut self) {
+        // Because we have a mutable reference, we can skip the atomic.
+        *self.0.get_mut() = 0;
+    }
+
+    /// Get the hash, returns zero if not computed yet.
+    #[inline]
+    fn get(&self) -> u128 {
+        // We only need atomicity and no synchronization of other operations, so
+        // `Relaxed` is fine.
+        self.0.load(Ordering::Relaxed)
+    }
+}
+
+impl Default for HashLock {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl Clone for HashLock {
+    fn clone(&self) -> Self {
+        Self(AtomicU128::new(self.get()))
+    }
+}

crates/typst-utils/src/lib.rs

@@ -15,7 +15,7 @@ mod scalar;
 pub use self::bitset::{BitSet, SmallBitSet};
 pub use self::deferred::Deferred;
 pub use self::duration::format_duration;
-pub use self::hash::{LazyHash, ManuallyHash};
+pub use self::hash::{HashLock, LazyHash, ManuallyHash};
 pub use self::pico::{PicoStr, ResolvedPicoStr};
 pub use self::round::{round_int_with_precision, round_with_precision};
 pub use self::scalar::Scalar;